Pure iron and laboratory synthesized iron oxides find applications in diverse areas and this has led to intensified research efforts for improved application [1]. These materials occur naturally in iron ores in the form of magnetite (Fe₃O₄), hematite (Fe₂O₃), goethite (FeO OH), limonite (FeO (OH) or siderite (FeCO₃) alongside some impurities [2]. Nigeria is among the African countries with vast iron ore deposits but do not currently have an efficient beneficiation process. A large deposit of iron ore with about 183 million metric tons is estimated for Itakpe Iron Ore [3], and been considered as the highest grade of iron ore [4]. Therefore, optimizing the process of production of the materials along the entire value chain further improves efficiency, reduce waste and further foster novel applications. As such, several studies are ongoing in this regard.

Longworth conducted ⁵⁷Fe Mössbauer spectroscopy study on the magnetic state of iron oxides in the soil collected from a site in England where they found that the percentage of non-magnetic compound in the soil varies between 50% and 60%. Similarly, Resende reported that the iron ore found in the tropical region of Brazilian soil contain (63%) of less magnetic hematite ore, Weheekama Siaplay (2017) analyzes five different samples of itakpe iron ore, which include coarse iron ore, middle fine iron ore, fine iron ore, gangue iron ore and concentrate iron ore. The sample analysis shows that Itakpe iron ore is hematite-rich with atomic % of Fe to be 40.00 and weight % to be 69.94.

Jiang in china studies iron ore tailings and found that a waste has an iron grade of approximately 8 wt% - 12 wt% on average and occasionally as high as 27 wt%. If this material contains 10 wt%, on average 750 million tons of metallic iron is lost during the disposal of Iron Ore Tailings (IOTs). There are about six followed stages involved in the beneficiation process which are ore extraction, concentration, smelting, refining, semi-manufacturing and manufacturing with associated wastes at every stage. The iron ore tailings which is the waste product of industries [5] is refined, reuse and recycle to extract valuable resource ingredients which in turn lead to economic benefits and address environmental concerns. This study will help quantify to magnetic and non-magnetic component in the ore with a view to investigate the magnetic properties of the concentrates and the wastes generated from the value chain and how such information could impact on the beneficiation process [6]. The study will provide essential information on the understanding of the changes in the magnetic parameters such as saturation magnetization, coercivity, remanence magnetization, isomer shift, hyperfine magnetic field and quadrupole splitting of the minerals [7]. The magnetization M is the total magnetic moments of a sample per unit volume determined usually under the influence of a magnetic field [8]. The response of magnetization of a sample under the influence of a magnetic field is called its susceptibility X which is defined as

                                                                                          (1)

Where,  is the permeability of free space and B₀ is the magnetic induction related to applied magnetic field H₀ by B_{0 =} µ₀ H₀. Saturation magnetization M_S, remanence magnetization M_R and coercivity H_C are characteristic features for examining the magnetic property of a materials. The M_S can be empirically calculated from the law approach to saturation magnetization [9].

               (2)

Where  and  are constants, M_S is the spontaneous magnetization and X _hf is the high field susceptibility. The a|H term is attributed to structural defects or nonmagnetic inclusions. The b|H² term is due to uniform magneto-crystalline anisotropy. The M_R measures the retentivity property of the material, this could be used to measure the earth magnetic field history in rock magnetism and provides relevant information for archeological study. The coercivity gives a measure of the anisotropy of a magnetic material [10].

This can be estimated by                                                                  

                                                                (3)

Where H_C1 and H_C2 are the negative and positive coactivity respectively along the magnetic field direction. A vibrating sample magnetometer is used to measure these parameters. Parameters such as magnetic hyperfine field , isomer shifts,  and quadrupole splitting  also significantly provide relevant information in characterizing the magnetic properties of Fe-bearing materials. Magnetic hyperfine field is the internal nuclear field responsible for magnetic hyperfine splitting. The splitting is caused by dipole interaction between the nuclear spin moment and a magnetic field referred to as Zeeman splitting. The Hamiltonian for the magnetic hyperfine dipole interaction can be written as

                                     (4)

Where  is the nuclear Bohr magneton,  is the nuclear magnetic moment, I is the nuclear spin and is the nuclear  [8]. Isomer shift is the change in nuclear energy levels which originates from electron charge densities different from the emitting and absorbing material [11]. It is measured relative to calibrated metallic α-Fe foil. Quadrupole splitting arises due to splitting of nuclear energy levels as a result of the interaction of nuclear quadrupole moment with electric field gradient.

Sample Collection/Preparation

Three waste samples of iron ore, was manually collected from the iron ore mining site in Itakpe, Kogi State. The iron ore lump was crushed using a ball milling machine into powder form for magnetic property measurements. Sample size of about 0.2 g is used each for Vibrating Sample Magnetometer (VSM) and ⁵⁷Fe Mössbauer measurements.

Vibrating sample magnetometer and ⁵⁷Fe Mössbauer spectroscopy was used for data collections for the analysis of the magnetic properties of the samples, for the VSM measurements, the sample was loaded in a cone sample holder, mounted into a Janis model helium cryostat. The sample was vibrated in a maximum magnetic field of 14 kOe generated by a LakeShore VSM machine.

The sample for ⁵⁷Fe Mössbauer spectroscopy was placed in a plastic sample holder and positioned in-between transmitted gamma ray corresponding to 14.4 keV energy, and a detector. The detected signals are amplified with amplifiers at high voltage and sent through a single channel analyzer and further processed by a multi-channel analyzer for data capturing. The accumulated data was interpreted as spectra and displayed on the output screen of a computer. Recoil software was used for the ⁵⁷Fe Mössbauer spectroscopy analysis. And Origin graphing software was sufficiently used to analyze the VSM magnetic data.

Fig 1 shows the room temperature magnetic hysteresis loop of the as-collected iron ore sample where parameters such as the saturation magnetization and coercivity were determined. The magnetization M, and coercivity HC were approximated form the loop. The hysteresis loops indicate mixed magnetic signatures of super paramagnetic and Paramagnetic [12]. Super paramagnetic features have been associated with small and single domain magnetic ferromagnetic particles. Iron oxides particles such as (Fe2O3) and (Fe3O4) are known to exhibit this feature. Therefore, it is not unusual to see the waste sample Itakpe iron ore display this characteristic. Similar magnetic features have been reported for natural occurring iron ore obtained outside Nigeria [13]. The paramagnetic component present in the sample is not unconnected with poorly magnetized phases in the iron ore. An approximate value of 5emu/g was obtained for the magnetization with a low coercivity of about 120 Oe.

Fig 2. Shows the magnified view of the coercivity of the waste sample. The law of approach to saturation magnetization, MS, according to equation (2) [14] was used to estimate the MS and the high field susceptibility χH. A fit to the equation was carried out on the initial magnetization as shown in Figure 3. Values of the MS, χH, and the constants a/H and b/H2 were extrapolated from the fit. However, it is interesting to see that the magnetization of the waste samples is high. This shows that waste samples from Itapke iron ore still contains a significant amount of iron content, therefore, requires an optimum beneficiation process for economic values.

Fig 1: Magnified view of the coercivity extracted from the magnetic hysteresis loop of Itapke iron ore waste sample.

Fig 2: A fit to high field magnetization of the initial magnetization of Itapke iron ore concentrate sample. The solid line is a fit to equation (2).

Fig 3: A fit to high field magnetization of the initial magnetization of Itapke iron ore waste sample. The solid line is a fit to equation (2).

Table 1. Saturation magnetization, M_S, fit parameters (a) and (b) high field susceptibility, χ, and goodness of fit χ² as obtained from the law of approach to saturation magnetization for Itapke iron ore. H_C is the coercivity as empirically derived from experimental data.

57Fe Mössbauer spectroscopy is a suitable technique that is capable of detecting Fe-oxide phases, the oxidation state of Fe and provides useful information such as the recoilless fraction population of the atoms at the atomic sites and the distribution of Fe cations between valence states. The spectra of the room temperature 57Fe Mössbauer measurements performed on the waste sample of Itapke iron ore is shown in (fig 4). The analysis of the spectra was done using recoil software. The software model is based on the Lorentzian multiplex analysis. It is a standard analysis that allows the fitting of several Lorentian singlets, doublets or sextets which correspond to paramagnetic sites with or without a quadrupole splitting and sites with a magnetic hyperfine field and a quadrupole shift [15]. The hyperfine parameters are quoted relative to α-Fe used for calibration. While, the waste sample exhibited a much different feature. The best fits were obtained using 2 sextets and a doublet. Although, the sample is still predominantly of the hematite α-Fe2O3 phase assigned to sextet 1 with a magnetic field of 515 kOe, a new a very small percentage Fe oxide phase of goethite α-FeOOH was detected and assigned to sextet 2. The obtained hyperfine magnetic field of 378 KOe is consistent with reported values [16].The doublet which is at about 5% accounts for super paramagnetic effects arising small magnetic particles that are probably as a result of the beneficiation. Therefore, a common feature to the three samples is the predominance of the hematite α-Fe2O3 phase.

Fig 4: Room temperature ⁵⁷Fe Mössbauer spectra for Itapke iron ore waste sample.

Table 2: ⁵⁷Fe Mössbauer hyperfine parameters such as, magnetic hyperfine field B_hf, isomer shift δ, quadrupole splitting, Δ_EQ, and Fe fraction population, f, for Itapke iron.

In the waste sample, the best fits were obtained using 2 sextets and a doublet. Although, the sample is still predominantly of the hematite α-Fe2O3 phase assigned to sextet 1 with a magnetic field of 515 kOe, a new a very small percentage Fe oxide phase of goethite α-FeOOH was detected and assigned to sextet 2. The obtained hyperfine magnetic field of 378 KOe is consistent with reported values [16]. The doublet which is at about 5% accounts for super paramagnetic effects arising small magnetic particles that are probably as a result of the beneficiation. Therefore, the samples is the predominance of the hematite α-Fe2O3 phase.

In this study of itakpe iron ore, the waste sample was successfully characterized, the magnetic properties of the sample were measured using two methods. The result indicate high magnetization for the waste and is predominantly hematite α-Fe₂O₃ and a new very small percentage Fe oxide phase of goethite was detected. Therefore, a feature of waste sample is predominantly hematite.

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