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Synthesis, structure and magnetic properties of spinel ferrite (Ni, Cu, Co)Fe2O4 from low nickel matte
Ceramics International 43 (2017) 16474–16481
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Synthesis, structure and magnetic properties of spinel ferrite (Ni, Cu, Co) Fe2O4 from low nickel matte Yu-jia Suna, Yi-fei Diaob, Hui-gang Wanga, Guangju Chena, Mei Zhanga, Min Guoa, a b
MARK
⁎
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China College of Engineering, Peking University, Beijing 100871, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Low nickel matte Spinel ferrite (Ni,Cu,Co)Fe2O4 Co-precipitation Calcination
Spinel ferrite (Ni, Cu, Co)Fe2O4 was synthesized from the low nickel matte by using a co-precipitation-calcination method for the first time. The influences of the added amount of NiCl2·6H2O, calcination temperature and time on the structure and magnetic properties of the as-prepared ferrites were studied in detail by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy, and Vibrating sample magnetometer (VSM). It is indicated that pure (Ni, Cu, Co)Fe2O4 with cubic phase could be obtained under the experimental conditions (NiCl2·6H2O added amount of 3.0: 100 g mL−1, calcination temperature from 800 to 1000 °C and calcination time from 1 to 3 h). With increasing calcination temperature and time, saturation magnetization (MS) of the synthesized (Ni, Cu, Co)Fe2O4 increased and the coercivity (HC) decreased. Under the optimum conditions (i.e. NiCl2·6H2O added amount of 3.0: 100 g mL−1, 1000 °C, 3 h), the MS and HC values of the product were approximately 46.1 emu g−1 and 51.0 Oe, respectively, which were competitive to those of other nickel ferrites synthesized from pure chemical reagents. This method explores a novel pathway for efficient and comprehensive utilization of the low nickel matte.
1. Introduction Low nickel matte, which is the intermediate product of the nickel sulphide smelting process, is richer in multi-metal elements, such as Ni, Cu, Co, and Fe, than nickel ore concentrate. The main mineral phases of the low nickel matte mostly consist of sulfides, such as pentlandite ((Fe,Ni)9S8) and bornite (Cu5FeS4), others are magnetite (Fe3O4) and NiFe alloy (FeNi3) [1,2]. Currently, nickel extraction from Ni-bearing sulfides with high efficiency has been realized by hydrometallurgical process normally including oxidation acid leaching [3–7], oxidation ammonia leaching [8,9] or other methods [10,11]. However, few methods [12–14] have been taken on leaching of nickel from the low nickel matte. In our previous works [14], by using a new hydrometallurgical process, namely, acid pre-leaching followed by oxidative ammonia leaching process ((NH4)2S2O8/NH3·H2O solution system), Ni, Cu, and Co were extracted efficiently and selectively from low nickel matte with high efficiencies of 98.03%, 99.13% and 85.60%, respectively. Till now, as for the treatment of metal ores, much more attention has been usually paid on only extracting Ni from the ores regardless of nickel loss and other co-existing valuable elements such as Co, Cu and Fe during the leaching and separation processes. This is not only a
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waste of resources, but a potentially dangerous source of environmental pollution. A more effective solution to solve this problem would come from increasing nickel leaching and separation efficiency, while simultaneously integrating the utilization of elements from metal ores. Unfortunately, to the best of our knowledge, the studies on the comprehensive utilization of metal elements from the low nickel matte leaching solution have rarely been reported, and the synthesis of metaldoped spinel ferrite ((Ni, Cu, Co)Fe2O4) with high magnetic properties from the low nickel matte is still a challenge issue. Spinel ferrite has generated much attention because of its unique properties and broad applicability in the areas of magnetic high density storage [15], magnetic media [16], microwave absorbers [17], ferrofluids [18], etc. Generally, the formula for spinel ferrite is MFe2O4 (M represents Ni, Cu, Co, Mg, Mn, etc.; Fe represents Al, Cr, etc.), and it can have normal and inverse spinel structures. Nickel ferrite (NiFe2O4) is an inverse spinel in which the tetrahedral (or A-sites) are occupied by half of ferric ions, and the octahedral (or B-sites) by ferric and nickel ions [19]. As a kind of soft magnetic material, nickel ferrite with moderate saturation magnetization (MS) and coercivity (HC), has been synthesized by various approaches such as co-precipitation method [20–23], sol-gel method [24–27], hydrothermal method [28–30], microemulsion method [31] and electrospinning-calcination method [32,33]. Different
Corresponding author. E-mail address: [email protected] (M. Guo).
http://dx.doi.org/10.1016/j.ceramint.2017.09.029 Received 21 July 2017; Received in revised form 25 August 2017; Accepted 4 September 2017 Available online 06 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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pure chemical reagents were selected to synthesize the ferrites by controlling the molar ratio of Fe to M (divalent ions) to approximately 2.0. Joshi et al. [21] reported that pure nickel ferrite nanoparticles with a size distribution of 8–20 nm were synthesized from nickel and ferric nitrate salts using co-precipitation method. They found that increasing the temperature would increase the values of MS and HC from 20.1 to 35.5 emu g−1 and 100 to 124.4 Oe, respectively. Xiang et al. [33] prepared uniform Co(1−x)Ni(x)Fe(2)O(4) (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanofibers by calcining the electrospun precursors. The results showed that the MS value of the obtained nickel ferrite was significantly improved from 29.3 to 56.4 emu g−1 by Co doping into the ferrite. Sharma et al. [34] presented that NiFe2O4, Ni0.95Cu0.05Fe2O4 and Ni0.94Cu0.05Co0.01Fe2O4 thin films can be synthesized by using metallorganic decomposition method (MOD) and spin coating technique. They found that with Cu, Co doping amounts increasing, the as-prepared Cu-doped NiFe2O4 and Cu-Co co-doped NiFe2O4 exhibited enhanced MS (180 and 196 emu cm−3) and HC (209 and 216 Oe) compared with undoped NiFe2O4 (150 emu cm−3, 205 Oe). Based on the above analysis, metal co-doped nickel ferrite could contribute to the improvement of magnetic performance. However, till now, none of studies have been done on synthesizing nickel ferrite, especially Cu, Co co-doped nickel ferrite from the low nickel matte. In this paper, the acid leaching solution of low nickel matte, which mainly contained Fe, Ni, Cu and Co, etc., was selected as the precursor for synthesizing metal co-doped nickel ferrite by co-precipitation-calcination method. The effects of experimental conditions including the added amount of NiCl2·6H2O, calcination temperature and time on the chemical compositions, structure evolution and magnetic properties of the prepared products were systematically investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy and vibrating sample magnetometer (VSM). The morphology of the asprepared (Ni, Cu, Co)Fe2O4 were characterized by scanning electron microscopy (SEM). This paper may pave a new approach to make use of low nickel matte for magnetic functional materials.
Fig. 1. XRD pattern of the low nickel matte.
2. Experimental 2.1. Materials The precursors of the spinel ferrite (Ni, Cu, Co)Fe2O4 were obtained from low nickel matte leaching solutions. The low nickel matte used in this study was obtained from a Jilin nickel smelter in China. The low nickel matte was firstly dried at 100 °C for 24 h, then was ground into powder with a particle size smaller than 0.75 µm. The typical chemical compositions of low nickel matte are listed in Table 1 and mainly included Fe, Ni, Cu, Co and S along with minor amounts of Al and Mn. The XRD pattern of the low-nickel matte is presented in Fig. 1, and the main minerals were pentlandite ((Fe,Ni)9S8), magnetite (Fe3O4), FeNi alloy (FeNi3) and bornite (Cu5FeS4). Analytical reagent grade hexahydrated ferric chloride (FeCl3·6H2O), hydrochloric acid (HCl) (36–38 wt%), hydrogen peroxide (H2O2) (30 wt%), and sodium hydroxide (NaOH) were purchased from the Beijing Reagent Factory of China. 2.2. Experimental procedure A flow chart for the synthesis of (Ni, Cu, Co)Fe2O4 from low nickel matte is shown in Fig. 2. The whole process can be divided into the following two parts. Table 1 Main chemical compositions and contents of low nickel matte by XRF analysis. Element
Fe
Ni
Cu
Co
S
Mg
Mn
Al
others
Content (wt%)
35.08
31.08
5.20
1.04
27.36
0.37
0.01
0.02
2.81
Fig. 2. A flow chart of the synthesis of (Ni, Cu, Co)Fe2O4 from low nickel matte.
2.2.1. Selective and efficient leaching of Ni, Cu, Co and Fe from the low nickel matte In our previous work, the nickel matte leaching solutions were prepared as follows: A total of 12.5 g low-nickel matte was added into a 500 mL three-neck round-bottomed flask, and then a mixture of HCl (0.5 mol L−1) and FeCl3 (1 mol L−1) was added (250 mL). The flask was connected to a mechanical electric stirrer (900 r·min−1) and condenser, and heated in a water bath at 90 °C for 7 h. After the leaching process, the leaching slurry was separated by centrifugation at a speed of 4500 r·min−1 for 10 min. Thus, leaching solutions containing Ni, Cu, Co and Fe etc. were obtained. The leaching residue was washed to neutral and dried at 95 °C. All the experiments were conducted in duplicate to ensure the repeatability. The obtained leaching solution was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES, America, PE), and the results are summarized in Table 2. The extraction efficiencies (ŋx) of Ni, Cu, and Co were calculated to be about 98.4%, 98.9%, and 97.3%, respectively, according to the Eq. (1).
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Table 2 Chemical analysis of the leaching solution under the optimum conditions. (FeCl3: 1 mol L−1, HCl: 0.5 mol L-1, liquid to solid ratio: 20 mL g-1, 7 h and 90 °C). Compositions
Fe
Ni
Cu
Co
Content (wt%) Content (mol L−1)
27.7 0.496
6.0 0.102
1.2 0.018
0.2 0.003
ŋx =
Cx V ×100% MWx
Table 3 Chemical compositions of the low nickel matte leaching solution, solubility product Ksp and pH value of completely precipitate for metal hydroxides at 25 °C.
(1)
where x represents the metal element in leaching solution, Ni, Cu, Co; Cx is the concentration of x, g·L−1; V is the volume of leaching solution, L; M is the mass of low nickel matte, g; Wx is the mass fraction of x, wt %. 2.2.2. Synthesis of (Ni, Cu, Co)Fe2O4 from acid leaching solution It can be calculated from Table 2 that the mole ratio of Fe to M (Ni, Cu, Co) in the leaching solution of low nickel matte was about 4.01 which was larger than 2.00, suggesting that the amount of M was insufficient for synthesizing single phase ferrite. Considering that Ni, Cu and Co ions co-exist in the leaching solution of low nickel matte besides Fe ions, some water-soluble salts which contain divalent metal ions such as NiCl2, NiSO4, Ni(NO3)2, CuSO4, CuCl2, etc. could be added into the reaction system to synthesize different spinel ferrites with unique properties. In this experiment, NiCl2·6H2O was chosen as the addictive to the leaching solution of low nickel matte to prepare pure (Ni,Cu,Co) Fe2O4 ferrite. In detail, firstly, leaching solution (20 mL) was placed in a conical flask under the atmospheric pressure and equipped with a magnetic stirrer. A certain amounts of NiCl2·6H2O (2.2: 100 g mL−1, 3.0: 100 g mL−1, 3.4: 100 g mL−1) were added into the leaching solution in order to adjust the molar of Fe: M. Secondly, hydrogen peroxide (H2O2) was added to oxidize the excess Fe2+ in the solution into Fe3+. Finally, co-precipitation processes were conducted at a pH value of 12 by using NaOH solution (5 mol L−1) as the precipitant. The solid-liquid separation was conducted in a centrifuge with the speed of 5000 r·min−1 for 5 min. Then the precipitate was rinsed with deionized water for five times and dried for 12 h in an oven at 95 °C. Finally, the precipitate was ground into powder and calcined at different temperatures (800 °C, 900 °C, 1000 °C) and time (1 h, 2 h, 3 h).
Element
Leaching solution concentration (mol L-1)
Solubility product constant Ksp
Fe Ni Cu Co
0.496 0.102 0.018 0.003
4. 2. 2. 1.
0 0 2 1
× × × ×
10–38 10–15 10–13 10–15
3.2 9.2 6.7 9.0
(Fex,Ni9−x)S8 + 18FeCl3 = (18 + x)FeCl2 + (9-x)NiCl2 + 8S°
(2)
Cu5FeS4 + 12FeCl3 = 5CuCl2 + 13FeCl2 + 4S°
(3)
FeNi3 + 8FeCl3 = 9FeCl2 + 3NiCl2
(4)
FeNi3 + 8HCl = FeCl2 + 3NiCl2 + 4H2
(5)
Fe3O4 + 8HCl = FeCl2 + 2FeCl3 + 4H2O
(6)
During the preparation process, NiCl2·6H2O was added into the solution in order to adjust the molar ratio of Fe: (Ni + Cu + Co) to approximately 2: 1. Because Fe3+ ions were reduced to Fe2+ ions (Eqs. (2)–(6)) during the leaching process, a sufficient amount H2O2 was added into the solution to ensure that all of the Fe2+ ions were oxidized to Fe3+ ions. Thereafter, NaOH (5 mol L−1) solution was used to adjust the pH value of the solution to 12.0 in order to completely precipitate Fe, Ni, Cu and Co (Eqs. (7)–(10)), as indicated in Table 3. In addition, thermodynamic equilibrium calculations were conducted to elucidate the possibility of reactions during the calcination process. The system was simplified into the Ni-Fe-O system. The changes in standard Gibbs free energy (ΔGθ) with temperature increasing [35] are shown in Fig. 3. Fe3+ + 3OH- = Fe(OH)3
(7)
Ni2+ + 2OH- = Ni(OH)2
(8)
Cu2+ + 2OH- = Cu(OH)2
(9)
Co2+ + 2OH- = Co(OH)2
(10)
2Fe(OH)3 → Fe2O3 + 3H2O θ
ΔG = −13.793 − 0.4217 T KJ mol
2.3. Analysis and characterization X-ray fluorescence (XRF-1800, Japan) and X-ray diffraction (XRD, Japan, Rigaku) were used to analyze the chemical composition and structure of low nickel matte and the synthesized (Ni, Cu, Co)Fe2O4. The elements concentrations of the leaching solution were analyzed by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, America, PE). The micro morphology of the as-prepared ferrites was characterized by Scanning electron microscope (SEM, Germany, Zeiss). Raman spectrometer (HR800, France, JY) was used to study the molecular interactions. Vibrating sample magnetometer (VSM, America, Quantum Design) was used to obtain the magnetic characterization of the as-prepared samples.
pH value of completely precipitation
Ni(OH)2 → NiO + H2O
(11) −1
(12)
3. Results and discussion 3.1. Leaching mechanism and preparation principle of (Ni, Cu, Co)Fe2O4 During the leaching process, the FeCl3-HCl solution system was chosen for extraction of metal ions from the low nickel matte (CuFeS2, CuS2, (Fe,Ni)9S8, Ni3S2, etc.) due to that the Fe3+ ions are strong oxidisers and HCl can dissolve FeNi alloy while restraining the hydrolysis of Fe3+ ions. The main reactions in the leaching process are depicted as Eqs. (2)–(6).
Fig. 3. Variation of ΔGθ with the change of temperature for different reactions during the calcination process.
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ΔGθ = 10.770 − 0.1387 T KJ mol−1 Ni(OH)2 + 2Fe(OH)3 → NiFe2O4 + 4H2O θ
ΔG = −22.671 −0.5549 T KJ mol Ni(OH)2 + Fe2O3 → NiFe2O4 + H2O θ
ΔG = −8.884 − 0.1332 T KJ mol
(13)
−1
(14) −1
NiO + 2Fe(OH)3 → NiFe2O4 + 3H2O
(15)
ΔGθ = −33.451 − 0.4162 T KJ mol−1 NiO + Fe2O3 → NiFe2O4 θ
(16) −1
ΔG = −19.657 − 0.0055 T KJ mol From the thermodynamic point of view, the more negative the value of ΔGθ is, the more likely the reaction occurs. Therefore, all the reactions ((11)–(16)) may occur with the calcination temperatures varying from 800 to 1000 °C, and NiFe2O4 or Fe2O3 can co-exist in the products based on reactions (13), (15) and (11), implying Fe2O3 may be the main impurity in the products.
Fig. 5. XRD spectra of the as-prepared (Ni, Cu, Co)Fe2O4 calcined at different temperatures for 2 h (a) 800 °C, (b) 900 °C and (c) 1000 °C.
3.2. Effect of added amount of NiCl2·6H2O
3.3. Effect of calcination temperature
In order to investigate the effect of added amounts of NiCl2·6H2O in the leaching solution (different solid-liquid ratios, g mL−1) on the structures and compositions of ferrites, XRD analysis of as-synthesized samples were conducted and the results were shown in Fig. 4. It can be clearly seen From Fig. 4 that the diffraction peaks located at 18.4°, 30.2°, 35.6°, 37.3°, 43.3°, 53.8°, 57.3° and 63.0° which existed in all of the XRD patterns, coincided well with the standard diffraction peaks of cubic NiFe2O4 (JCPDS: 01-074-2081) for the crystal faces of (111), (200), (311), (222), (400), (422), (511) and (440). Decreasing the added amount of NiCl2·6H2O from 3.0: 100 to 2.2: 100 g mL−1 led to the formation of impurity Fe2O3 (JCPDS: 01-087-1164). However, with NiCl2·6H2O amount increasing from 3.0: 100 to 3.4: 100 g mL−1, the diffraction peaks of Fe2O3 disappeared, while that of NiO (JCPDS: 01089-7130) phase occurred, suggesting that pure spinel ferrite NiFe2O4 could not be synthesized under the two experiment conditions. When the amount of NiCl2·6H2O was controlled at 3.0: 100 g mL−1, only single phase (Ni, Cu, Co)Fe2O4 ferrite was obtained, further confirming that the amount of addictive played a key role in determining the structure and composition of as-prepared ferrites.
Fig. 5 illustrates the XRD spectra of the as-prepared metal-doped ferrites calcined at different temperatures (800, 900, and 1000 °C) for 2 h. All the diffraction peaks located at 18.4°, 30.2°, 35.6°, 37.3°, 43.3°, 53.8°, 57.3° and 63.0° corresponded well with the standard diffraction peaks of NiFe2O4 (JCPDS: 01-074-2081, cubic crystal system) for the crystal faces of (111), (200), (311), (222), (400), (422), (511) and (440). No other phases peaks were detected, indicating that the pure (Ni, Cu, Co)Fe2O4 were obtained as the temperature increased from 800 to 1000 °C. In addition, the enhanced diffraction peak intensities of the spinel ferrites confirmed the better crystallinity of the samples. The average grain sizes calculated based on Eq. (17) [36] increased from 59.92 to 86.01 nm with the temperature rising from 800 to 1000 °C, suggesting that the as-synthesized samples were nano sized (Ni, Cu, Co) Fe2O4.
D = (Kλ )/(B cos θ)
(17)
where D is the thickness along the crystal face (also regarded as a the grain size); λ is the wavelength of X-ray radiation, 0.15406 nm; θ is the Prague diffraction angle and B is the half-width of the diffraction peak. Fig. 6 shows the SEM image of (Ni, Cu, Co)Fe2O4 prepared at 1000 °C for 3 h. The obtained ferrite consisted of aggregated particles, and the particle size distribution range was wide, spanning dozens to
Fig. 4. XRD patterns of as-synthesized samples with different added amounts of NiCl2·6H2O into the leaching solutions (a) 2.2: 100 g mL−1, (b) 3.0: 100 g mL−1, (c) 3.4: 100 g mL−1 calcined at 1000 °C for 2 h.
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Fig. 6. SEM image of (Ni, Cu, Co)Fe2O4 synthesized at 1000 °C for 3 h.
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Fig. 7. Raman spectra of (Ni, Cu, Co)Fe2O4 synthesized at different calcination temperatures (a) 800 °C, (b) 900 °C, (c) 1000 °C.
hundreds of nanometers. The molecular formula of the as-prepared nickel ferrite can be denoted as Ni0.913Cu0.074Co0.013Fe2O4 according to ICP analysis, as indicated in the inset of Fig. 6, confirming that metal co-doped ferrite was obtained. In order to further reveal the structural properties of the (Ni, Cu, Co) Fe2O4, Raman spectra of the ferrites calcined at different temperatures were recorded, as depicted in Fig. 7, and the corresponding Raman shifts are summarized in Table 4. Generally, spinel ferrite is a cubic spinel structure, and belongs to the space group Oh7 (Fd-3m). Tetrahedron and octahedron in the lattice are made up of oxygen anion (O2-), and metal ions occupy both the tetrahedral and octahedral sites. Five Raman feature peaks, (A1g + Eg + 3T2g), correspond to the motion of O ions and ions at A-sites and B-sites [37]. The A1g matches to the symmetric stretching vibration of the oxygen anion, whereas Eg matches to the symmetric bending vibration of the oxygen anion, and T2g corresponds to asymmetric stretching vibration between oxygen anions and cations in the A-site and B-site [30]. It can be seen from Fig. 7 that all of the Raman peaks were assigned to the A1g, T2g and Eg modes reflecting the vibration of spinel structures, indicating that the pure (Ni, Cu, Co) Fe2O4 were obtained when the calcination temperature varied from 800 to 1000 °C. Especially, the peak intensity ratio of I (A1g(1)): I(A1g(2)) which represents the quantitative surface composition ratio increased gradually from 3.07 to 3.15 and 3.65, suggesting that some divalent cations might shift from the A-site to the B-site, meanwhile the same numbers of Fe3+ cations might migrate from the B-site to A-site with the calcination temperature increasing. Fig. 8 presents the magnetic hysteresis loops of the samples Table 4 Raman modes of the (Ni, Cu, Co)Fe2O4 synthesized at different calcination temperatures for 2 h. Assignment
A1g(1) [30] A1g(2) [30] T2g(3) [30] T2g(2) [30] Eg [39] T2g(1) [39]
Vibrations [30]
symmetric stretching symmetric stretching asymmetric stretching asymmetric stretching bending asymmetric stretching
Ions involved
Raman modes (cm-1) 800 °C
900 °C
1000 °C
A-O (Fe-O) [30,38] A-O (Fe-O) [38,39] A-O [38,39]
687
684
687
640
638
640
548
544
548
A-O [30,38]
470
467
469
A-O [38] A-O [38,39]
313 197
310 196
313 197
A (Ni, Cu, Co) represents metal ions in tetrahedral sites.
prepared at different calcination temperatures for 2 h, and MS and HC values of the obtained (Ni, Cu, Co)Fe2O4 and the sample in reference [21] are given in Fig. 9. It can be observed that MS value increased from 36.0 to 44.2 emu g−1 as the temperature increased from 800 to 1000 °C, agreeing well with the variation trend of other literature's [40–42]. All the obtained samples exhibited larger MS values compared with samples synthesized from pure reagents [21]. According to the Neel's two sublattice models [43], the magnetic moment per formula unit (M) of the lattice is the difference between the magnetic moments of B and A sublattices, namely, M = MB - MA. According to the analysis of Raman spectra of the prepared (Ni, Cu, Co)Fe2O4 as indicated in Fig. 7, some divalent cations including Cu2+ (1 μB), Ni2+ (2 μB), and Co2+ (3 μB), might migrate from the A-site to the B-site. Simultaneously, the same quantity of Fe3+ (5 μB) moved from the B-site to the A-site. Therefore, considering the differences of these migrating ions magnetic moments, the MB decreased while MA increased, thus the MS values of the formed ferrites decreased accordingly with the increase of temperature. However, the factors that determine the magnetic property of ferrite were not just the internal structure. With the increase of calcination temperature, the crystallinity became higher and average grain size became larger, leading to the decrease of the proportion of surface atoms. That made the magnetic moment tend to the external field, and the magnetic anisotropy weaken, which finally enhanced the Ms values of as-synthesized ferrites. In addition, the HC values of the (Ni, Cu, Co)Fe2O4 decreased from 158.5 to 54.1 Oe with increasing the temperature (Fig. 8(b) and Fig. 9). This phenomenon may be ascribed to the following reasons. With the increase of grain size and ferrite magnetic domain, the displacement resistance of domain wall decreased, which would increase the uniformity and weaken the magnetic anisotropy of the ferrites, thus, HC values decreased accordingly [43,44]. 3.4. Effect of calcination time Fig. 10 shows the XRD spectra of the as-prepared metal-doped ferrites calcined at 1000 °C for different time (1 h, 2 h and 3 h). All the diffraction peaks were well indexed to the standard diffraction peaks (18.4°, 30.2°, 35.6°, 37.3°, 43.3°, 53.8°, 57.3° and 63.0°) of NiFe2O4 (JCPDS: 01-074-2081, cubic crystal system) for the crystal faces of (111), (200), (311), (222), (400), (422), (511) and (440). No peak for other phase was detected, which indicated that pure (Ni, Cu, Co)Fe2O4 were obtained at 1000 °C for 1–3 h. The average grain sizes calculated based on Eq. (17) increased from 61.29 to 92.32 nm with the time rising from 1 to 3 h. Raman spectra and the corresponding Raman shift changes of the (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time (1 h, 2 h and 3 h) are illustrated in Fig. 11 and Table 5. All of the Raman peaks which corresponded to A1g, T2g and Eg modes reflected the vibrations of (Ni, Cu, Co)Fe2O4, indicating that pure (Ni, Cu, Co)Fe2O4 were obtained at different calcination time. In addition, the peak intensity ratio, I (A1g(1))：I(A1g(2)), increased rapidly from 1.90 to 3.15 with calcination time increasing from 1 to 2 h, then remain unchanged between 2 to 3 h, suggesting that some Fe3+ cations moved from the B-site to the Asite, as the same amount of divalent cations shifted from the A-site to the B-site. After 2 h reaction, the migration of metal ions reached equilibrium. The magnetic hysteresis loops and HC values of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time are illustrated in Fig. 12, and the corresponding comparison of MS and HC is summarized in Fig. 13. The MS increased from 44.0 to 46.1 emu g−1 and HC decreased from 82.2 to 51.0 Oe as the calcination time changed from 1 to 3 h. All the obtained samples exhibited larger MS and smaller HC values compared with the product synthesized from pure reagents (35.5 emu g−1 and 124.4 Oe) [21]. This phenomenon may be ascribed to the changes in the internal structure of the ferrite, which caused by the ion movement in the obtained (Ni, Cu, Co)Fe2O4 (Fig. 11) and other factors discussed in Section
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Fig. 8. (a) The magnetic hysteresis loops of the as-prepared (Ni, Cu, Co)Fe2O4 calcined at different temperatures and (b) HC values of the products.
Fig. 11. Raman spectra of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time (a) 1 h, (b) 2 h and (c) 3 h.
Fig. 9. MS and HC values of as-synthesized (Ni, Cu, Co)Fe2O4 at different temperatures and the sample in reference [21].
Table 5 Raman parameters of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time. Assignment
Vibrations [30]
A1g(1) [30]
symmetric stretching
A1g(2) [30]
symmetric stretching
T2g(3) [30]
asymmetric stretching asymmetric stretching bending asymmetric stretching
T2g(2) [30] Eg [39] T2g(1) [39]
Ions involved
Raman modes (cm-1) 1h
2h
3h
A-O (Fe-O) [30,38] A-O (Fe-O) [38,39] A-O [38,39]
690
687
688
649
640
642
552
548
549
A-O [30,38]
473
469
471
A-O [38] A-O [38,39]
316 190
313 197
315 196
A (Ni, Cu, Co) represents metal ions in tetrahedral sites.
Fig. 10. XRD spectra of the as-prepared (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time (a) 1 h, (b) 2 h and (c) 3 h.
3.3, such as changes of the magnetic moment and the magnetic anisotropy. In addition, the larger average grain size (from 1 to 3 h of calcination) may also lead to a higher MS value. Moreover, with the
increase of the grain size and ferrite magnetic domain, the uniformity increased and the magnetic anisotropy decreased, which all resulted in the decreased HC values of the formed ferrites.
4. Conclusions
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In this paper, the pure (Ni, Cu, Co)Fe2O4 was successfully
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Fig. 12. (a) The magnetic hysteresis loops of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time and (b) HC values of the products.
References
Fig. 13. MS and HC values of as-synthesized (Ni, Cu, Co)Fe2O4 synthesized at 1000 °C for different time and the sample in reference [21].
synthesized via the co-precipitation-calcination method from the leaching solutions of low nickel matte for the first time. All the added amount of NiCl2·6H2O, calcination temperature and time played important roles in determining the structure and magnetic properties of the as-prepared ferrites. When the added amount of NiCl2·6H2O was controlled at 3.0: 100 g mL−1, pure (Ni, Cu, Co)Fe2O4 ferrite could be obtained. In addition, with the increase of calcination temperature and time, the MS values of formed (Ni, Cu, Co)Fe2O4 increased while HC values decreased. The magnetic property of the ferrite were not only affected by the ion movement, but also by the magnetic moment and the magnetic anisotropy caused by crystalline and average grain size. Under the optimum conditions (NiCl2·6H2O added amount of 3.0: 100 g mL−1, 1000 °C and 3 h), the obtained (Ni, Cu, Co)Fe2O4 exhibited a higher MS of 46.1 emu g−1 and a lower HC of 51.0 Oe compared with other nickel ferrites synthesized from pure chemical reagents. The process proposed in this work realized the comprehensive utilization of the low nickel matte for synthesis of functional metal-doped spinel ferrite.
Acknowledgements This work was financially supported by the National Basic Research Priorities Program of China (No. 2014CB643401, No. 2013AA032003), and the National Natural Science Foundation of China (No. 51372019).
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Synthesis, structure and magnetic properties of spinel ferrite (Ni, Cu, Co) Fe2O4 from low nickel matte Yu-jia Suna, Yi-fei Diaob, Hui-gang Wanga, Guangju Chena, Mei Zhanga, Min Guoa, a b
MARK
⁎
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China College of Engineering, Peking University, Beijing 100871, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Low nickel matte Spinel ferrite (Ni,Cu,Co)Fe2O4 Co-precipitation Calcination
Spinel ferrite (Ni, Cu, Co)Fe2O4 was synthesized from the low nickel matte by using a co-precipitation-calcination method for the first time. The influences of the added amount of NiCl2·6H2O, calcination temperature and time on the structure and magnetic properties of the as-prepared ferrites were studied in detail by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Raman spectroscopy, and Vibrating sample magnetometer (VSM). It is indicated that pure (Ni, Cu, Co)Fe2O4 with cubic phase could be obtained under the experimental conditions (NiCl2·6H2O added amount of 3.0: 100 g mL−1, calcination temperature from 800 to 1000 °C and calcination time from 1 to 3 h). With increasing calcination temperature and time, saturation magnetization (MS) of the synthesized (Ni, Cu, Co)Fe2O4 increased and the coercivity (HC) decreased. Under the optimum conditions (i.e. NiCl2·6H2O added amount of 3.0: 100 g mL−1, 1000 °C, 3 h), the MS and HC values of the product were approximately 46.1 emu g−1 and 51.0 Oe, respectively, which were competitive to those of other nickel ferrites synthesized from pure chemical reagents. This method explores a novel pathway for efficient and comprehensive utilization of the low nickel matte.
1. Introduction Low nickel matte, which is the intermediate product of the nickel sulphide smelting process, is richer in multi-metal elements, such as Ni, Cu, Co, and Fe, than nickel ore concentrate. The main mineral phases of the low nickel matte mostly consist of sulfides, such as pentlandite ((Fe,Ni)9S8) and bornite (Cu5FeS4), others are magnetite (Fe3O4) and NiFe alloy (FeNi3) [1,2]. Currently, nickel extraction from Ni-bearing sulfides with high efficiency has been realized by hydrometallurgical process normally including oxidation acid leaching [3–7], oxidation ammonia leaching [8,9] or other methods [10,11]. However, few methods [12–14] have been taken on leaching of nickel from the low nickel matte. In our previous works [14], by using a new hydrometallurgical process, namely, acid pre-leaching followed by oxidative ammonia leaching process ((NH4)2S2O8/NH3·H2O solution system), Ni, Cu, and Co were extracted efficiently and selectively from low nickel matte with high efficiencies of 98.03%, 99.13% and 85.60%, respectively. Till now, as for the treatment of metal ores, much more attention has been usually paid on only extracting Ni from the ores regardless of nickel loss and other co-existing valuable elements such as Co, Cu and Fe during the leaching and separation processes. This is not only a
⁎
waste of resources, but a potentially dangerous source of environmental pollution. A more effective solution to solve this problem would come from increasing nickel leaching and separation efficiency, while simultaneously integrating the utilization of elements from metal ores. Unfortunately, to the best of our knowledge, the studies on the comprehensive utilization of metal elements from the low nickel matte leaching solution have rarely been reported, and the synthesis of metaldoped spinel ferrite ((Ni, Cu, Co)Fe2O4) with high magnetic properties from the low nickel matte is still a challenge issue. Spinel ferrite has generated much attention because of its unique properties and broad applicability in the areas of magnetic high density storage [15], magnetic media [16], microwave absorbers [17], ferrofluids [18], etc. Generally, the formula for spinel ferrite is MFe2O4 (M represents Ni, Cu, Co, Mg, Mn, etc.; Fe represents Al, Cr, etc.), and it can have normal and inverse spinel structures. Nickel ferrite (NiFe2O4) is an inverse spinel in which the tetrahedral (or A-sites) are occupied by half of ferric ions, and the octahedral (or B-sites) by ferric and nickel ions [19]. As a kind of soft magnetic material, nickel ferrite with moderate saturation magnetization (MS) and coercivity (HC), has been synthesized by various approaches such as co-precipitation method [20–23], sol-gel method [24–27], hydrothermal method [28–30], microemulsion method [31] and electrospinning-calcination method [32,33]. Different
Corresponding author. E-mail address: [email protected] (M. Guo).
http://dx.doi.org/10.1016/j.ceramint.2017.09.029 Received 21 July 2017; Received in revised form 25 August 2017; Accepted 4 September 2017 Available online 06 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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pure chemical reagents were selected to synthesize the ferrites by controlling the molar ratio of Fe to M (divalent ions) to approximately 2.0. Joshi et al. [21] reported that pure nickel ferrite nanoparticles with a size distribution of 8–20 nm were synthesized from nickel and ferric nitrate salts using co-precipitation method. They found that increasing the temperature would increase the values of MS and HC from 20.1 to 35.5 emu g−1 and 100 to 124.4 Oe, respectively. Xiang et al. [33] prepared uniform Co(1−x)Ni(x)Fe(2)O(4) (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanofibers by calcining the electrospun precursors. The results showed that the MS value of the obtained nickel ferrite was significantly improved from 29.3 to 56.4 emu g−1 by Co doping into the ferrite. Sharma et al. [34] presented that NiFe2O4, Ni0.95Cu0.05Fe2O4 and Ni0.94Cu0.05Co0.01Fe2O4 thin films can be synthesized by using metallorganic decomposition method (MOD) and spin coating technique. They found that with Cu, Co doping amounts increasing, the as-prepared Cu-doped NiFe2O4 and Cu-Co co-doped NiFe2O4 exhibited enhanced MS (180 and 196 emu cm−3) and HC (209 and 216 Oe) compared with undoped NiFe2O4 (150 emu cm−3, 205 Oe). Based on the above analysis, metal co-doped nickel ferrite could contribute to the improvement of magnetic performance. However, till now, none of studies have been done on synthesizing nickel ferrite, especially Cu, Co co-doped nickel ferrite from the low nickel matte. In this paper, the acid leaching solution of low nickel matte, which mainly contained Fe, Ni, Cu and Co, etc., was selected as the precursor for synthesizing metal co-doped nickel ferrite by co-precipitation-calcination method. The effects of experimental conditions including the added amount of NiCl2·6H2O, calcination temperature and time on the chemical compositions, structure evolution and magnetic properties of the prepared products were systematically investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman spectroscopy and vibrating sample magnetometer (VSM). The morphology of the asprepared (Ni, Cu, Co)Fe2O4 were characterized by scanning electron microscopy (SEM). This paper may pave a new approach to make use of low nickel matte for magnetic functional materials.
Fig. 1. XRD pattern of the low nickel matte.
2. Experimental 2.1. Materials The precursors of the spinel ferrite (Ni, Cu, Co)Fe2O4 were obtained from low nickel matte leaching solutions. The low nickel matte used in this study was obtained from a Jilin nickel smelter in China. The low nickel matte was firstly dried at 100 °C for 24 h, then was ground into powder with a particle size smaller than 0.75 µm. The typical chemical compositions of low nickel matte are listed in Table 1 and mainly included Fe, Ni, Cu, Co and S along with minor amounts of Al and Mn. The XRD pattern of the low-nickel matte is presented in Fig. 1, and the main minerals were pentlandite ((Fe,Ni)9S8), magnetite (Fe3O4), FeNi alloy (FeNi3) and bornite (Cu5FeS4). Analytical reagent grade hexahydrated ferric chloride (FeCl3·6H2O), hydrochloric acid (HCl) (36–38 wt%), hydrogen peroxide (H2O2) (30 wt%), and sodium hydroxide (NaOH) were purchased from the Beijing Reagent Factory of China. 2.2. Experimental procedure A flow chart for the synthesis of (Ni, Cu, Co)Fe2O4 from low nickel matte is shown in Fig. 2. The whole process can be divided into the following two parts. Table 1 Main chemical compositions and contents of low nickel matte by XRF analysis. Element
Fe
Ni
Cu
Co
S
Mg
Mn
Al
others
Content (wt%)
35.08
31.08
5.20
1.04
27.36
0.37
0.01
0.02
2.81
Fig. 2. A flow chart of the synthesis of (Ni, Cu, Co)Fe2O4 from low nickel matte.
2.2.1. Selective and efficient leaching of Ni, Cu, Co and Fe from the low nickel matte In our previous work, the nickel matte leaching solutions were prepared as follows: A total of 12.5 g low-nickel matte was added into a 500 mL three-neck round-bottomed flask, and then a mixture of HCl (0.5 mol L−1) and FeCl3 (1 mol L−1) was added (250 mL). The flask was connected to a mechanical electric stirrer (900 r·min−1) and condenser, and heated in a water bath at 90 °C for 7 h. After the leaching process, the leaching slurry was separated by centrifugation at a speed of 4500 r·min−1 for 10 min. Thus, leaching solutions containing Ni, Cu, Co and Fe etc. were obtained. The leaching residue was washed to neutral and dried at 95 °C. All the experiments were conducted in duplicate to ensure the repeatability. The obtained leaching solution was analyzed by inductively coupled plasma atomic emission spectrometer (ICP-AES, America, PE), and the results are summarized in Table 2. The extraction efficiencies (ŋx) of Ni, Cu, and Co were calculated to be about 98.4%, 98.9%, and 97.3%, respectively, according to the Eq. (1).
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Table 2 Chemical analysis of the leaching solution under the optimum conditions. (FeCl3: 1 mol L−1, HCl: 0.5 mol L-1, liquid to solid ratio: 20 mL g-1, 7 h and 90 °C). Compositions
Fe
Ni
Cu
Co
Content (wt%) Content (mol L−1)
27.7 0.496
6.0 0.102
1.2 0.018
0.2 0.003
ŋx =
Cx V ×100% MWx
Table 3 Chemical compositions of the low nickel matte leaching solution, solubility product Ksp and pH value of completely precipitate for metal hydroxides at 25 °C.
(1)
where x represents the metal element in leaching solution, Ni, Cu, Co; Cx is the concentration of x, g·L−1; V is the volume of leaching solution, L; M is the mass of low nickel matte, g; Wx is the mass fraction of x, wt %. 2.2.2. Synthesis of (Ni, Cu, Co)Fe2O4 from acid leaching solution It can be calculated from Table 2 that the mole ratio of Fe to M (Ni, Cu, Co) in the leaching solution of low nickel matte was about 4.01 which was larger than 2.00, suggesting that the amount of M was insufficient for synthesizing single phase ferrite. Considering that Ni, Cu and Co ions co-exist in the leaching solution of low nickel matte besides Fe ions, some water-soluble salts which contain divalent metal ions such as NiCl2, NiSO4, Ni(NO3)2, CuSO4, CuCl2, etc. could be added into the reaction system to synthesize different spinel ferrites with unique properties. In this experiment, NiCl2·6H2O was chosen as the addictive to the leaching solution of low nickel matte to prepare pure (Ni,Cu,Co) Fe2O4 ferrite. In detail, firstly, leaching solution (20 mL) was placed in a conical flask under the atmospheric pressure and equipped with a magnetic stirrer. A certain amounts of NiCl2·6H2O (2.2: 100 g mL−1, 3.0: 100 g mL−1, 3.4: 100 g mL−1) were added into the leaching solution in order to adjust the molar of Fe: M. Secondly, hydrogen peroxide (H2O2) was added to oxidize the excess Fe2+ in the solution into Fe3+. Finally, co-precipitation processes were conducted at a pH value of 12 by using NaOH solution (5 mol L−1) as the precipitant. The solid-liquid separation was conducted in a centrifuge with the speed of 5000 r·min−1 for 5 min. Then the precipitate was rinsed with deionized water for five times and dried for 12 h in an oven at 95 °C. Finally, the precipitate was ground into powder and calcined at different temperatures (800 °C, 900 °C, 1000 °C) and time (1 h, 2 h, 3 h).
Element
Leaching solution concentration (mol L-1)
Solubility product constant Ksp
Fe Ni Cu Co
0.496 0.102 0.018 0.003
4. 2. 2. 1.
0 0 2 1
× × × ×
10–38 10–15 10–13 10–15
3.2 9.2 6.7 9.0
(Fex,Ni9−x)S8 + 18FeCl3 = (18 + x)FeCl2 + (9-x)NiCl2 + 8S°
(2)
Cu5FeS4 + 12FeCl3 = 5CuCl2 + 13FeCl2 + 4S°
(3)
FeNi3 + 8FeCl3 = 9FeCl2 + 3NiCl2
(4)
FeNi3 + 8HCl = FeCl2 + 3NiCl2 + 4H2
(5)
Fe3O4 + 8HCl = FeCl2 + 2FeCl3 + 4H2O
(6)
During the preparation process, NiCl2·6H2O was added into the solution in order to adjust the molar ratio of Fe: (Ni + Cu + Co) to approximately 2: 1. Because Fe3+ ions were reduced to Fe2+ ions (Eqs. (2)–(6)) during the leaching process, a sufficient amount H2O2 was added into the solution to ensure that all of the Fe2+ ions were oxidized to Fe3+ ions. Thereafter, NaOH (5 mol L−1) solution was used to adjust the pH value of the solution to 12.0 in order to completely precipitate Fe, Ni, Cu and Co (Eqs. (7)–(10)), as indicated in Table 3. In addition, thermodynamic equilibrium calculations were conducted to elucidate the possibility of reactions during the calcination process. The system was simplified into the Ni-Fe-O system. The changes in standard Gibbs free energy (ΔGθ) with temperature increasing [35] are shown in Fig. 3. Fe3+ + 3OH- = Fe(OH)3
(7)
Ni2+ + 2OH- = Ni(OH)2
(8)
Cu2+ + 2OH- = Cu(OH)2
(9)
Co2+ + 2OH- = Co(OH)2
(10)
2Fe(OH)3 → Fe2O3 + 3H2O θ
ΔG = −13.793 − 0.4217 T KJ mol
2.3. Analysis and characterization X-ray fluorescence (XRF-1800, Japan) and X-ray diffraction (XRD, Japan, Rigaku) were used to analyze the chemical composition and structure of low nickel matte and the synthesized (Ni, Cu, Co)Fe2O4. The elements concentrations of the leaching solution were analyzed by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, America, PE). The micro morphology of the as-prepared ferrites was characterized by Scanning electron microscope (SEM, Germany, Zeiss). Raman spectrometer (HR800, France, JY) was used to study the molecular interactions. Vibrating sample magnetometer (VSM, America, Quantum Design) was used to obtain the magnetic characterization of the as-prepared samples.
pH value of completely precipitation
Ni(OH)2 → NiO + H2O
(11) −1
(12)
3. Results and discussion 3.1. Leaching mechanism and preparation principle of (Ni, Cu, Co)Fe2O4 During the leaching process, the FeCl3-HCl solution system was chosen for extraction of metal ions from the low nickel matte (CuFeS2, CuS2, (Fe,Ni)9S8, Ni3S2, etc.) due to that the Fe3+ ions are strong oxidisers and HCl can dissolve FeNi alloy while restraining the hydrolysis of Fe3+ ions. The main reactions in the leaching process are depicted as Eqs. (2)–(6).
Fig. 3. Variation of ΔGθ with the change of temperature for different reactions during the calcination process.
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ΔGθ = 10.770 − 0.1387 T KJ mol−1 Ni(OH)2 + 2Fe(OH)3 → NiFe2O4 + 4H2O θ
ΔG = −22.671 −0.5549 T KJ mol Ni(OH)2 + Fe2O3 → NiFe2O4 + H2O θ
ΔG = −8.884 − 0.1332 T KJ mol
(13)
−1
(14) −1
NiO + 2Fe(OH)3 → NiFe2O4 + 3H2O
(15)
ΔGθ = −33.451 − 0.4162 T KJ mol−1 NiO + Fe2O3 → NiFe2O4 θ
(16) −1
ΔG = −19.657 − 0.0055 T KJ mol From the thermodynamic point of view, the more negative the value of ΔGθ is, the more likely the reaction occurs. Therefore, all the reactions ((11)–(16)) may occur with the calcination temperatures varying from 800 to 1000 °C, and NiFe2O4 or Fe2O3 can co-exist in the products based on reactions (13), (15) and (11), implying Fe2O3 may be the main impurity in the products.
Fig. 5. XRD spectra of the as-prepared (Ni, Cu, Co)Fe2O4 calcined at different temperatures for 2 h (a) 800 °C, (b) 900 °C and (c) 1000 °C.
3.2. Effect of added amount of NiCl2·6H2O
3.3. Effect of calcination temperature
In order to investigate the effect of added amounts of NiCl2·6H2O in the leaching solution (different solid-liquid ratios, g mL−1) on the structures and compositions of ferrites, XRD analysis of as-synthesized samples were conducted and the results were shown in Fig. 4. It can be clearly seen From Fig. 4 that the diffraction peaks located at 18.4°, 30.2°, 35.6°, 37.3°, 43.3°, 53.8°, 57.3° and 63.0° which existed in all of the XRD patterns, coincided well with the standard diffraction peaks of cubic NiFe2O4 (JCPDS: 01-074-2081) for the crystal faces of (111), (200), (311), (222), (400), (422), (511) and (440). Decreasing the added amount of NiCl2·6H2O from 3.0: 100 to 2.2: 100 g mL−1 led to the formation of impurity Fe2O3 (JCPDS: 01-087-1164). However, with NiCl2·6H2O amount increasing from 3.0: 100 to 3.4: 100 g mL−1, the diffraction peaks of Fe2O3 disappeared, while that of NiO (JCPDS: 01089-7130) phase occurred, suggesting that pure spinel ferrite NiFe2O4 could not be synthesized under the two experiment conditions. When the amount of NiCl2·6H2O was controlled at 3.0: 100 g mL−1, only single phase (Ni, Cu, Co)Fe2O4 ferrite was obtained, further confirming that the amount of addictive played a key role in determining the structure and composition of as-prepared ferrites.
Fig. 5 illustrates the XRD spectra of the as-prepared metal-doped ferrites calcined at different temperatures (800, 900, and 1000 °C) for 2 h. All the diffraction peaks located at 18.4°, 30.2°, 35.6°, 37.3°, 43.3°, 53.8°, 57.3° and 63.0° corresponded well with the standard diffraction peaks of NiFe2O4 (JCPDS: 01-074-2081, cubic crystal system) for the crystal faces of (111), (200), (311), (222), (400), (422), (511) and (440). No other phases peaks were detected, indicating that the pure (Ni, Cu, Co)Fe2O4 were obtained as the temperature increased from 800 to 1000 °C. In addition, the enhanced diffraction peak intensities of the spinel ferrites confirmed the better crystallinity of the samples. The average grain sizes calculated based on Eq. (17) [36] increased from 59.92 to 86.01 nm with the temperature rising from 800 to 1000 °C, suggesting that the as-synthesized samples were nano sized (Ni, Cu, Co) Fe2O4.
D = (Kλ )/(B cos θ)
(17)
where D is the thickness along the crystal face (also regarded as a the grain size); λ is the wavelength of X-ray radiation, 0.15406 nm; θ is the Prague diffraction angle and B is the half-width of the diffraction peak. Fig. 6 shows the SEM image of (Ni, Cu, Co)Fe2O4 prepared at 1000 °C for 3 h. The obtained ferrite consisted of aggregated particles, and the particle size distribution range was wide, spanning dozens to
Fig. 4. XRD patterns of as-synthesized samples with different added amounts of NiCl2·6H2O into the leaching solutions (a) 2.2: 100 g mL−1, (b) 3.0: 100 g mL−1, (c) 3.4: 100 g mL−1 calcined at 1000 °C for 2 h.
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Fig. 6. SEM image of (Ni, Cu, Co)Fe2O4 synthesized at 1000 °C for 3 h.
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Fig. 7. Raman spectra of (Ni, Cu, Co)Fe2O4 synthesized at different calcination temperatures (a) 800 °C, (b) 900 °C, (c) 1000 °C.
hundreds of nanometers. The molecular formula of the as-prepared nickel ferrite can be denoted as Ni0.913Cu0.074Co0.013Fe2O4 according to ICP analysis, as indicated in the inset of Fig. 6, confirming that metal co-doped ferrite was obtained. In order to further reveal the structural properties of the (Ni, Cu, Co) Fe2O4, Raman spectra of the ferrites calcined at different temperatures were recorded, as depicted in Fig. 7, and the corresponding Raman shifts are summarized in Table 4. Generally, spinel ferrite is a cubic spinel structure, and belongs to the space group Oh7 (Fd-3m). Tetrahedron and octahedron in the lattice are made up of oxygen anion (O2-), and metal ions occupy both the tetrahedral and octahedral sites. Five Raman feature peaks, (A1g + Eg + 3T2g), correspond to the motion of O ions and ions at A-sites and B-sites [37]. The A1g matches to the symmetric stretching vibration of the oxygen anion, whereas Eg matches to the symmetric bending vibration of the oxygen anion, and T2g corresponds to asymmetric stretching vibration between oxygen anions and cations in the A-site and B-site [30]. It can be seen from Fig. 7 that all of the Raman peaks were assigned to the A1g, T2g and Eg modes reflecting the vibration of spinel structures, indicating that the pure (Ni, Cu, Co) Fe2O4 were obtained when the calcination temperature varied from 800 to 1000 °C. Especially, the peak intensity ratio of I (A1g(1)): I(A1g(2)) which represents the quantitative surface composition ratio increased gradually from 3.07 to 3.15 and 3.65, suggesting that some divalent cations might shift from the A-site to the B-site, meanwhile the same numbers of Fe3+ cations might migrate from the B-site to A-site with the calcination temperature increasing. Fig. 8 presents the magnetic hysteresis loops of the samples Table 4 Raman modes of the (Ni, Cu, Co)Fe2O4 synthesized at different calcination temperatures for 2 h. Assignment
A1g(1) [30] A1g(2) [30] T2g(3) [30] T2g(2) [30] Eg [39] T2g(1) [39]
Vibrations [30]
symmetric stretching symmetric stretching asymmetric stretching asymmetric stretching bending asymmetric stretching
Ions involved
Raman modes (cm-1) 800 °C
900 °C
1000 °C
A-O (Fe-O) [30,38] A-O (Fe-O) [38,39] A-O [38,39]
687
684
687
640
638
640
548
544
548
A-O [30,38]
470
467
469
A-O [38] A-O [38,39]
313 197
310 196
313 197
A (Ni, Cu, Co) represents metal ions in tetrahedral sites.
prepared at different calcination temperatures for 2 h, and MS and HC values of the obtained (Ni, Cu, Co)Fe2O4 and the sample in reference [21] are given in Fig. 9. It can be observed that MS value increased from 36.0 to 44.2 emu g−1 as the temperature increased from 800 to 1000 °C, agreeing well with the variation trend of other literature's [40–42]. All the obtained samples exhibited larger MS values compared with samples synthesized from pure reagents [21]. According to the Neel's two sublattice models [43], the magnetic moment per formula unit (M) of the lattice is the difference between the magnetic moments of B and A sublattices, namely, M = MB - MA. According to the analysis of Raman spectra of the prepared (Ni, Cu, Co)Fe2O4 as indicated in Fig. 7, some divalent cations including Cu2+ (1 μB), Ni2+ (2 μB), and Co2+ (3 μB), might migrate from the A-site to the B-site. Simultaneously, the same quantity of Fe3+ (5 μB) moved from the B-site to the A-site. Therefore, considering the differences of these migrating ions magnetic moments, the MB decreased while MA increased, thus the MS values of the formed ferrites decreased accordingly with the increase of temperature. However, the factors that determine the magnetic property of ferrite were not just the internal structure. With the increase of calcination temperature, the crystallinity became higher and average grain size became larger, leading to the decrease of the proportion of surface atoms. That made the magnetic moment tend to the external field, and the magnetic anisotropy weaken, which finally enhanced the Ms values of as-synthesized ferrites. In addition, the HC values of the (Ni, Cu, Co)Fe2O4 decreased from 158.5 to 54.1 Oe with increasing the temperature (Fig. 8(b) and Fig. 9). This phenomenon may be ascribed to the following reasons. With the increase of grain size and ferrite magnetic domain, the displacement resistance of domain wall decreased, which would increase the uniformity and weaken the magnetic anisotropy of the ferrites, thus, HC values decreased accordingly [43,44]. 3.4. Effect of calcination time Fig. 10 shows the XRD spectra of the as-prepared metal-doped ferrites calcined at 1000 °C for different time (1 h, 2 h and 3 h). All the diffraction peaks were well indexed to the standard diffraction peaks (18.4°, 30.2°, 35.6°, 37.3°, 43.3°, 53.8°, 57.3° and 63.0°) of NiFe2O4 (JCPDS: 01-074-2081, cubic crystal system) for the crystal faces of (111), (200), (311), (222), (400), (422), (511) and (440). No peak for other phase was detected, which indicated that pure (Ni, Cu, Co)Fe2O4 were obtained at 1000 °C for 1–3 h. The average grain sizes calculated based on Eq. (17) increased from 61.29 to 92.32 nm with the time rising from 1 to 3 h. Raman spectra and the corresponding Raman shift changes of the (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time (1 h, 2 h and 3 h) are illustrated in Fig. 11 and Table 5. All of the Raman peaks which corresponded to A1g, T2g and Eg modes reflected the vibrations of (Ni, Cu, Co)Fe2O4, indicating that pure (Ni, Cu, Co)Fe2O4 were obtained at different calcination time. In addition, the peak intensity ratio, I (A1g(1))：I(A1g(2)), increased rapidly from 1.90 to 3.15 with calcination time increasing from 1 to 2 h, then remain unchanged between 2 to 3 h, suggesting that some Fe3+ cations moved from the B-site to the Asite, as the same amount of divalent cations shifted from the A-site to the B-site. After 2 h reaction, the migration of metal ions reached equilibrium. The magnetic hysteresis loops and HC values of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time are illustrated in Fig. 12, and the corresponding comparison of MS and HC is summarized in Fig. 13. The MS increased from 44.0 to 46.1 emu g−1 and HC decreased from 82.2 to 51.0 Oe as the calcination time changed from 1 to 3 h. All the obtained samples exhibited larger MS and smaller HC values compared with the product synthesized from pure reagents (35.5 emu g−1 and 124.4 Oe) [21]. This phenomenon may be ascribed to the changes in the internal structure of the ferrite, which caused by the ion movement in the obtained (Ni, Cu, Co)Fe2O4 (Fig. 11) and other factors discussed in Section
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Fig. 8. (a) The magnetic hysteresis loops of the as-prepared (Ni, Cu, Co)Fe2O4 calcined at different temperatures and (b) HC values of the products.
Fig. 11. Raman spectra of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time (a) 1 h, (b) 2 h and (c) 3 h.
Fig. 9. MS and HC values of as-synthesized (Ni, Cu, Co)Fe2O4 at different temperatures and the sample in reference [21].
Table 5 Raman parameters of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time. Assignment
Vibrations [30]
A1g(1) [30]
symmetric stretching
A1g(2) [30]
symmetric stretching
T2g(3) [30]
asymmetric stretching asymmetric stretching bending asymmetric stretching
T2g(2) [30] Eg [39] T2g(1) [39]
Ions involved
Raman modes (cm-1) 1h
2h
3h
A-O (Fe-O) [30,38] A-O (Fe-O) [38,39] A-O [38,39]
690
687
688
649
640
642
552
548
549
A-O [30,38]
473
469
471
A-O [38] A-O [38,39]
316 190
313 197
315 196
A (Ni, Cu, Co) represents metal ions in tetrahedral sites.
Fig. 10. XRD spectra of the as-prepared (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time (a) 1 h, (b) 2 h and (c) 3 h.
3.3, such as changes of the magnetic moment and the magnetic anisotropy. In addition, the larger average grain size (from 1 to 3 h of calcination) may also lead to a higher MS value. Moreover, with the
increase of the grain size and ferrite magnetic domain, the uniformity increased and the magnetic anisotropy decreased, which all resulted in the decreased HC values of the formed ferrites.
4. Conclusions
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In this paper, the pure (Ni, Cu, Co)Fe2O4 was successfully
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Fig. 12. (a) The magnetic hysteresis loops of (Ni, Cu, Co)Fe2O4 calcined at 1000 °C for different time and (b) HC values of the products.
References
Fig. 13. MS and HC values of as-synthesized (Ni, Cu, Co)Fe2O4 synthesized at 1000 °C for different time and the sample in reference [21].
synthesized via the co-precipitation-calcination method from the leaching solutions of low nickel matte for the first time. All the added amount of NiCl2·6H2O, calcination temperature and time played important roles in determining the structure and magnetic properties of the as-prepared ferrites. When the added amount of NiCl2·6H2O was controlled at 3.0: 100 g mL−1, pure (Ni, Cu, Co)Fe2O4 ferrite could be obtained. In addition, with the increase of calcination temperature and time, the MS values of formed (Ni, Cu, Co)Fe2O4 increased while HC values decreased. The magnetic property of the ferrite were not only affected by the ion movement, but also by the magnetic moment and the magnetic anisotropy caused by crystalline and average grain size. Under the optimum conditions (NiCl2·6H2O added amount of 3.0: 100 g mL−1, 1000 °C and 3 h), the obtained (Ni, Cu, Co)Fe2O4 exhibited a higher MS of 46.1 emu g−1 and a lower HC of 51.0 Oe compared with other nickel ferrites synthesized from pure chemical reagents. The process proposed in this work realized the comprehensive utilization of the low nickel matte for synthesis of functional metal-doped spinel ferrite.
Acknowledgements This work was financially supported by the National Basic Research Priorities Program of China (No. 2014CB643401, No. 2013AA032003), and the National Natural Science Foundation of China (No. 51372019).
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