Hematite Samples Research Articles

Application of (U-Th)/He hematite geochronology to the Çaldağ lateritic Ni-Co deposit, Western Anatolia: Implications for multi-stage weathering events during interglacial periods/segments

Lateritic Ni-Co deposits are supergene deposits that develop due to intense weathering of the underlying ultramafic parent rocks and/or their serpentinized equivalents under tropical to sub-tropical climatic conditions. These deposits provide not only important sources of valuable metals such as iron, aluminium, nickel, and cobalt, but they also provide a proxy for paleoclimatic conditions. In this regard, geochronological studies on these deposits are critical to determine the timing of the paleoclimatic conditions, which is important for a better understanding of the paleoclimate at regional scale and for identification of favourable weathering periods that can be targeted in the exploration of undiscovered lateritic deposits in a region. In this contribution, we report hematite (U-Th)/He geochronology results from a well-preserved Çaldağ lateritic Ni-Co deposit in Western Türkiye. We sampled the different parts of the lateritic profile from the different laterite zones at the Çaldağ deposit and carried out analyses of polished thin section to determine and identify the phases of hematite formation and primary hematite formed during primary lateritization. Then, we applied scanning electron microscopy (SEM) analysis and TESCAN Integrated Mineral Analyser (TIMA) mineral mapping to identify the suitable areas on primary hematites for subsequent (U-Th)/He dating.Primary hematites are detected at the base of the lateritic profiles (transition between the limonite zone and altered serpentinite) that are only in-situ parts of the laterites exposed in two different pits in the deposit. The obtained (U-Th)/He dates of ca. 502, 206, 178, and 63 ka correspond to the interglacial periods. The three youngest (U-Th)/He dates (206, 178, and 63 ka) from the weathering front of the hematite pit become younger towards the deeper parts of the profile in the direction of weathering. The ages indicate the weathering processes should have lasted until the Quaternary with some interruptions during interglacial periods/segments, despite the fact that the main intensive lateralization period is thought to be during the Middle Eocene.This study presents the first hematite (U-Th)/He dating of lateritic Ni-Co deposits and demonstrates the reliable use of this method after a careful selection of hematite samples. Our results suggest that, in contrast to the short-living lateritization model for lateritic Ni-Co deposits, they may have a multi-stage weathering history throughout their long-lasting development. In addition, the study showed that it has huge potential for the dating of lateritization and weathering processes that are related to paleoclimatic events. Finally, knowledge of the favourable paleoclimatic periods of chemical weathering in a region may help in determining the potential areas for ultramafic exposures for discovering new lateritic deposits.

Effect of Sodium Hexametaphosphate on Flotation of Quartz and Hematite with Starch/Etheramine in the Presence of Ca and Mg Ions

ABSTRACT The aim of this study was to evaluate the effect of sodium hexametaphosphate (SHMP), a complexing reagent for Ca and Mg ions, on iron ore reverse cationic flotation. Quartz and hematite samples contaminated with both Ca and Mg were subjected to microflotation assays at pH 10.5 with starch and etheramine, which showed that SHMP did not provide selectivity in the separation of these minerals. Zeta potential measurements, Fourier transform infrared spectroscopy of quartz and hematite without and with the studied reagents (starch, amine, SHMP, CaCl2/MgCl2) and published studies in the literature indicated that although the HMP anions removed the Ca and Mg ions from both minerals, it also formed complexes with the Fe3+ ions on the hematite surface, which prevented subsequent adsorption of starch and consequently decreased the selectivity for reverse flotation of iron ore.

The change of coordination environments induced by vacancy defects in hematite leads to a contrasting difference between cation Pb(II) and oxyanion As(V) immobilization

Hematite is an iron oxide commonly found in terrestrial environments and plays an essential role in controlling the migration of heavy metal(loid)s in groundwater and sediments. Although defects were shown to exist both in naturally occurring and laboratory-synthesized hematite, their influences on the immobilization of heavy metal(loid)s remain poorly understood. In this study, hematite samples with tunable vacancy defect concentrations were synthesized to evaluate their adsorption capacities for the cation Pb(II) and for the oxyanion As(V). The defects in hematite were characterized using XRD, TEM-EDS mapping, position annihilation lifetime spectroscopy, and XAS. The surface charge characteristics in defective hematite were investigated using zeta potential measurements. We found that Fe vacancies were the primary defect type in the hematite structure. Batch experiments confirmed that Fe vacancies in hematite promoted As(V) adsorption, while they decreased Pb(II) adsorption. The reason for the opposite effects of Fe vacancies on Pb(II) and As(V) immobilization was investigated using DFT calculations and EXAFS analysis. The results revealed that Fe vacancies altered As–Fe coordination from a monodentate to a bidentate complex and increased the length of the Pb–Fe bond on the hematite surface, thereby leading to an increase in As(V) bonding strength, while decreasing Pb(II) adsorption affinity. In addition, the zeta potential analysis demonstrated that the presence of Fe vacancies led to an increase in the isoelectric point (IEP) of hematite samples, which therefore decreased the attraction for the cation Pb(II) and increased the attraction for the oxyanion As(V). The combination of these two effects caused by defects contributed to the contrasting difference between cation Pb(II) and oxyanion As(V) immobilization by defective hematite. Our study therefore provides new insights into the migration and fate of toxic heavy metal(loid)s controlled by iron minerals.

In situ Fe isotope analysis of Cr-rich iron oxides using pure chromium metal for isobaric interference corrections by femtosecond LA–MC–ICP–MS

Iron oxide minerals, such as hematite and magnetite, are commonly found in a wide range of geological environments. Their Fe isotopic composition acts as a potent geochemical tracer, finding applications across various realms of Earth sciences. However, natural iron oxide minerals often have a high Cr content up to percentile levels, necessitating correction of isobaric interference caused by 54Cr+ on 54Fe+ to ensure accurate and precise in situ Fe isotopic compositions. In this study, a pure chromium metal with homogeneous isotopic compositions was examined within the analysis sequence to obtain the fractionation factor of Cr (βCr) for isobaric interference correction. A femtosecond laser ablation system combined with wet plasma conditions was used to decrease the matrix effect. We synthesized a series of magnetite and hematite samples with simple matrix as well as natural magnetite samples with complex matrix to evaluate the feasibility of Cr interference correction. By employing our proposed correction method, the deviation caused by the isobaric interference of 54Cr+ was effectively eliminated. The corrected δ56Fe values for hematite and magnetite samples, even with a Cr/Fe ratio of as high as 1.27, exhibited good agreement with the reference values within the long-term reproducibility uncertainty of 0.10‰. These results indicate the robustness of Cr interference correction in obtaining accurate Fe isotopic compositions of Cr-rich iron oxides.

Doping of large amount tetravalent Ge ions into Fe2O3 structure and experimental results on modified structural, optical and electronic properties

We report the doping of high concentration of tetravalent Ge4+ ions (5 mol % for x = 0.05–30 mol % for x = 0.30) at the Fe3+ sites of Fe2-xGexO3 system by chemical coprecipitation route. The charge state of Fe and Ge ions has been modified into lower values, in addition to their normal +3 and + 4 states, to stabilize the rhombohedral phase of hematite (α-Fe2O3) structure. X-ray photoelectron spectra and optical band gap measurements indicated a combination of ionic and covalence character of metal-oxygen bonds as an effect of Ge doping in the hematite structure. The Ge doped hematite samples have exhibited semiconductor properties with band gap of 1.50–4.70 eV and remarkably enhanced electrical conductivity (σ ∼ 10−4 S/m) in comparison to α-Fe2O3 (10−11 S/m). The results confirmed the strategy of enhancing electrical conductivity in hematite structure by doping of tetravalent cations. The thermo-conductivity measurements using warming and cooling modes showed highly irreversible feature at higher temperatures. Some of the samples indicated metal-like state at lower temperature, in addition to semiconductor state at higher temperatures. It has been understood that combination of ionic and covalence character of the metal-oxygen bonds has played an important role in modifying the semiconductor properties in Ge doped Fe2O3 system.

Prediction of Thermogravimetric Data in the Thermal Recycling of e-waste Using Machine Learning Techniques: A Data-driven Approach.

The release of bromine-free hydrocarbons and gases is a major challenge faced in the thermal recycling of e-waste due to the corrosive effects of produced HBr. Metal oxides such as Fe2O3 (hematite) are excellent debrominating agents, and they are copyrolyzed along with tetrabromophenol (TBP), a lesser used brominated flame retardant that is a constituent of printed circuit boards in electronic equipment. The pyrolytic (N2) and oxidative (O2) decomposition of TBP with Fe2O3 has been previously investigated with thermogravimetric analysis (TGA) at four different heating rates of 5, 10, 15, and 20 °C/min, and the mass loss data between room temperature and 800 °C were reported. The objective of our paper is to study the effectiveness of machine learning (ML) techniques to reproduce these TGA data so that the use of the instrument can be eliminated to enhance the potential of online monitoring of copyrolysis in e-waste treatment. This will reduce experimental and human errors as well as improve process time significantly. TGA data are both nonlinear and multidimensional, and hence, nonlinear regression techniques such as random forest (RF) and gradient boosting regression (GBR) showed the highest prediction accuracies of 0.999 and lowest prediction errors among all the ML models employed in this work. The large data sets allowed us to explore three different scenarios of model training and validation, where the number of training samples were varied from 10,000 to 40,000 for both TBP and TBP + hematite samples under N2 (pyrolysis) and O2 (combustion) environments. The novelty of our study is that ML techniques have not been employed for the copyrolysis of these compounds, while the significance is the excellent potential of enhanced online monitoring of e-waste treatment and extension to other characterization techniques such as spectroscopy and chromatography. Lastly, e-waste recycling could greatly benefit from ML applications since it has the potential to reduce total and operational costs and improve overall process time and efficiency, thereby encouraging more treatment plants to adopt these techniques, resulting in reducing the increasing environmental footprint of e-waste.

A Spectroscopic Study of Mars-analog Materials with Amorphous Sulfate and Chloride Phases: Implications for Detecting Amorphous Materials on the Martian Surface

The Chemistry and Mineralogy X-ray diffraction (XRD) instrument aboard the Curiosity rover consistently identifies amorphous material at Gale Crater, which is compositionally variable, but often includes elevated sulfur and iron, suggesting that amorphous ferric sulfate (AFS) may be present. Understanding how desiccating ferric sulfate brines affect the spectra of Martian material analogs is necessary for interpreting complex/realistic reaction assemblages. Visible and near-infrared reflectance (VNIR), mid-infrared attenuated total reflectance (MIR, FTIR-ATR), and Raman spectra, along with XRD data are presented for basaltic glass, hematite, gypsum, nontronite, and magnesite, each at three grain sizes (<25, 25–63, and 63–180 μm), mixed with ferric sulfate (+/−NaCl), deliquesced, then rapidly desiccated in 11% relative humidity or via vacuum. All desiccated products are partially or completely XRD amorphous; crystalline phases include starting materials and trace precipitates, leaving the bulk of the ferric sulfate in the amorphous fraction. Due to considerable spectral masking, AFS detectability is highly dependent on spectroscopic technique and minerals present. This has strong implications for remote and in situ observations of Martian samples that include an amorphous component. AFS is only identifiable in VNIR spectra for magnesite, nontronite, and gypsum samples; hematite and basaltic glass samples appear similar to pure materials. Sulfate features dominate Raman spectra for nontronite and basaltic glass samples; the analog material dominates Raman spectra of hematite and gypsum samples. MIR data are least affected by masking, but basaltic glass is almost undetectable in MIR spectra of those mixtures. NaCl produces similar FTIR-ATR and Raman features, regardless of analog material.

Quantitative Contribution of Oxygen Vacancy Defects to Arsenate Immobilization on Hematite.

Hematite is a common iron oxide in natural environments, which has been observed to influence the transport and fate of arsenate by its association with hematite. Although oxygen vacancies were demonstrated to exist in hematite, their contributions to the arsenate immobilization have not been quantified. In this study, hematite samples with tunable oxygen vacancy defect (OVD) concentrations were synthesized by treating defect-free hematite using different NaBH4 solutions. The vacancy defects were characterized by positron annihilation lifetime spectroscopy, Doppler broadening of annihilation radiation, extended X-ray absorption fine structure (EXAFS), thermogravimetric mass spectrometry (TG-MS), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The results revealed that oxygen vacancy was the primary defect type existing on the hematite surface. TG-MS combined with EPR analysis allowed quantification of OVD concentrations in hematite. Batch experiments revealed that OVDs had a positive effect on arsenate adsorption, which could be quantitatively described by a linear relationship between the OVD concentration (Cdef, mmol m-2) and the enhanced arsenate adsorption amount caused by defects (ΔQm, μmol m-2) (ΔQm = 20.94 Cdef, R2 = 0.9813). NH3-diffuse reflectance infrared Fourier transform (NH3-DRIFT) analysis and density functional theory (DFT) calculations demonstrated that OVDs in hematite were beneficial to the improvement in adsorption strength of surface-active sites, thus considerably promoting the immobilization of arsenate.

The Relationship between the Structural Characteristics of α-Fe2O3 Catalysts and Their Lattice Oxygen Reactivity Regarding Hydrogen.

In this paper, the relationship between the structural features of hematite samples calcined in the interval of 800-1100 °C and their reactivity regarding hydrogen studied in the temperature-programmed reaction (TPR-H2) was studied. The oxygen reactivity of the samples decreases with the increasing calcination temperature. The study of calcined hematite samples used X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and Raman spectroscopy, and their textural characteristics were studied also. According to XRD results, hematite samples calcined in the temperature range under study are monophase, represented by the α-Fe2O3 phase, in which crystal density increases with increasing calcination temperature. The Raman spectroscopy results also register only the α-Fe2O3 phase; the samples consist of large, well-crystallized particles with smaller particles on their surface, having a significantly lower degree of crystallinity, and their proportion decreases with increasing calcination temperature. XPS results show the α-Fe2O3 surface enriched with Fe2+ ions, whose proportion increases with increasing calcination temperature, which leads to an increase in the lattice oxygen binding energy and a decrease in the α-Fe2O3 reactivity regarding hydrogen.

Görünür-Işık ile Çalışan Fotokatalizör Hematitin Çinko ve Bakır ile Katkılanmasıyla Geliştirilmesinin Karşılaştırmalı İncelenmesi

The low cost, earth abundance, nontoxic, and efficient photocatalysts materials have a critical role in order to solve environmental issues. In this regard, hematite (α-Fe2O3) has received significant attention due to its desirable properties. In the present study, zinc-doped and copper-doped hematite nanoparticles were synthesized by the hydrothermal method. The photocatalytic features of produced nanopowders were investigated. The evaluations of photocatalytic activities of synthesized nanoparticles were executed by monitoring the degradation rate of Rhodamine B (RhB) under the solar simulator in heterogeneous photocatalysis. Compared to commercial Degussa TiO2 powder, the transition metal doped hematite (α-Fe2O3) samples showed better photocatalytic activities against RhB under the solar simulator. It was observed that even though there were no significant differences in their characteristic properties strongly affecting photocatalytic activity such as morphological features, optical absorption characteristics, and band gaps, Cu-doped α-Fe2O3 nanoparticles exhibited higher photocatalytic activity, which is %20 higher than the Zn-doped α-Fe2O3. The synthesized Cu-doped hematite nanoparticles are hopeful materials as a visible-light-driven photocatalytic material to degrade organic pollutants in aquatic media.

Effect of water vapor on the reduction kinetics of hematite powder by hydrogen-water vapor in different stages

The powder of the hematite sample was isothermally reduced with a hydrogen-water vapor gas mixture at 1023K-1273K. The results indicate that the overall reduction process of hematite can be divided into three stages (Fe2O3-Fe3O4-FeO-Fe) each of which should be investigated. At 1023K, the average reaction rate decreased by 53.6% in stage 1 when the water vapor content of the reaction gas increased from 0% to 50%, and it decreased by about 77.2% in stage 2. However, in stage 3, when the water vapor content increased only from 0% to 20%, it decreased by about 78.1%. The results also show that the influence of water vapor on the reduction reaction increases with increasing reaction temperature in all stages of the reduction reaction. The microstructure of the reduction products showed that they still had some holes; that did not seriously block the channel for hydrogen diffusion. Various models were considered to further clarify the influence of water vapor in the reduction stage The range of apparent activation energy of the different stages obtained by the model fitting was about 20-70 kJ/mol, which also confirmed the absence of the solid-state diffusion phenomenon.

Deep Insights into the Radiation Shielding Features of Heavy Minerals in Their Native Status: Implications for Their Physical, Mineralogical, Geochemical, and Morphological Properties

Barite and hematite are the most common heavy-weight minerals applied as aggregates in radiation shielding concrete (RSC). Therefore, to limit the cement consumption and reduce the CO2 emissions accompanying its production, the aim of this study is to use Egyptian barite and hematite minerals in their native status and evaluate their attenuation efficiency against fast neutrons and γ-rays. This was implemented through the measurement of their radiation attenuation against fast neutrons and γ-rays in the energy ranges of 0.80–11 and 0.40–8.30 MeV, respectively, employing a Pu-Be source and a stilbene scintillator. Theoretical calculations were prepared using the NXcom program to validate the fast neutron attenuation measurements. Furthermore, the implications of the physical, mineralogical, geochemical, and morphological characteristics of these heavy-weight minerals with respect to their attenuation efficiencies were considered. We found that barite has superior radiation attenuation efficiency for fast neutrons and γ-rays compared to hematite by 9.17 and 51% for fast neutrons and γ-rays, respectively. This was ascribed to the superior physical, mineralogical, geochemical, and morphological properties of the former relative to those of the latter. Furthermore, a satisfactory agreement between the experimental and theoretical results was achieved, with a deviation of 16 and 19.25% for the barite and hematite samples, respectively. Eventually, barite and hematite can be successful candidates for their use as sustainable alternatives to common RSC.

Adsorption of sodium oleate at the microfine hematite/aqueous solution interface and its consequences for flotation

The adsorption of sodium oleate (NaOL) at the microfine hematite/aqueous solution interface was investigated in this paper. Experimental research indicated that negative effects stemmed from the dissolution of the microfine hematite (D50=19.21 μm) could be effectively eliminated via the appropriate dosage of NaOL at alkali pH conditions. Solution chemistry calculation and adsorption test results indicated that RCOO− and (RCOO)22− ions were responsible for microfine hematite flotation at pH 8.2. Zeta potential and FTIR measurements confirmed the co-adsorption of molecular and ionic oleate species occurred at pH 8.2. X-ray photoelectron spectroscopy (XPS) results further indicated that oleate species interacted with hematite surfaces mainly through chemisorption, giving rise to molecule/colloid formation of oleate and Fe―OL complex compound. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) results demonstrated that oleate species adsorbed onto the hematite surfaces with a thickness of a few nanometers. Furthermore, the normalized peak intensity of C4H7+ ions on the hematite sample at pH 8.2 increased remarkably comparing with corresponding result of hematite sample at pH 6.8. The new findings of the present study well revealed the dissolution of microfine hematite and the pH effects on the hematite flotation, as well as the adsorption characteristics of oleate species.

Mimosine facilitates Fe uptake by Leucaena leucocephala subsp. glabrata in alkaline soils by solubilizing Fe-oxides

PurposeGiant leucaena is a tropical tree-legume that thrives in alkaline and calcareous soils. It contains large amounts of mimosine, the physiological role of which is not well understood. The goals of this study were to determine i) if mimosine is released by giant leucaena into the rhizosphere ii) if mimosine enhances solubility of Fe present in various forms in alkaline soil, and iii) if mimosine enhances transition metal uptake in giant leucaena.MethodsMimosine in the extracts of rhizosphere soil and root exudates was analyzed by high performance liquid chromatography (HPLC) and liquid chromatography/mass spectrometry (LC/MS) to determine if it is released by giant leucaena. Iron concentration in tissue and aqueous samples was determined by ICP-OES analysis.ResultsMimosine was detected in both the rhizosphere eluates and root exudates. Mimosine added to samples of goethite, hematite and FeCl3 increased Fe solubility in water. Giant leucaena plants grown in alkaline soils and supplied with mimosine had higher levels of chlorophyll, Fe and Mn than the control plants.ConclusionMimosine is released by giant leucaena roots into the rhizosphere, where it interacts with Fe. Mimosine enhances solubility of insoluble Fe-oxides and prevents conversion of ferric Fe to insoluble oxides at an alkaline pH. Mimosine enhances Fe and Mn uptake in plants growing in alkaline soil conditions.

THE PROPERTIES OF SONICATED IMMERSION GROWN HEMATITE FILMS AT VARIOUS ANNEALING TEMPERATURES

In this research, hematite (α-Fe2O3) film was synthesised to study the effect of annealing temperature on its crystallinity, optical and electrical properties. Through a sonicated solution immersion technique, hematite films were deposited on a fluorine-doped tin oxide (FTO) glass substrate. In the synthesis process, 0.2 M ferric chloride (FeCl3·6H2O) was used as a precursor, 0.2 M urea (NH2CONH2) as the stabilizer, and de-ionized (DI) water as a solvent to produce 200 ml of aqueous solution. During the annealing treatment, we varied the temperatures at 350 ºC, 450 ºC and 500 ºC. The X-ray diffraction (XRD) pattern revealed the presence of peaks of 2θ angles between 20° to 90°, corresponding to (104), (110), (214), (125) and (128) planes, which exhibited crystalline structures of rhombohedral with diffraction peaks of hematite phase (α-Fe2O3). Optical characterisations showed that the transmittances of all samples were close to 100% in the high wavelength level of the visible light region, which is close to the infrared spectrum. Absorption of hematite samples was found to be more than 0.6 a.u. in the low wavelength level of the visible light region close to the ultraviolet spectrum and close to 0 in the high wavelength level of the visible light region close to the infrared spectrum. A sample with an annealing temperature of 500 °C has the lowest transmission and the highest absorbance in the visible region due to dim pigments in the hematite film.

Electron diffraction characterization of nanocrystalline materials using a Rietveld-based approach. Part II. Application to microstructural analysis

This is the second in a two-paper series concerning the quantitative characterization of nanocrystalline materials using an electron-diffraction-based approach, in which a full-pattern fitting Rietveld-based refinement is applied to electron powder diffraction data in transmission electron microscopy (TEM). Part I [Sinha et al. (2022). J. Appl. Cryst. 55, 953–965] established a standard calibration protocol to determine the instrumental effects, with special emphasis on the camera length and the diameter of the selected area apertures. Possible application cases are now considered to demonstrate the capabilities of the approach, including the evaluation of the phase composition of TEM specimens, an operation that reveals new application fields for this powerful materials characterization technique. In this regard, different types of material specimen are examined: nanocrystalline yttrium oxide, silicon, titanium dioxide and debris from disc brake wear, each one featuring specific aspects to be tackled with the proposed methodology. To demonstrate the limits of the proposed approach as concerns the material characteristics, an analysis of a hematite sample obtained from the heat treatment of natural goethite, displaying a relatively coarse crystallite size, is performed and a critical discussion of the results is given.

Whether magnetization roasting requires complete phase reconstruction of iron minerals: A study of phase transition and microstructure evolution

Previous magnetization roasting studies usually pursued whole phase reconstruction of iron minerals. This research innovatively proposed a surface magnetization roasting technology requiring only partial phase reconstitution of hematite to decrease carbon emissions. Two hematite samples with narrow particle size distributions (45–74 μm and 74–150 μm) were adopted to better control the magnetite layer thickness. The optimal magnetic separation index was obtained when the relative thickness of the magnetite layer was only approximately 0.3000 and the magnetite content in the roasted product was only about 40%. The remaining hematite still needed a long time to be reduced to magnetite. The hematite on the particle surface was preferentially reduced to magnetite. The surface magnetite can provide magnetic force during the magnetic separation process to recover the roasted particles. The efficient iron recovery was realized through surface magnetization roasting, which only required partial reduction of hematite, consuming less energy and reductant.

Effect of the Precursor and Synthesis Mode on the Properties of Hematite for the Preparation of Promoted Iron Oxide Catalysts

The fine crystal structure of hematite samples used to prepare potassium-promoted iron oxide dehydrogenation catalysts has been studied by X-ray diffraction and scanning electron microscopy. Samples of α-Fe2O3 were obtained under nonequilibrium conditions from several precursors under different thermolysis regimes. The most important characteristic of hematite, which determines the activity and selectivity of the catalyst based on it, is the fine crystal structure (TCS). The TCS of hematite determines the phase composition of the catalyst. The TCS of hematite is formed during the synthesis of hematite and is determined by the nature of the precursor, the temperature of sample synthesis, and the temperature gradient of the rate of removal of gaseous thermolysis products. The highest activity was demonstrated by a catalyst prepared on the basis of hematite with mosaic blocks of 70–90 nm, with a minimum concentration of SF due to half and quaternary dislocations. Such hematite was obtained by thermolysis of iron sulfate at 950 K under fluidized bed conditions and a low temperature gradient. Hematite from iron carbonate is not recommended for the synthesis of a catalyst due to the high concentration of low-temperature SF, which leads to the formation of catalytically inactive potassium β-polyferrite.

Disentangling the size-dependent redox reactivity of iron oxides using thermodynamic relationships

Nanoparticles often exhibit size-dependent redox reactivities, with smaller particles being more reactive in some cases, while less reactive in others. Predicting trends between redox reaction rates and particle sizes is often complicated because a particle's dimensions can simultaneously influence its aggregation state, reactive surface area, and thermodynamic properties. Here, we tested the hypothesis that interfacial redox reaction rates for nanoparticles with different sizes can be described with a single linear free-energy relationship (LFER) if size-dependent reactive surface areas and thermodynamic properties are properly considered. We tested this hypothesis using a well-known interfacial redox reaction: the reduction of nitrobenzene to aniline by iron-oxide-bound Fe2+. We measured the reduction potential (EH) values of nano-particulate hematite suspensions containing aqueous Fe2+ using mediated potentiometry and characterized the size and aggregation states of hematite samples at circumneutral pH values. We used the measured EH values to calculate surface energies and reactive surface areas using thermodynamic relationships. Nitrobenzene reduction rates were lower for smaller particles, despite their larger surface areas, due to their higher surface energies. When differences in surface areas and thermodynamic properties were considered, nitrobenzene reduction kinetics for all particle sizes was described with a LFER. Our results demonstrate that when Fe2+ serves as a reductant, an antagonistic effect exists, with smaller particles having larger reactive surface areas but also more positive reduction potentials. When Fe3+ serves as an oxidant, however, these two effects work in concert, which likely explains past discrepancies regarding how iron oxide particle sizes influence redox reaction rates.

Growth behavior and kinetics of magnetite during magnetization roasting

The magnetization roasting of hematite with two particle sizes was performed to investigate the growth behavior and kinetics of magnetite. The magnetite exhibited comparable growth trends throughout the magnetization roasting process at different roasting temperatures, and enhancing the roasting temperature can facilitate the growth of magnetite. The nascent magnetite nuclei appeared acicular, predominantly developed along the edges and fissures of the hematite particles before progressively growing inward. Moreover, the temperature stress and magnetite formation led to the loose and porous structure of the nascent magnetite particles. The growth kinetics demonstrated that the growth process of magnetite could be divided into induction period and growth period based on the different growth rates. For the same hematite sample, the growth rate of magnetite in the induction period was lower than that in the growth period. The magnetization roasting of hematite with a smaller particle size may be completed in a shorter time, even if increasing the particle size of hematite can accelerate the growth rate of magnetite. The growth kinetic models were established based on the parabolic law, and the reliability of describing the growth process of magnetite was verified by comparing the predicted values with the experimental values.