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. 2026 Jan 15;11(3):4304–4313. doi: 10.1021/acsomega.5c09490

Recovery of Ferromagnetic Hematite Submicron Particles from Pyrrhotite Ash and Comprehensive Study of Their Structural, Morphological, and Magnetic Properties

Youssef Esshouba †, Tarik Mahfoud †, Youssef Tamraoui †, El Kébir Hlil ‡, Hassan El Moussaoui §, Mounir El Achaby †,*
PMCID: PMC12854513  PMID: 41626496

Abstract

The rapid advancement of modern civilization has increased the demand for heavy metals, while the exhaustion of high-grade natural ore supplies compels the investigation of alternate metal sources. Simultaneously, industrial activities cause substantial amounts of inorganic solid waste every year globally, and pyrrhotite ash (PA) represents a notable case for study. PA is an inorganic solid waste produced during the manufacture of phosphoric acid from roasted pyrrhotite ore and contains valuable metals, including Fe. Currently, PA is stored in large open spaces without treatment, posing significant environmental challenges. In response, this study aimed to extract iron oxide particles, identified as a potential raw material for the preparation of new materials. In this work, we develop a full synthesis process of ferromagnetic hematite particles from PA, through acid leaching, followed by a hydrothermal and co-precipitation reaction. Both techniques were effective and led to the production of α-Fe2O3 (hematite structure) as confirmed by XRD, SEM, TGA, and Raman spectroscopy techniques, while magnetic properties were determined by using VSM at room temperature. The results revealed that the extracted hematite particles exhibit ferromagnetic behavior.

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1. Introduction

Industrial activities generate large amounts of inorganic solid waste globally each year, including steel production, phosphoric acid production, metal extraction, cement manufacturing, and other process. Among those, pyrrhotite ash (PA) represents a notable case study. Pyrrhotite ash is an inorganic solid waste produced during the manufacture of phosphoric acid from roasted pyrrhotite ore in southwestern Morocco. The chemical composition of PA consists of iron­(III) oxide (α-Fe2O3) and silicon dioxide (quartz, SiO2), along with traces of sulfur trioxide, calcium oxide, aluminum oxide, phosphorus pentoxide (P2O5), and titanium dioxide. Currently, PA is stored in large open spaces without treatment, posing significant environmental challenges. The presence of heavy metals in PA may lead to leaching into the environment, contaminating water and subsoil, and negatively impacting ecosystems and human health. Additionally, PA presents substantial challenges in handling and disposal due to its large volume, and variable mineralogical composition. These challenges also include elevated salinity, the occupation of valuable land resources, and the disfigurement of the landscape. Nevertheless, PA has not been employed for any particular purpose up to this point, a few initiatives have been undertaken in recent years to use PA directly in various applications, such as raw material in the manufacturing of clay bricks, elaboration of membranes for wastewater treatment, and as an adsorbent. − Therefore, an urgent need exists to address this challenge and develop an economic and sustainable solution for managing this inorganic solid waste.

Iron­(III) oxide is generally classified into five crystal structures: α-Fe2O3 (hematite), β-Fe2O3, γ-Fe2O3 (maghemite), ε-Fe2O3 and ζ-Fe2O3. Hematite (α-Fe2O3) is considered a highly significant material due to its wide range of unique electrical and magnetic properties, as well as its chemical and physical stability. Structurally, hematite crystallizes in the rhombohedral system, with space group R̅3c and specific lattice parameters of a = b = 0.49 nm and c = 1.35 nm in addition to being antiferromagnetic at room temperature, it can exhibit ferromagnetic behavior, weak ferromagnet or a canted antiferromagnet above approximately 260 K. Moreover, it exhibits excellent corrosion resistance. These properties make it widely applicable in various fields, including magnetic devices, batteries, catalysis, pigments, and sensor. − Different techniques are used to produce hematite particles, including hydrothermal, sol–gel, microemulsions, co-precipitation, and others. − The synthesis method has a significant impact on the amount of sample produced, the accuracy of particle size control, morphology, the extent of agglomeration and the resulting hematite particles.

Iron oxide minerals play a significant role. It is used a as raw materials to synthesize magnetic spinel ferrites (CoFe2O4, MnFe2O4, and NiFe2O4), which are crucial in magnetic materials and magnetic components, particularly in advanced applications involving ferrites, such as soft magnetic materials, magnetic shielding materials, and microwave components. − Thus, the second resource of α-Fe2O3 has gained interest due to the increasing demand for hematite in magnetic materials, such as ferrites. , As a result, the supply and demand chain for iron­(III) oxide could benefit from the use of industrial waste as a secondary source of iron oxide. Furthermore, it is desirable and cost-effective to use industrial waste to create a product of additional value. Iron­(III) oxide (α-Fe2O3) has been extracted from industrial waste using a range of technologies and methods, including hydrometallurgical processes (acid leaching) and physical methods (flotation, gravity separation, and magnetic separation). − The above-mentioned physical methods are not currently used in large-scale industrial applications. This is due to low recovery yields, prohibitive costs, or the difficulty of integrating the process directly into existing systems. Another solution is to recover iron from industrial waste by leaching with inorganic acid. − Therefore, due to its hydrometallurgical advantages including high leaching efficiency and the ability to extract multiple elements simultaneously the hydrometallurgical method involving acid leaching is currently considered one of the most promising technologies. Subsequently, the next challenge is to extract iron with an acceptable level of purity from the leachate. Various methods have been investigated for the separation of iron from leachate, including chemical precipitation, solvent extraction, antisolvent approaches, and hydrothermal processes. Pepper et al. extracted and synthesized iron oxide from bauxite residue (an industrial waste) using an acid leaching process followed by precipitation. Chakraborty et al. reported the low-temperature processing of iron oxide nanoflakes from red mud extract, followed by a hydrothermal reaction. Karimi et all. studied a simple, cost-effective, and stepwise leaching and precipitation method for the recovery of nanosized hematite (α-Fe2O3) and mesoporous gamma alumina (γ-Al2O3) from red mud (industrial waste).

In this work, hematite (α-Fe2O3) particles were extracted from pyrrhotite ash (PA)-based industrial waste using HCl leaching. Two different methods, hydrothermal and co-precipitation synthesis, were then employed to produce pure α-Fe2O3 from the extracted iron leachate denoted as Hyd-HPs and Cop-HPs, respectively. The resulting particles were thoroughly characterized for their structural, morphological, optical, and magnetic properties using various techniques, including X-ray fluorescence (XRF), thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV–Visible (UV–Vis) spectroscopy, and vibrating sample magnetometry (VSM).

2. Experimental Section

2.1. Chemicals and Materials

2.1.1. Materials: Chemical Reagents

37% HCl, 30% ammonia, and distilled water were used in all experiments.

2.1.2. Pyrrhotite Ash

A ∼1 kg raw, representative Pyrrhotite ash (PA) in powder form was collected from an abandoned sulfuric acid plant site in the Office Cherifian of Phosphates (OCP) group plant located in Morocco’s Safi. The as-received PA powder was ground and sieved through a <100 μm standard sieve using ball milling. The chemical and mineralogical composition of PA was determined using XRD (Figure a) and XRF (Table ). Results reveal the presence of three main solid phases: 71.2% iron­(III) oxide (α-Fe2O3) and 16.77% silicon dioxide (quartz SiO2), besides some traces of sulfur trioxide, calcium, aluminum oxide, phosphorus oxide, and titanium oxide (3.97, 1.95, 1.27, and 0.11% respectively). Figure c shows an optical microscope image of the PA powder. The predominant colors observed are reddish-brown, gray, white, and yellow, which correspond to hematite, quartz, calcium oxide or phosphorus pentoxide, and sulfur trioxide, respectively. Scanning Electron Microscopy (SEM) was used to examine the PA particle morphology. Figure b shows that the PA particles were large and irregular in shape. According to the particle size distribution shown in Figure d, the particles have an average size of 24 μm, Half of them is smaller than 15.76 μm, and 90% of the particles (D0.9) are below 59.04 μm.

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(a) XRD diffractogram, (b) SEM image, (c) Optical microscope image, and (d) Particle size distribution of PA powder.

1. XRF Elemental Analysis of PA.
oxides α-Fe2O3 SiO2 SO3 CaO Al2O3 P2O5 TiO2 others
wt % 71.21 16.77 3.97 3.97 1.95 1.27 0.11 1.78
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2.2. Leaching Process Optimization for Iron Recovery from PA Using HCl

Previous studies have shown that several factors such as temperature, reaction time, acid concentration, and liquid-to-solid (L/S) ratio affect the leaching efficiency. Accordingly, the One-Factor-At-a-Time (OFAT) approach was employed to optimize the leaching parameters for iron recovery from PA. Before performing each optimization experiment, the PA powder was pretreated with diluted HCl and subsequently used in the leaching tests. The effect of HCl concentration was investigated by leaching 10 g of PA powder in 100 mL of HCl solutions (1–6 M) at 90 °C for 3 h under continuous stirring (300 rpm). The influence of temperature was examined in the range of 25–120 °C for 3 h while keeping other parameters constant (6 M HCl, L/S = 10:1, 300 rpm). To study the effect of leaching time, experiments were carried out for 30–300 min under identical conditions (6 M HCl, 90 °C, L/S = 10:1, 300 rpm). Finally, the effect of the liquid-to-solid ratio was evaluated by varying the ratio from 4:1 to 10:1 using 6 M HCl at 90 °C for 90 min with a stirring speed of 400 rpm.

2.3. Synthesis of Hematite Particles from Pyrrhotite Ash

Acid leaching followed by hydrothermal and co-precipitation syntheses methods were employed to extract and synthesize iron­(III) oxide from pyrrhotite ash (PA) (Figure ). Approximately 100 g of roasted and sieved PA powder was dissolved in concentrated hydrochloric acid (6 M) under reflux conditions, with continuous stirring at 300 rpm and gentle heating at 90 °C for 3 h as the optimum conditions. The resulting iron chloride solution was filtered and transferred to a beaker. At this stage, a 2 M solution of NH4OH was gradually added under stirring until the pH reached eight. For the hydrothermal method, approximately 10 mL of the solution was transferred into a Teflon-lined hydrothermal autoclave and heated at 160 °C for 24 h. After cooling to room temperature, the precipitate was collected by centrifugation, repeatedly washed with distilled water and ethanol to remove impurities, and dried at 60 °C for 6 h. For the co-precipitation method, 10 mL of the solution was heated in a beaker at 80 °C with continuous stirring. Brown precipitates gradually formed, and after 2 h of stirring at 80 °C, the precipitate was collected by centrifugation, washed with distilled water and ethanol to remove impurities, and dried at 60 °C for 6 h. Finally, the dried powder was annealed at 700 °C for 4 h to obtain the desired particles.

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Flowchart for the extraction and synthesis of hematite particles by hydrothermal and precipitation method.

2.4. Analysis Methods

The X-ray diffraction patterns of the samples were obtained using powder X-ray diffraction with a BRUKER D8-Discover diffractometer with Cu-Kα radiation (λ = 1.5418 Å), operating at a scan rate of 2°/min over a 2θ of 5–80° range.

The chemical and mineralogical compositions of the PA powder and the extracted hematite particles were determined using a Malvern Panalytical XRF spectrometer (Epsilon 4 Model). Morphology was analyzed using a scanning electron microscope (Zeiss EVO 10 SEM, Carl Zeiss Microscopy, GmbH, Jena, Germany) at an accelerating voltage of 15 kV.

Thermogravimetric analysis (TGA) curves of the synthesized product were obtained using a TA Instruments (Discovery TGA). TGA tests were conducted in an air atmosphere on samples of approximately 5 mg, using a heating rate of 10 °C/min from room temperature to 700 °C.

Raman spectra were acquired using a HORIBA Scientific instrument using a 532 nm laser, an 1800 g/mm grating, and a 0.95 numerical aperture objective with a laser excitation intensity of 3 mW.

Fourier Transform Infrared (FTIR) Spectrometer was recorded with a Bruker (ALPHA II) spectrometer on neat samples (ATR FT-IR), in the 4000–400 cm–1 range with a resolution of 4 cm–1 and an accumulation of 24 scans.

The Ultraviolet–Visible (UV–Vis) absorption of the samples was recorded using a SHIMADZU UV–Vis Spectrophotometer (UV-3101 PC).

The magnetic properties of the selected samples were determined using a vibrating sample magnetometer (VSM).

3. Result and Discussion

3.1. Results of Leaching Process Optimization for Iron Recovery from PA Using HCl

3.1.1. Effect of HCl Concentration

Figure a presents the optimization of hydrochloric acid concentration. Ten grams of PA powder pretreated with diluted HCl and subsequently immersed in 100 mL of HCl solution with concentrations ranging from 1 to 6 M at 90 °C for 3 h under continuous agitation at 300 rpm. The leaching efficiency increased progressively with higher acid concentration, reaching up to 80%. Based on these findings, a 6 M HCl concentration was determined to be the optimal condition for the leaching of PA powder, confirming the complete dissolution of iron hematite.

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Effect of HCl concentration (a), leaching temperature (b), dissolution time (c), and liquid-to-solid ratio (d), on leaching efficiency of iron from PA powder.

3.1.2. Effect of Temperature

To elucidate the influence of temperature on iron leaching efficiency, experiments were performed over a temperature range of 25–120 °C for 3 h while maintaining other parameters constant (HCl concentration of 6 M, liquid-to-solid ratio of 10:1 and stirring speed of 300 rpm). As depicted in Figure b, temperature exhibited a pronounced effect on the leaching performance. Specifically, when the temperature increased from 25 to 90 °C, iron leaching efficiency improved from 10 to 80%. Further increases beyond 90 °C resulted in marginal improvement, indicating that the leaching efficiency approached a plateau at higher temperatures.

3.1.3. Effect of Leaching Time

The effect of leaching duration on iron extraction was examined by varying the leaching time from 30 to 300 min, while keeping all other variables constant (HCl concentration = 6 M, temperature = 90 °C, liquid-to-solid ratio = 10:1 and stirring speed = 300 rpm). As illustrated in Figure c, iron leaching efficiency exceeded 82% after 90 min, increased slightly with extended leaching time, and stabilized beyond 90 min, indicating equilibrium conditions.

3.1.4. Effect of Liquid-to-Solid Ratio

The impact of liquid-to-solid ratio on the leaching efficiency was studied by varying the ratio from 4:1 to 10:1. The PA powder was washed with dilute HCl before being immersed in a 6 M HCl solution at 90 °C for 90 min, with the liquid-to-solid ratio varied from 4:1 to 10:1, and a stirring speed of 400 rpm. Figure d shows the optimization of the liquid–solid ratio for the leaching of iron. As a result, by increasing the liquid–solid-ratio up to 10 mL per gram of solid, the leaching efficiency of iron species rises from 44.18 to 82.02%. Therefore, we conclude that the liquid-to-solid ratio of 10:1 is preferred in this study to achieve a higher leaching efficiency of iron.

The optimal parameters determined for maximizing the leaching efficiency of iron­(III) oxide were a liquid-to-solid ratio of 10:1, an HCl concentration of 6 M, a leaching temperature of 90 °C for 3 h, and a stirring rate of 300 rpm. Furthermore, following the leaching process with the optimum parameters a comprehensive characterization analysis was conducted to verify the successful extraction of iron oxide from PA. The insoluble residue (Figure S1, Supporting Information) obtained after filtration was characterized using various analytical techniques, including X-ray fluorescence (XRF) and X-ray diffraction (XRD). As presented in Table S1 (Supporting Information), the chemical composition of the insoluble residue consists of 95.05% silicon dioxide (SiO2), 1.92% phosphorus oxide, and 2.43% sulfur trioxide. Moreover, Figure S2 (Supporting Information) illustrates that the XRD patterns of the residual material exhibit diffraction peaks corresponding to insoluble phases such as silicon dioxide (SiO2), phosphorus oxide, and sulfur trioxide. The complementary results obtained from XRF and XRD analyses confirm the complete removal of iron, providing convincing evidence for its successful leaching from PA waste. This characterization not only validates the efficiency of the leaching process under optimum conditions but also provides valuable insights for further refinement and optimization of the extraction methodology.

3.2. Hematite Particles Properties

3.2.1. Thermal Properties

To assess the thermal stability of Cop-HPs and Hid-HPs powder TGA test were conducted in the temperature range 25–750 °C under an oxidative atmosphere (air) as shown in Figure S3a,b. The TGA curves showed negligible mass loss, attributed to the dehydration of free water, across the entire temperature range, indicating the purity and high thermal stability of the samples. As a result, the TGA data confirmed that the HPs synthesized using the two different methods are in a thermodynamically stable phase.

3.2.2. Structural Properties

The phase purity and crystallinity of the extracted and synthesized HPs, prepared using co-precipitation and hydrothermal techniques, were confirmed by X-ray diffraction analysis. Figure shows the XRD patterns of samples (Cop-HPs and Hid-HPs), all indexed peaks of the two samples (0 1 2), (1 0 4), (1 1 0), (0 0 6), (1 1 3), (2 0 2), (0 2 4), (1 1 6), (1 2 2), (2 1 4), (3 0 0), (2 0 8), (1 0 10), (1 1 9), (2 2 0), and (3 0 6) corresponding to 24.12, 33.11, 35.61, 39.22, 40.83, 43.47, 49.41, 54.00, 57.50, 62.38, 63.96, 69.50, 71.82, 72.17, 75.40, and 77.66 plans, respectively, belongs to the rhombohedral crystal structure of α-Fe2O3 (hematite) [space group: R̅3c] which are good agreement with literature (JCPDS No. 00-024-0072). No additional diffraction peaks were detected, confirming the high purity of the extracted hematite submicron particles. A notable feature of the XRD patterns is the significant differences in peak intensities, along with small shifts (as shown by the captions in Figure ) observed between the peaks of the Cop-HPs and Hid-HPs samples. These differences can be attributed to variations in nucleation centers and the growth of crystalline submicron particles influenced by the calcination temperature in the co-precipitation method. Furthermore, the broad peaks observed in the XRD pattern can be attributed to the smaller crystallite size, while the presence of sharp peaks indicates the good crystallinity of the prepared samples. The average crystallite sizes (D) were estimated to be from the most intense peaks using the Debye–Scherrer formula

D=0.9λβcos(θ) 1

where λ is the X-ray wavelength (1.5406 Å), β is the full width at half-maximum of the diffraction peak, θ is the Braggs’ angle. The crystallite size obtained from co-precipitation method (31.97 nm) found to be slightly higher compared to hydrothermal (31.62 nm).

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XRD diffractogram of samples.

To confirm the phase composition and enhance understanding of the structural properties, the XRD data were analyzed using the Rietveld refinement method with FullProf software. Figure a,b show the Rietveld refinement of XRD patterns of hematite submicron particles (α-Fe2O3) synthesized by co-precipitation and hydrothermal methods, respectively, using Fullprof software. The structural model used for the refinement was generated with VESTA software. The positions of the allowed Bragg peaks are indicated by vertical bars, and the lower curve shows the difference profile between the calculated and the observed XRD pattern. The Rietveld refinement fit shows good agreement between the experimental and calculated spectra, with a refinement factor of RF = 4.1501. The close agreement indicates the single-phase of hematite structure (α-Fe2O3, space group: R̅3c) with a Rhombohedral primitive cell.

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Rietveld refinement profile of XRD data of extracted and synthesis hematite particles (a) Hyd-HPs and (b) Cop-HPs.

Raman spectroscopy measurements were performed at room temperature to confirm the results obtained from XRD measurement. Figure a illustrates the Raman spectra recorded in the range of 100–800 cm–1 for samples extracted and synthesized using two different methods precipitation (Cop-HPs) and hydrothermal (Hyd-HPs). From the literature, α-Fe2O3 exhibits seven phonon modes that are anticipated to emerge in the Raman spectrum, namely two A1g phonon modes and five Eg phonon modes. − The Raman peaks that appear at around 220–226 cm–1 and 494–504 cm–1 belong to the A1g phonon mode, and those at around 242–246 cm–1, 287–292 cm–1, 401–408 cm–1, 601–614 cm–1 and 660–664 cm–1 belong to the Eg phonon mode. − The Raman spectra of the synthesized samples (Cop-HPs and Hyd-HPs) are identical, showing no significant shifts in peak positions. According to the literature, the FTIR spectrum of hematite particles typically exhibits two characteristic absorption bands corresponding to the stretching and bending vibrations of iron–oxygen (Fe–O) bonds. Figure b shows the FTIR spectra of the extracted samples, where two distinct bands at 519 and 434 cm–1 are observed. These bands clearly indicate the presence of Fe–O bonds. , Overall, the presence of various peaks corresponding to seven Raman modes as well the observed Fe–O bonds from FTIR spectra corroborates the XRD results, confirming the successful extraction and synthesis of hematite particles with high purity.

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(a) Raman spectra, and (b) FTIR spectra of extracted and synthesis hematite particles.

3.2.3. Chemical Composition, Surface Morphologies, and Size Distribution

The chemical composition of the extracted samples was analyzed using X-ray fluorescence (XRF), and the results are summarized in Table S2. The samples synthesized by the hydrothermal and co-precipitation methods contained 98.76 and 98.48 wt % α-Fe2O3, respectively, with trace impurities of P2O5, SiO2, and Al2O3.

The surface morphologies of the extracted and synthesized HPs through co-precipitation and hydrothermal were performed using scanning electron microscopy (SEM). Figure S4a,b shows the SEM images of the samples (Cop-HPs and Hyd-HPs) synthesized by co-precipitation and hydrothermal, respectively. The image of the sample (Cop-HPs) exhibits highly consolidated grains, resulting in irregularly shaped, high-density aggregates with coarse surfaces and a random particle size distribution, in contrast to the reference image of the sample (Hyd-HPs). The EDX spectra of the samples indicates that they consist primarily of iron and oxygen, with trace amounts of aluminum and phosphorus, as also shown by the XRF results (Figure S4a,b). The synthesized samples are therefore considered pure, as their EDX spectra display only minor peaks corresponding to impurities.

Further study of the morphological properties and size distribution of extracted HPs was observed using the STEM. Figure shows detailed morphologies and particle size distribution of the Hyd-HPs and Cop-HPs samples synthesized by precipitation and hydrothermal methods, respectively. The morphologies of particles are found to be small oblate grains shape for HPs extracted and synthesized by two methods. When HPs are synthesized by the hydrothermal process, the average size of the submicron particles is small, around 154.34 nm. However, when HPs are synthesized via the precipitation approach, the average size of submicron particles is large, around 200.35 nm. As a result, the synthesized hematite particles’ submicron size depends on the synthesis method. These results are in good agreement with the literature. For instance, Lassoued et al. studied the effect of synthesis methods on hematite nanoparticles prepared by hydrothermal and co-precipitation routes and found an average particle size of 21 nm for the hydrothermal samples and 33 nm for the precipitation samples.

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STEM observation and size distribution of samples prepared by two different methods: (a) hydrothermal and (b) co-precipitation.

3.2.4. Optical Properties

Figure S5a displays the UV–Vi’s absorption spectra of HPs extracted and synthesized by co-precipitation and hydrothermal methods. As evident in Figure S5a the extracted samples (Hyd-HPs, and Cop-HPs) showed intense absorption in the visible wavelength region with typical band at 543 nm, 554 nm (labeled a1 and a2), indicating a red shift in the band gap transition. The Cop-HPs sample shows significantly higher absorption intensity compared to the Hyd-HPs sample. This change in the absorption spectra might be attributed to size-dependent effects. Mizuno et al. demonstrated that the size and morphology of hematite nanoarchitectures significantly influence their optical absorption properties. The optical band gap energy (E g) of the extracted HPs has been calculated using the Tauc plot eq .

αhν=A(hν−Eg)n 2

where α is the absorption coefficient, A is constant, hν is the energy of light and n is constant depending on the nature of the electron transition. Hematite is n-type semiconductor with a direct band gap (n = 2). Figure S5b shows the plot of (αhν)2 versus energy. The obtained results indicate that the extracted samples (Cop-HPs and Hyd-HPs) have band gap values of 2.18 and 2.07 eV, respectively. This suggests that the optical band gap of hematite submicron particles changes with respect to the synthesis method used as well as the particle size effect. Furthermore, the band gap energy values of hematite synthesized via both co-precipitation and hydrothermal methods were closely aligned with those reported in the literature. ,

3.2.5. Magnetic Properties

The magnetic properties of the extracted HPs were measured at 5 and 300 K using vibrating sample magnetometer (VSM). Hysteresis loops for magnetization (M) and magnetic field (H) of the extracted HPs are shown in Figure , and the related magnetic parameters, including Ms, H C, and M r, are summarized in Table . Here M S is the saturation magnetization, H C is the coercive field, and M r is the remanent magnetization (at H = 0 Oe). All synthesized hematite particles exhibited narrow hysteresis loops, indicating typical ferromagnetic behavior. Among the synthesis routes investigated, the co-precipitation method yielded hematite particles with enhanced ferromagnetic properties, characterized by relatively high saturation magnetization values of 9.02 emu/g and 8.26 emu/g, remanent magnetizations of 2.25 emu/g and 4.05 emu/g, and low coercive fields of 0.29 kOe and 0.28 kOe at room temperature and 5 K, respectively. The synthesis method significantly influences particle size and magnetic parameters, including saturation magnetization (M S), remanent magnetization (M r), and coercivity (H C). These variations are mainly attributed to differences in particle size, as reported in previous studies. ,− The saturation magnetization values of hematite particles synthesized by both co-precipitation and hydrothermal methods are consistent with those reported in the literature. −

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Hysteresis loops measured at 5 K and 300 K for the extracted and synthesis hematite particles: (a) Hyd-HPs and (b) Cop-HPs.

2. Magnetic Properties of Prepared Samples at 300 and 5 K.
samples measurement temperature saturation magnetization M S (emu/g) Remanent magnetization M r (emu/g) Coercivity H c (kOe)
Hyd-HPs 300 K 6.74 2.18 0.53
Hyd-HPs 5 K 6.53 2.71 0.66
Cop-HPs 300 K 9.02 2.25 0.29
Cop-HPs 5 K 8.26 4.05 0.68
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4. Conclusions

This work explored a new route for iron recovery from pyrrhotite ash. Initially, the feasibility of extracting iron through cost-effective HCl leaching was demonstrated, followed by synthesis via hydrothermal and co-precipitation methods. Characterization of the resulting iron oxide confirmed the formation of high-purity (98%) α-Fe2O3 with ferromagnetic behavior. Overall, the results highlight that hematite submicron-particles synthesized by co-precipitation exhibited enhanced ferromagnetic properties, characterized by relatively high saturation magnetization values of 9.02 emu/g and 8.26 emu/g, remanent magnetizations of 2.25 emu/g and 4.05 emu/g, and low coercive fields of 0.29 kOe and 0.28 kOe at room temperature and 5 K, respectively. In addition to these favorable magnetic characteristics, the synthesis process offers notable economic and environmental benefits, contributing to sustainability through the mitigation of industrial waste.

Future research should focus on developing effective methods for treating the residual solution generated after iron oxide precipitation, as its limited renderability significantly increases the overall cost of powder production. This includes exploring environmentally friendly modification strategies and performing comprehensive life-cycle assessments to better evaluate the process’s environmental impact. Moreover, subsequent studies should investigate the use of Cop-HPs as precursors for synthesizing soft magnetic powders (CoFe2O4, MnFe2O4, and NiFe2O4) via solid–state reactions for modern technological applications.

Supplementary Material

ao5c09490_si_001.pdf (947.7KB, pdf)

Acknowledgments

This research receives support from the Moroccan Ministry of Higher Education, Scientific Research, and Innovation, along with funding from the OCP Foundation through the APRD research program.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c09490.

  • XRF elemental analysis of insoluble residue powder (Table S1); image of residual powder (Figure S1); XRD patterns of residual powder (Figure S2); TGA of extracted and synthesized hematite particles, including Hyd-HPs and Cop-HPs (Figure S3); XRF elemental analysis of extracted α-Fe2O3 (Table S2); SEM images of extracted and synthesized hematite particles (Hyd-HPs and Cop-HPs) and corresponding EDX images (Figure S4); UV–Vis spectra and Tauc plot of samples (Figure S5) (PDF)

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the last version.

The authors declare no competing financial interest.

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