Synthesis of nanosized nickel zinc ferrite using electric arc furnace dust and ferrous pickle liquor

Electric arc furnace dust (EAFD) and waste pickle liquor (WPL); two major side products of the steel industry with negative environmental impact were used for the synthesis of nickel zinc ferrite (NZF); the important magnetic ceramic material of versatile industrial applications. The structural and magnetic properties of the prepared material were examined which showed good magnetic properties (high saturation magnetization and low coercivity) compared with those synthesized from pure reagents. In the applied process, nano sized nickel zinc ferrite (NZF) with a composition of Nix(Zn + impurities)1−xFe2O4 (where x = 0, 0.25, 0.5, 0.75 and impurities of manganese, magnesium, and calcium were prepared using zinc-containing electric arc furnace dust (EAFD) and waste pickle liquor (WPL). The chemical compositions of the prepared samples were determined using X-ray fluorescence (XRF) analysis. The optimum acetic acid concentration for EAFD treatment was found 2% v/v that decreased Ca content of EAFD by 70.6% without loss of Fe and Zn. The structural and morphological characterization was done by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Field Emission Scanning Electron Microscope (FESEM) to confirm the formation of Ni–Zn ferrite nanoparticles and estimate the particle sizes. The maximum saturation magnetization (Ms) of 73.89 emu/g was achieved at 0.5 Ni content and the minimum coercivity of 2.55 Oe was obtained at 0.25 Ni content.

www.nature.com/scientificreports/ the co-precipitation process. Second is making use of the dissolving step of EAFD at HCl in its purification from undesired impurities like Al and Si oxides, which have poor solubility at HCl solutions, so such impurities will not be transferred to the EAFD leach liquor that will be used for the co-precipitation process. And, finally the advantages of Co-precipitation process as a low cost method that doesn't require high temperatures and provides crystalline nano particles with high purity in short duration. The effect of acetic acid treatment on EAFD composition was studied and the influences of Ni ratio on the structural and magnetic properties of the prepared Ni-Zn ferrite was investigated using XRD, FTIR, FESEM and vibrating sample magnetometer (VSM) examinations. By the applied process in this paper, EAFD and WPL could be transformed from waste environmentally hazardous materials to a high added value material with both economic and environmental benefits.

Experimental details
EAFD treatment with acetic acid. Treatment of the EAFD with acetic acid was done to reduce the calcium oxide content. To determine the optimum acetic acid concentration for EAFD treatment, 150 mL of five different concentration acetic acid solutions (1, 2, 3, 4, 5 v/v%) were used for the treatment of five EAFD samples (15 g). The acetic acid treatment for each EAFD sample was carried at 80 °C for 1 h with a continuous stirring. The EAFD samples treated with 1, 2, 3, 4 and 5 v/v% acetic acid were denoted ATEAFD1, ATEAFD2, ATEAFD3, ATEAFD4, and ATEAFD5, respectively. The treated EAFD samples were collected by centrifugal separation at 2000 rpm and the five samples were washed with distilled water and dried at 250 °C for 4 h.
Dissolving of the acetic acid-treated EAFD in hydrochloric acid. A 300 g sample of 2% acetic acidtreated EAFD was dissolved in 3000 mL of 15% HCl at 80 °C with a continuous stirring for 1 h. The undissolved particles were separated with centrifugal separation at 2000 rpm, and leach liquor of the dissolved sample was obtained.
Preparation of the NZF samples by co-precipitation process. Four NZF samples with the formula Ni x (Zn + impurities) 1-x Fe 2 O 4 (where x = 0, 0.25, 0.5 and 0.75 and the impurities contains manganese, magnesium and calcium were prepared by co-precipitation technique. First, the mixed solutions were prepared by adding NiCl 2 ·6H 2 O and WPL in stoichiometric ratios to four 200 mL solutions of the leach liquor obtained from dissolving of the acetic acid-treated EAFD in HCl. Each mixed solution was added to 0.6 M NaOH solution dropwise with stirring of speed 50 rpm at room temperature. NaOH solutions were prepared taking the ratio Fe:Na to be 1:4 for each NZF composition. pH 10 was kept during the precipitation process by continuous titration with NaOH. During the addition, dark grey precipitates were obtained. The supernatant liquors were decanted, and the remaining suspensions were placed into a dryer at 200 °C for 4 h to complete ferritization reaction. The dried precipitates were washed several times with distilled water and dried at 200 °C for 4 h. The flow chart of the preparation process is shown in Fig. 1. Characterization. Chemical compositions of the WPL and the leach liquor obtained by dissolving of the acetic acid-treated EAFD in HCl were obtained using ICP analysis (ICP-OES Prodigy, Teledyne Leeman Labs, USA).
X-ray Fluorescence (Rigaku Supermini 200, Japan) was used to get the chemical composition of the EAFD before and after acetic acid treatment. It was also used to determine the chemical composition of the prepared nickel-zinc ferrite samples.
For phase identification, estimation of crystallite size, and other structural parameters. XRD examination was performed using X ray-diffractometer (Shimadzu-XRD 6100) using Cu K∝ (λ = 1.54060 Å) radiation,  www.nature.com/scientificreports/ with continuous scan mode and scanning speed of 12 degree/minute for scanning range of 5-80 degrees. FTIR examination for the prepared nickel-zinc ferrite samples was carried out in the range of 4000-200 cm −1 using FTIR-8400S, Shumadzu, Japan device. The dried samples were pressed with KBr matrix, and spectra measurements were according to transmittance method. Magnetic measurements were carried out at room temperature with a maximum magnetic field of 20,000 Oe using a Lakeshore vibrating sample magnetometer (model: 8600 series VSM, lakeshore, USA) and magnetic parameters including M s , H c and remanence (M r ) were evaluated. NZF samples were subjected to SEM examinations using JEOL scanning microscope (JSM-IT200 Series).

Results and discussion
Chemical composition of WPL. The chemical analysis of WPL is shown in Table 1. It is seen that the WPL contains mainly Fe in the form of ferrous/ferric chlorides and that Mn concentration is very low so it can be used directly for the synthesis of nickel-zinc ferrite.
Effect of acetic treatment on EAFD composition. The chemical composition of the EAFD before and after leaching with acetic acid is given in Table 2. It is seen that the effect of acetic acid treatment on Fe, Zn and Ca concentrations is most obvious than its effect on the other EAFD components. Figure 2 shows the effect of acetic acid concentration on Fe, Zn and Ca concentration in the EAFD. With increase of acetic acid concentration, iron concentration is increased while calcium concentration is decreased. For low acetic acid concentrations of 1 and 2 v/v%, Zn concentration is raised while Zn concentration is declined at higher acetic acid concentrations of 3, 4, and 5 v/v%. CaO is removed upon treatment with acetic acid due to its reaction with acetic acid according to reactions (1) and (2) 8,9 .
In addition, acetic acid treatment depresses Pb weight percent from 1.1% ± 0.024% in the raw EAFD sample to 0.06% ± 0.024 in the 2% acetic acid treated sample due to formation of lead acetate.
The EAFD material yield after the acetic acid treatment with different concentrations was calculated from the weight difference between the EAFD samples before and after treatment. As indicated from Fig. 3, the highest yield is 88.3%, which obtained at acetic acid concentrations of 1 and 2% v/v.
The best acetic acid concentration accordingly is 2 v/v% which reduced the Ca content by 70.6% ± 0.40% (from 8.5 to 2.5%) with the maximum zinc enrichment (24.5% ± 0.35) in addition to the maximum EAFD material yield (88.3%).
The XRD patterns of the EAFD before and after 2% v/v acetic acid-treatment are shown in Fig. 4a  Chemical composition of the dissolved acetic acid-treated EAFD leach liquor. The chemical analysis of the obtained liquor after dissolving of the acetic acid-treated EAFD sample in 15% HCl solution is illustrated in Table 3. It is noticed that Si and Al didn't exist which indicates that their oxides have poor solubility in HCl. The weight of the undissolved particles is 29.23 g representing 9.74% of the acetic acid-treated EAFD sample weight. Incomplete dissolving of the sample could be due to the impurities of metal oxides with a lower solubility in HCl solution mainly silicon and aluminum.
Chemical analysis of the prepared NZF precipitates. The chemical analysis of the prepared samples is shown in Table 4. Although Cu and Pb are detected in dissolved acetic acid treaded EAFD solution, they are disappeared in the prepared NZF samples due to their very low concentration in the precipitation solution.
Structural properties of the prepared NZF samples. XRD patterns of the as prepared NZF samples with different Ni contents are shown in Figure 5.  www.nature.com/scientificreports/ The results indicate that the reflections in all the cases are characteristic of the spinel structure without any impurity phases. There is a slight shifting of the 2θ of the diffraction peaks to higher values with increase of Ni ratio. This shifting is attributed to the fact that the size of Ni 2+ cation is smaller than that of Zn 2+ cation (69 Å and 74 Å respectively), so the lattice parameter of the spinel unit cell decreases with increasing of Ni content. This results in a larger diffraction angle because the diffraction angle θ is inversely proportional to the lattice   where β is the peaks full width at half maximum in radians, and λ is the wavelength of the used X-ray beam and 0.90 is the shape factor. Peak of (311) reflection is used for calculations because it is the sharpest one. The crystallites size are 10.8, 10.7, 12.0 and 16.7 nm for samples N 0.0 ZF, N 0.25 ZF, N 0.50 ZF and N 0.75 ZF, respectively. The crystallite size is almost the same at Ni = 0.0 and 0.25, while further increase of Ni to 0.5 and 0.75 showed an obvious increase in the crystallite size. The observed increase of the crystallite size with increase of Ni content was observed by Chuan et al. 16 and Upadhyay et al. 17 . It is attributed to Zn-ferrite formation is more exothermic than Ni-ferrite formation, so the surface temperature of the formed crystals becomes higher with increase of the Zn content which reduces the molecules concentration at the surface of the growing crystals.
The lattice parameters for each sample was calculated using the formula 18,19 .
where a is lattice parameter, d is the inter planer spacing and h, k, l are the miller indices of (311). The lattice parameter shows a slight decrease in its value with increase of Ni content which is attributed to its lower cation size compared to Zn cation.
The theoretical XRD density ρ x was calculated using the formula 20,21 .
where M is NZF samples molecular weight, A N is Avogadro's number (mol −1 ) and V is the lattice volume. A slight increase of ρ x is observed with increase of Ni ratio due to the accompanied reduced lattice parameter. The lattice strain ε was calculated using Williamson-Hall equation 22,23 .
the lattice strain can be estimated with the aid of Eq. (6) from the slope of β hkl cosθ versus 4sinθ plot as in Fig. 6. The estimated lattice stain values declines with the increase of Ni content due to the smaller Ni 2+ cation size which makes less distortion for the spinel lattice than the larger Zn 2+ cations, a similar phenomenon was reported by Abu El-Fadl et al. 13 in the study of NZF prepared by microwave combustion method and by Srinivas et al. 24 in co-precipitated NZF and by Batoo et al. in nanoparticles of Ni-Cu-Mg ferrite prepared by sol-gel method 25 . The XRD calculated crystallites size, lattice parameter, lattice strain and XRD density for all samples are listed in Table 5. FTIR spectra of the prepared NZF samples are shown in Fig. 7. Two main broad absorption bands are appeared in the range of 390 to 586 cm −1 . The highest wavenumber one is designated ʋ1, observed in the wavenumber range of 565 to 586 cm −1 and this band corresponds to the metal ions intrinsic stretching vibrations at www.nature.com/scientificreports/ the tetrahedral site. The other band appeared in the range of 392 to 409 cm −1 and it is designated as ʋ2 and corresponds to metal ions stretching vibrations in the octahedral sites. These two bands are observed in previous studies about spinel ferrites [24][25][26][27][28] . It is seen from Table 6 that there is a slight shifting of ʋ1 towards higher wavenumbers with increase of Ni content. This can be explained the in view of the cation distribution within NZF spinel structure. Zn ferrite has a normal spinel structure where Zn 2+ cations occupy the tetrahedral sites (A sites) and Fe 3+ cations occupy the octahedral sites (B sites). On the other hand, Ni ferrites have inverse spinel structure in which octahedral sites are occupied by both Ni 2+ and Fe 3+ cations. Accordingly, when Ni 2+ cations are added in expense of Zn 2+ cations, they occupy the octahedral sites pushing part of Fe 3+ cations from the octahedral to the tetrahedral sites. As Fe 3+ cations are smaller in size and lighter than Zn 2+ cations, atomic vibration of the tetrahedral sites increases as   www.nature.com/scientificreports/ well giving rise to shifting of the absorption band towards higher wavenumbers. The same shifting of ʋ1 towards higher wavenumbers with increase of Ni was also reported by Tahrani et al. 12 and Batoo et al. 29 . Other bands are observed around 1630 and 3420 cm −1 and the first is attributed to deformation vibrations of adsorbed water during preparation while the second is attributed to O-H stretching indicating the remaining of some hydroxyl groups 12,30 . The intensity of these two bands are decreased with increase in heat treatment temperature. Figure 8 (from a to d) display the SEM images of the prepared NZF samples. The sample with zero Ni content has non-uniform hexagonal and spherical particles. As Ni content increase, the particles become more spherical in shape and finer in size. A similar effect of Ni content on NZF morphology was reported by Khan et al. 31 in the study of the effect of Zn 2+ dopping on Nickel ferrites. Figure 9 depicts the magnetization hysteresis loops obtained by VSM measurement for the prepared NZF samples. The obtained values of M s , H c and retentivity (M r ) are shown in Table 7. It is seen from Table 7 that the specific Ms values of 19.8, 28.64, 73.89 and 45 emu/g are observed for the NZF samples N 0.0 ZF, N 0.25 ZF, N 0.50 ZF and N 0.75 ZF, respectively. The saturation magnetization of magnetic ferrites depends on their chemical composition and the cations distribution within their spinel structure. In NZF spinel lattice, Ni 2+ cations prefer to occupy the octahedral sites whereas, Zn 2+ cations prefer the tetrahedral sites, and Fe 3+ cations are distributed between both octahedral and tetrahedral sites 32 . The net magnetic moment of the spinel lattice is the difference between the magnetic moments of the octahedral B and the tetrahedral A sublattices (M = M B − M A ) where M B is the magnetic moment of B sub-lattice and M A is the magnetic moment of A sub-lattice [32][33][34][35] . As Ni 2+ content at the octahedral sites increases, more Fe 3+ cations are forced to move from  www.nature.com/scientificreports/ octahedral to tetrahedral sites. As the magnetic moments of Fe 3+ cations are higher than that of Ni 2+ cations (5 μ B and 2 μ B, respectively ) , the replacement of Ni 2+ cations for Fe 3+ cations at octahedral sites results in a decrease of M B and an increase of M A . The increase of M A with Ni addition produces a higher and super-exchange interaction between A and B sub-lattices which leads to an increase of the magnetization value. However, at a higher content of Ni up to 0.75 the reduction of net magnetic moments between A and B sub-lattices is observed and this can be elucidated based on the high super exchange interaction between the two cations sites. It is also noticed from the magnetization examinations that sample N 0.0 ZF with zero Ni content achieves 19.8 emu/g of Ms, which is abnormal as zinc ferrite should be paramagnetic at room temperature because all Fe 3+ cations are located in the octahedral sites and all Zn 2+ cations are localized in the tetrahedral sites. Therefore, the magnetic spins of the octahedral Fe 3+ cations can't be co-aligned because of the absence of super-exchange between octahedral and tetrahedral sub-lattices 36 .

Magnetic characterization.
The existence of Mn, Mg and Ca impurities in the prepared NZF samples can explain the saturation magnetization of the zero Ni sample where the presence of Ni modifies the cations distribution and the magnetic moments in the NZF spinel lattice. Mn 2+ and Ca 2+ cations prefer the tetrahedral sites in the spinel lattice, whereas Mg 2+ cations prefer the octahedral sites [37][38][39][40] . As Mn 2+ cations with high magnetic moment (5 μ B ) occupy the tetrahedral sites and Mg 2+ cations with zero magnetic moment occupy octahedral sites, M A increases and M B decreases. The increase of the magnetic moments of the tetrahedral A sub-lattices increases their capability to align antiparallel to the magnetic moments of the octahedral B sub-lattices and accordingly enhances the super-exchange interaction between A and B sub-lattices and increases the net magnetic moment. The abnormal saturation magnetization of the sample N 0.0 ZF can also be attributed to the cations miss distribution in nanosized ferrites. In nanosized NZF some Zn 2+ cations may occupy the octahedral B sites instead of their most favorable tetrahedral A sites, leading to shifting of some Fe 3+ cations to the tetrahedral sites. This abnormal cation distribution illustrates the increase of saturation magnetization in the zinc ferrite sample 13 .
On the other hand, the minimum coercivity of 2.55 Oe is observed at Ni content of 0.25 for sample N 0.25 ZF whereas sample N 0.75 ZF exhibits the maximum coercivity of 105 Oe. The high value of the coercivity at Ni content was also obtained by Abu El-Fadl et al in NZF prepared by microwave combustion method, and by Chaudhari et al in NZF synthesized by oxalate precursor method. The phenomenon of coercivity increase with increase of Ni content is attributed to the high magnetocrystalline anisotropy energy of Ni ions in comparison to Zn ions 13,40 . It is also observed from Table 7 that sample N 0.0 ZF of zero Ni content exhibits higher coercivity compared to N 0.25 ZF sample (11.41 and 2.55, respectively). It was also observed by Abu El-Fadl et al in the previous mentioned study that the sample with 0.2 Ni content exhibited lower coercivity than that with zero Ni content. This was attributed to the fact that Fe 3+ content on the A-site is the highest for Ni content of 0.2 in comparison with Ni content of 0.0 13 .  www.nature.com/scientificreports/

Conclusion
Single phase nanosized NZF was produced using EAFD and WPL. Treatment of the EAFD with 2% v/v acetic acid reduced its Ca content by about 70% and enriched its content of Fe and Zn. Dissolving of the acetic acidtreated EAFD in hydrochloric acid resulted in removal of non-soluble SiO 2 and Al 2 O 3, which improved the purity of the prepared samples. The structural and magnetic properties of the prepared NZF samples are affected by their Ni content and the highest saturation magnetization of 73.89 emu/g was achieved at 0.5 Ni ratio and the lowest coercivity of 2.55 Oe was achieved for Ni content 0.25. The presence of Ca, Mn, and Mg impurities affected the magnetic properties of the prepared samples by readjustment of the cation's distribution and the magnetic moments within the spinel NZF lattice. The applied process in this paper realized the transformation of the EAFD and WPL form environmentally hazardous waste by products to value-added magnetic ferrites.