Rheology and sedimentation velocity of alkaline suspensions of hematite particles at elevated temperature

This study aims to characterize the sedimentation velocity and the rheology of suspensions of hematite particles suspended in strongly alkaline media at 100 and 110 °C, as done for an alternative electrochemical process in development for iron production by direct electrode reduction of hematite. Considering the medium used in the process, i.e. 12% (v/v) suspension of hematite particles in 50% sodium hydroxide aqueous, the sedimentation velocity of hematite particle at 110 °C is 0.010 mm/s, which is very slow because the average size of the solid particles is around 10 μm and the significant collisions and interactions occuring between the particles in the concentrated suspension. Two geometries were used to characterize the rheological behavior of the apparent viscosity of the suspension of 12% (v/v) (i.e. 33 wt%) at 100 °C: a conventional Couette geometry and a helical ribbon mixer. The suspension was found shear thinning in the range of shear rate studied. The rheological behavior of the suspension can be described by a power-law model. The apparent viscosity of the hematite suspension estimated at a shear rate between 0.5 and 10 s⁻¹ is between 100 and 20 mPa s for the two geometries. The apparent viscosity calculated from the terminal velocity of 10 μm particles is of the same order of magnitude of the results obtained with the two rheometer configurations. The effect of the particle concentration on the sedimentation velocity and viscosity of the hematite suspensions was also studied.     

Thermally oxidized iron oxide nanoarchitectures for hydrogen production by solar-induced water splitting

Thermally oxidized iron oxide (α-Fe₂O₃, Hematite) nanostructures are investigated as photoanodes that convert solar energy into hydrogen by splitting water. α-Fe₂O₃ is stable for water photo-oxidation, it has a favorable band gap energy and is a non-toxic common material. However, α-Fe₂O₃ photoanodes suffer from high loss due to electron-hole recombination; therefore nanoarchitectures with high aspect ratio that allows photons to be absorbed close to the photoanode/electrolyte interface are preferred. The thermal oxidation of iron is a simple way to produce nanostructured iron oxide electrodes. Different morphologies, aspect ratios, and oxide thicknesses result depending on the process parameters. Nanorod structures were obtained by annealing iron foils in oxygen rich atmosphere, whereas annealing in oxygen lean atmosphere resulted in nanocoral-like morphology. The nanorod-structured photoanodes achieved moderate photocurrent density of 0.9 mA/cm² while the nanocoral morphology achieved 2.6 mA/cm² (both at 1.8 V vs. the reversible hydrogen electrode). The effect of the oxidation process and oxide layer on performance is discussed.     

Photoactivity of MWCNTs modified α-Fe2O3 photoelectrode towards efficient solar water splitting

MWCNTs (Multiwalled Carbon Nanotubes) modified α-Fe₂O₃ (hematite) photoelectrodes have been investigated for their possible application in hydrogen generation via photoelectrochemical (PEC) splitting of water. Enhanced photoresponse seen in comparison to the pristine α-Fe₂O₃ films is credited to the effective charge facilitation and charge separation provided by MWCNT conducting support. 0.2 wt% MWCNTs modified α-Fe₂O₃ thin film exhibited the maximum photocurrent density of 2.8 mA/cm² at 0.75 V/SCE. Measured values of flat band potential, donor density, resistance, Applied bias photon-to-current efficiency (ABPE) and Incident-photon-to current-conversion efficiency (IPCE) support the observed enhancement in photocurrent.     

Solid solutions of mixed metal Mn3−xMgxFe4(PO4)6 orthophosphates: Colouring performance within a double-firing ceramic glaze

Solid solutions of mixed metal Mn_3−xMg_xFe₄(PO₄)₆ orthophosphates (x = 0, 0.5, 1, 1.5, 2, 2.5, and 3) were prepared for the first time (from coprecipitate powders calcined up to 1000 °C) and characterized by thermal analysis, XRD, SEM/EDX, UV-vis-NIR spectroscopy and colour measurements (CIE-L*a*b*). Orthophosphate (Mn,Mg)_3−yFe_4+z(PO₄)₆ solid solutions isotypic to Fe₇(PO₄)₆ structure (triclinic P-1(2) spatial group) formed successfully as the major crystalline phase within the studied range of compositions, accompanied only by variable quantities of α- and/or β-Mg₂P₂O₇ diphosphates as secondary phases. Noteworthy, the obtained solid solutions were tested as potential ceramic dyes and exhibited an interesting interaction upon enamelling within a double-firing ceramic glaze: considerable amounts of Fe segregated from the solid solutions to be stabilized as hematite particles (α-Fe₂O₃) in the ceramic glaze and conferring the glaze an intense dark-brown colouration, which was almost independent of the amount of Mg doping. Thus, the obtained solid solutions with a minimized Mn content (especially Mg₃Fe₄(PO₄)₆ composition, without Mn) could serve as low-toxicity Fe reservoirs to stabilize hematite in double-firing glazes and produce an interesting dark-brown colouration, being an alternative to other brown ceramic pigments containing hazardous metals (i.e. Cr, Ni, Zn, or Sb).     

磺化氧化石腊皂的合成及其对赤铁矿捕收性能的研究

介绍了新型磺化氧化石腊皂捕收剂的合成方法及用于包钢选矿厂1、3系列正浮选强磁铁精矿中赤铁太等含铁矿物，在40℃和14℃时，粗选精矿品位分别为53．18％和54．32％，回收率分别为90．93％和85．35％。试验表明该新型捕收剂具有较好的选择捕收作用和耐低温性能。     

Physical and photo-electrochemical characterizations of α-Fe2O3. Application for hydrogen production

The optical, electrical and photo-electrochemical properties of dense hematite α-Fe₂O₃ have been studied for the photo-catalytic hydrogen production. The band gap was evaluated at 1.96 eV from the diffuse reflectance spectrum and the transition is directly allowed; further indirect transition occurs at 2.04 eV. The oxygen deficiency permits the altering of the transport properties and the oxide exhibits n type behavior with activation energy of 0.11 eV. α-Fe₂O₃ is found to be photo-electrochemically active. The flat band potential V_fb (−0.51 V_SCE) and the density N_D (19.12 × 10¹⁹ cm⁻³) were obtained respectively by extrapolating the linear part to C⁻² = 0 and the slope of the Mott–Schottky plot. The complex impedance pattern is circular in the high frequency region followed by a straight line in the low frequency one, a behavior attributed to the Warburg ionic diffusion. The conduction band edge (−0.62 V_SCE) lies below the H₂O/H₂ level (−0.50 V_SCE) and Fe₂O₃ offers the possibility to be used as hydrogen photocathode. The best activity was obtained in SO₃²⁻ (0.5 M, pH 13.8) solution with a rate evolution of 6 ml (g catalyst)⁻¹ min⁻¹.     

Pluronic-assisted modifications of FeOOH precursor facilitate morphology adjustment and performance enhancement of Fe2O3 photoanodes

Morphology regulation and surface modification are crucial strategies to improving the photoelectrochemical water oxidation performance of Fe₂O₃ photoanodes. In this study, Pluronic F127-assisted synthesis and post-treatment were adopted to achieve surface modification of FeOOH nanorods prepared by hydrothermal technique, thereby adjusting the morphology and surface properties of Fe₂O₃ photoanodes after calcination. Although the morphology of FeOOH barely changed, the creation of porous nanorods through F127-assisted synthesis and morphological change from worm-like nanorods into nanoplates by F127-assisted post-treatment were realized, and the electrochemically active surface area, crystallinity, number of surface disorders, and photoabsorption property were affected. Furthermore, relatively high intensity of lattice defects and low-valent ferrous ions (Fe²⁺) were generated after F127-assisted synthesis, and charge transfer from the surface states was increased. Consequently, Fe₂O₃ photoanode subjected to F127-assisted synthesis exhibited a reduction in the onset potential by 60 mV. The photocurrent density of Fe₂O₃ increased by 77% at 1.23 V versus reversible hydrogen electrode following a synergistic effect of F127-assisted synthesis and post-treatment.     

Kinetic characterization of flash reduction process of hematite ore fines under hydrogen atmosphere

In order to attain comprehensive utilization of hematite ore fines and reduce carbon dioxide emission, the flash reduction of hematite ore fines under hydrogen atmosphere is studied. The changes of phase composition at 1450 K–1800 K are investigated. A mathematical model is developed to accurately evaluate the reduction kinetic parameters of hematite ore fines based on experiment. The complex flash reduction is accurately described by considering the multiple reaction mechanisms, including hematite thermal decomposition, gas-solid reduction and gas-liquid reduction. The activation energies of the gas-solid reduction and gas-liquid reduction at high temperature are obtained as 223 kJ mol⁻¹ and 180 kJ mol⁻¹, respectively. The developed kinetic model can well describe and predict the flash reduction process of hematite ore fines. The effects of particle size and temperature on the flash reduction process, and the contributions of the thermal decomposition, gas-solid reduction and gas-liquid reduction to the overall reaction process are clarified.     

Investigation of photo-electrochemical response of iron oxide/mixed-phase titanium oxide heterojunction toward possible solar energy conversion

Photocatalysts are part of key strategies to enable green fuel. Photocatalysis and water splitting could be a promising solution to challenges associated with the intermittent nature of sunlight as a huge energy source on Earth. In this study, photo-electrochemical performance and behavior of mixed-phase titanium oxide and iron oxide heterojunction (Ti-TiO_x (High-voltage)-FeO_x electrode) are compared to the photo-electrochemical performance and behavior of titanium oxide nanotubes with the rutile phase and iron oxide heterojunction (TiO_x-nanotubes (H₂SO₄/KF)-FeO_x electrode). The results of photo-electrochemical experiments show that the application of stabilization potential and the presence/absence of dissolved oxygen could not be considered as significant factors affecting the photo-electrochemical properties of the Ti-TiO_x (High-voltage)-FeO_x and TiO_x-nanotubes (H₂SO₄/KF)-FeO_x electrodes. The Ti-TiO_x (High-voltage)-FeO_x electrode shows an anodic photo-electrochemical response in wavelengths shorter than 530 nm and cathodic photo-electrochemical response in wavelengths longer than 530 nm. However, the Ti-nanotubes (H₂SO₄/KF)-FeO_x electrode consistently exhibits the anodic photo-electrochemical response. Both of the prepared heterojunctions are further characterized through Scanning Electron Microscopy, Energy-dispersive X-ray Spectroscopy, Diffuse Reflectance UV–Vis Spectroscopy, X-ray Diffraction, and Attenuated Total Reflectance Spectroscopy methods. These experiments show that despite different morphologies observed in SEM imaging data, the deposited iron oxide layers on both mixed-phase titanium oxide and titanium oxide nanotubes share the same hematite phase structure. However, only iron oxide electro-deposited on the surface of the mixed-phase titanium oxide, which contains both anatase and rutile phases, with vacant sites of oxygen, exhibits un-expected anodic and cathodic photo-electrochemical responses. Furthermore, according to the results of the characterization and photo-electrochemical investigations, the different chemical environment of mixed-phase titanium oxide, and the possible formation of different types of heterojunction structures in mixed-phase titanium oxide and iron oxide, in contrast to the titanium oxide nanotubes and iron oxide, might be considered the possible discernible reasons for the observed different photo-electrochemical responses. This paper sheds new light on photo-electrochemistry of iron oxide/mixed-phase titanium oxide heterojunction for possible solar energy conversion.     

An investigation on room temperature synthesis of vertically oriented arrays of iron oxide nanotubes by anodization of iron

Formation of iron oxide nanotubes on to pure iron substrate by an electrochemical anodization method was investigated in fluoride containing electrolytes. Anodization of iron foil in fluoride containing borate solution resulted in stacked nano-ring type oxide morphology. Nanoporous oxide layer was observed at low pH and a granular oxide layer was formed at higher pH of phosphate + fluoride solutions. Formation of either nanoporous or nanotubular oxide layer was observed in ethylene glycol (EG) solution containing 0.05-0.1 M fluoride + 1.5-3.0 vol.% water. Transition from nanoporous structure to nanotubular structure was critically controlled by anodization potential, water addition and fluoride concentration of the EG solution. The potential required for this transition decreased with increase in the water content up to 7 vol.% beyond which enhanced dissolution occurred. Annealing of the nanotubes at 500 °C resulted in predominantly α-Fe₂O₃ crystal structure. The annealed Fe₂O₃ samples consisting of a single layer of nanotubular structure showed a photo current density of 0.4 mA/cm² at 0.5 V Ag/AgCl in 1 M KOH solution under simulated solar light illumination.