Search Results (15)

The complete dissolution of the Ti and Fe content of ilmenite is a necessary first step for the production of TiO₂ directly from ilmenite. Hydrochloric acid is one of the possible solubilizing agents. However, the ability to dissolve ilmenite in hydrochloric acid depends on the nature of the source material. Here, we consider the effect that the oxidation state of Fe in the ilmenite has on the dissolution rate. Three placer ilmenite fractions from New Zealand and India were dissolved in concentrated hydrochloric acid in a stirred vessel. The dissolution rate constants for Fe and Ti for each fraction based on a shrinking sphere model were compared with the Fe(III)/Fe(II) ratio. Pre-edge Fe K-edge XANES as a measure of Fe(III)/Fe(II) has been shown to extend to ilmenite, which has a complex pre-edge region due to the involvement of Ti electronic levels. It was found that there is a relationship between the oxidation state of Fe and the dissolution rate, with a higher Fe(II) content resulting in more rapid dissolution. A higher Fe(II) content reflects a younger, less weathered material, closer to the “standard” stoichiometry of ilmenite. These data and the presented correlation may support the design of industrial processes to digest ilmenite in hydrochloric acid from varying feedstocks. Full article

In this work, we investigate spacetime and photon dynamics around a charged Hayward black hole, focusing on the effects of electric charge Q and the length factor l. Our analysis shows that the maximum charge for black hole existence decreases as l increases, vanishing at $l/M\simeq 0.77$. The black hole has both inner and outer horizons, with the outer horizon shrinking and the inner horizon expanding as spacetime parameters increase. The spacetime curvature, measured by the Kretschmann scalar, is most pronounced when both parameters are small, resembling the Schwarzschild black hole. The electric charge strongly influences the curvature and photon sphere size, while the effect of the length factor is less significant. Additionally, the gravitational redshift of photons is more sensitive to the electric charge of the compact object than the length factor, diminishing as Q increases and with greater radial distance from the black hole. Overall, while both spacetime parameters affect black hole properties, the electric charge has a slightly stronger impact, especially on gravitational redshift and photon behavior. Full article

This work investigates the dissolution of CaO into three different compositions of SiO₂-CaO-Al₂O₃ slag similar to those found in industrial Si furnaces. It was found that CaO dissolution into the slag is fast. During the dissolution process, a layer containing 35–42% CaO was formed between the CaO particle and the slag, which corresponded to the phases CaO·Al₂O₃·2SiO₂ or 2CaO·Al₂O₃·SiO₂ in this study. Two models were investigated to determine the dissolution rate of the three slags. In the first model, the CaO particle is assumed to be a smooth shrinking sphere, and the rate controlled by the chemical reaction rate. The second model assumes that the rate is controlled by mass transport and depends on the diffusion rate of CaO through a boundary layer on the surface of the CaO. Both models gave similar accuracy to the experimental values, and a proportional relationship between the rate constants and the viscosities was obtained. At 1500 °C, the diffusion coefficients were found to be in the order of 10⁻⁶ cm²/s. Full article

A Computational Fluid Dynamic study was carried out to match the measured outer ash deposition rates associated with the combustion of petroleum coke (PC)–natural gas in AIR and O₂/CO₂ (70/30 vol%, OXY70). The fly ash PSD associated with high-fixed-carbon, non-porous fuel was estimated using a shrinking sphere burnout model and employed in conjunction with particle kinetic energy (PKE), particle viscosity (µ_P), and a critical Weber-number-based capture criterion. Deposition rate predictions were sensitive to the fly ash composition employed for estimating µ_P due to the significant enrichment of Fe in the deposits. Predictions were insensitive to the specific µ_P model formulation employed or whether the V₂O₅ in the ash was assumed to play the role of a glass former or a glass modifier. OXY70 scenario impaction rates were significantly lower than the measured deposition rates when the fly ash PSD associated with the AIR scenario was employed in the calculations. This necessitated an ad hoc modification of the OXY70 fly ash PSD to a coarser range to match the measurements and attributing it to agglomeration resulting from longer residence times and higher temperatures. This shift in PSD was in line with AIR and OXY70 fly ash PSD measurements reported previously. Full article

Planets are surrounded by fractal surfaces (traditionally called Hill spheres), separating the inner zones of long-term stable orbital motion of their satellites from the outer space where the gravitational pull from the Sun takes over. Through this surface, external minor bodies in trajectories loosely co-orbital to a planet can be stochastically captured by the planet without any assistance from external perturbative forces, and can become moons chaotically orbiting the planet for extended periods of time. Using state-of-the-art orbital integrators, we simulate such capture events for Venus, resulting in long-term attachment phases by reversing the forward integration of a moon initially attached to the planet and escaping it after an extended period of time. Chaotic capture of a retrograde moon from a prograde heliocentric orbit appears to be more probable because the Hill sphere is almost four times larger in area for a retrograde orbit than for a prograde orbit. Simulated capture trajectories include cases with attachment phases up to 860,000 years for prograde moons and up to 370,000 years for retrograde moons. Although the probability of a long-term chaotic capture from a single encounter is generally low, the high density of co-orbital bodies in the primordial protoplanetary disk makes this outcome possible, if not probable. The early Venus was surrounded by a dusty gaseous disk of its own, which, coupled with the tidal dissipation of the kinetic energy in the moon and the planet, could shrink the initial orbit and stabilize the captured body within the Hill surface. The tidal torque from the moon, for which we use the historical name Neith, gradually brakes the prograde rotation of Venus, and then reverses it, while the orbit continues to decay. Neith eventually reaches the Roche radius and disintegrates, probably depositing most of its material on Venus’ surface. Our calculations show that surface density values of about 0.06 kg m⁻² for the debris disk may be sufficient to stabilize the initial chaotic orbit of Neith and to bring it down within several radii of Venus, where tidal dissipation becomes more efficient. Full article

The paper reports a facile synthesis of novel anionic spherical polymer brushes which was based on grafting sodium 2-acrylamido-2-methylpropane-1-sulfonate from the surface of 4,4′-Azobis (4-cyanopentanoyl chloride)-modified carbon spheres. Various characterization methods involving a scanning electron microscope, energy dispersive X-ray spectroscopy, Fourier transform infrared spectrum, and thermo-gravimetric analysis were utilized to analyze the morphology, chemical composition, bonding structure, and thermal stability, respectively. The molecular weight (Mw) and polydispersity (Mw/Mn) of brushes were 616,000 g/mol and 1.72 determined by gel permeation chromatography experiments. Moreover, the dispersibility of ASPB in water and in the presence of aqueous NaCl solutions of different concentrations was investigated. Results show that the dispersibility of carbon spheres has been enhanced owing to grafted polyelectrolyte chains, while the zeta potential of the particle decreases and its brush layer shrinks upon exposure to sodium ions (Na⁺). Full article

Limestone with a particle size of less than 5 mm was rapidly calcined in a high-temperature resistance furnace at 1623 K to simulate the conditions of rapid calcination of limestone at ultra-high temperature in a converter. In this study, the decomposition mechanism and calcination characteristics of small-sized limestone at steelmaking temperature were investigated. The study shows that the shrinking sphere or cylinder models with phase boundary reaction were found to be the best representation of limestone kinetic data, and the mechanism function equation is G(α) = 1 − (1 − α)ⁿ, n = 1/2 or 1/3. Limestone particles with a size of 0.18–1.0 mm can be quickly calcined to obtain a typical active lime microstructure and a high activity of more than 350 mL, which is the preferred limestone particle size range in the steelmaking process in which limestone powder is injected into the converter. Full article

This work aims at investigating the kinetic mechanisms of the reduction/oxidation (redox) reactions of iron oxide/iron pellets under different operating conditions. The reaction principle is the basis of a thermochemical hydrogen storage system. To simulate the charging phase, a single pellet consisting of iron oxide (90% Fe₂O₃, 10% stabilising cement) is reduced with different hydrogen (H₂) concentrations at temperatures between 600 and $800 °\mathrm{C}$. The discharge phase is initiated by the oxidation of the previously reduced pellet by water vapour (H₂O) at different concentrations in the same temperature range. In both reactions, nitrogen (N₂) is used as a carrier gas. The redox reactions have been experimentally measured in a thermogravimetric analyser (TGA) at a flow rate of $250 mL/min$. An extensive literature review has been conducted on the existing reactions’ kinetic mechanisms along with their applicability to describe the obtained results. It turned out that the measured kinetic results can be excellently described with the so-called shrinking core model. Using the geometrical contracting sphere reaction mechanism model, the concentration- and temperature-dependent reduction and oxidation rates can be reproduced with a maximum deviation of less than 5%. In contrast to the reduction process, the temperature has a smaller effect on the oxidation reaction kinetics, which is attributed to 71% less activation energy ($E_{\mathrm{a},\mathrm{Re}}=56.9 \mathrm{kJ/mol}$ versus $E_{\mathrm{a},\mathrm{Ox}}=16.0 \mathrm{kJ/mol}$). The concentration of the reacting gas showed, however, an opposite trend: namely, to have an almost twofold impact on the oxidation reaction rate constant compared to the reduction rate constant. Full article

Knowledge on the effects of minerals on soil water stability and wettability is mostly gained from experiments on natural soils of different mineral composition. To gain a “clearer” picture, the water stability and wettability of artificial aggregates composed of soil silt and various proportions of pure minerals: kaolinite, montmorillonite, illite, zeolite and goethite, were examined. The wettability was attributed to contact angles measured goniometrically and to the water drop penetration time (WDPT). The water stability was measured by monitoring of air bubbling after aggregate immersion in water and the shrinking sphere model was used to analyse aggregates’ destruction kinetics. The rate of aggregate destruction in water increased with increasing mineral content and it slightly decreased for aggregates composed of all pure minerals except goethite. An apparent hydrophobicity period (a period where the bubbling stopped for some time), resulted most probably from the wavy shape of pores, was observed mainly for aggregates with low mineral proportions. Among all studied minerals, kaolinite increased the water contact angle and water repellency to the greatest extent. With increasing the mineral content in the aggregates up to 8%, contact angles decreased and then increased. Contact angles did not correlate with aggregates’ stability. Aggregates more rapidly penetrated by water (shorter WDPT) were destroyed faster. Water stability of aggregates containing all minerals except illite appeared to be higher for the more mechanically resistant aggregates. Full article

The most promising source of alumina in the 21st century is the coal fly ash (CFA) waste of coal-fired thermal plants. The methods of alumina extraction from CFA are often based on the pressure alkaline or acid leaching or preliminary roasting with different additives followed by water leaching. The efficiency of the alumina extraction from CFA under atmospheric pressure leaching is low due to the high content of acid-insoluble alumina phase mullite (3Al₂O₃·2SiO₂). This research for the first time shows the possibility of mullite leaching under atmospheric pressure after preliminary desilication using high liquid to solid ratios (L:S ratio) and Na₂O concentration. The analysis of the desilicated CFA (DCFA) chemical and phase composition before and after leaching has been carried out by inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD). The morphology and elemental composition of solid product particles has been carried out by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). An automated neural network and a shrinking core model (SCM) were used to evaluate experimental data. The Al extraction efficiency from DCFA has been more than 84% at T = 120 °C, leaching time 60 min, the L/S ratio > 20, and concentration of Na₂O—400 g L⁻¹. The kinetics analysis by SCM has shown that the surface chemical reaction controls the leaching process rate at T < 110 °C, and, at T > 110 °C after 15 min of leaching, the process is limited by diffusion through the product layer, which can be represented by titanium compounds. According to the SEM-EDX analysis of the solid residue, the magnetite spheres and mullite acicular particles were the main phases that remained after NaOH leaching. The spheric agglomerates of mullite particles with non-porous surface have also been found. Full article

The current analysis aims to exhibit the nanoparticles of Al₂O₃ + Cu-water hybrid nanofluid flow for Darcy–Forchheimer with heterogeneous–homogeneous chemical reactions and magnetic field aspects past a stretching or shrinking cylinder with Joule heating. This paper performed not only with the hybrid nanofluid but also the shape of Al₂O₃ and Cu nanoparticles. The model of single-phase hybrid nanofluid due to thermophysical features is utilized for the mathematical formulation. In the present exploration equal diffusions factors for reactants and auto catalyst are instituted. The system of governing equations has been simplified by invoking the similarity transformation. The numerical computations are invoked due to the function bvp4c of Matlab, with high non-linearity. Numerical outcomes illustrated that; sphere shape nanoparticles presented dramatic performance on heat transfer of hybrid nanofluid movement; an opposite behavior is noticed with lamina shape. The local Nusselt number strengthens as the transverse curvature factor becomes larger. In addition, the homogeneous–heterogeneous reactions factors lead to weaken concentration fluctuation. Full article

Due to functional impairment and low mobility, the sphere of activities of older adults often shrinks and they rely on their living environment more. Especially for urban community-dwelling older adults who are aging in place, the urban neighborhood environment affects their quality of life (QoL) heavily. This study aims to explore how the urban neighborhood environment affects QoL of community-dwelling older adults and develop a mediation model called “Neighborhood Environment-Quality of Life (NE-QoL)” for community-dwelling older adults. The reliability test is applied to test and modify the questionnaire based on cross-sectional data collected from the survey, the multiple regression analysis is used to identify significant influence relations between variables of neighborhood environment and dimensions of the QoL, mediation effects are assumed and tested by the mediation analysis in SPSS, and then the “NE-QoL” is developed to reveal the detailed influence path between the urban neighborhood environment and QoL of community-dwelling older adults. The “NE-QoL” model reveals seven variables of the urban neighborhood environment, which influences the QoL of community-dwelling older adults significantly, and three mediation effects exist in the influence path, making clear the understanding about the relationship between neighborhood environment and the QoL of community-dwelling older adults. It provides valuable retrofit guidelines of the neighborhood environment for improving QoL of community-dwelling older adults. Full article

Most inorganic material surfaces exposed to ambient air can adsorb water, and hydrogen bonding interactions among adsorbed water molecules vary depending on, not only intrinsic properties of material surfaces, but also extrinsic working conditions. When dimensions of solid objects shrink to micro- and nano-scales, the ratio of surface area to volume increases greatly and the contribution of water condensation on interfacial forces, such as adhesion (F_a) and friction (F_t), becomes significant. This paper reviews the structural evolution of the adsorbed water layer on solid surfaces and its effect on F_a and F_t at nanoasperity contact for sphere-on-flat geometry. The details of the underlying mechanisms governing water adsorption behaviors vary depending on the atomic structure of the substrate, surface hydrophilicity and atmospheric conditions. The solid surfaces reviewed in this paper include metal/metallic oxides, silicon/silicon oxides, fluorides, and two-dimensional materials. The mechanism by which water condensation influences F_a is discussed based on the competition among capillary force, van der Waals force and the rupture force of solid-like water bridge. The condensed meniscus and the molecular configuration of the water bridge are influenced by surface roughness, surface hydrophilicity, temperature, sliding velocity, which in turn affect the kinetics of water condensation and interfacial F_t. Taking the effects of the thickness and structure of adsorbed water into account is important to obtain a full understanding of the interfacial forces at nanoasperity contact under ambient conditions. Full article

Over the past three decades, there has been a steady increase of funds by the international community to support civil society organizations (CSOs) in fragile states. Surprisingly, this growing attention has not strengthened local civil society landscapes in a way that it would lead to processes of social transformation. On the contrary, civic freedom and space is shrinking around the globe. In analyzing prominent international aid-effectiveness frameworks and donor strategies towards civil society, this paper will put forward one central argument. The way in which civil society actors and functions are currently appropriated threatens deep-rooted social transformation thereby impeding processes of structural and political change—necessary for the transition from conflict to sustainable peace. In delineating, how actors and functional approaches informed peacebuilding and development policy and practice, their strengths and limitations will be examined. Doing so, we draw on different case studies and examples from the literature. We find that existing frameworks for fragile states operate on a presumed model of a public sphere and civil society that may or may not exist. Such an approach disregards an organic formation of a civil society landscape thereby impeding processes of structural, social, and political change in times of fragility. Full article

A three-dimensional inverse opal (3DIO) WO₃ architecture has been synthesized via a simple sacrificial template method. Morphology features of the 3DIO were characterized by scanning electron microscope (SEM) and its structure was characterized by X-ray diffraction (XRD). The shrinking ratio of the PMMA spheres was ~28.2% through measuring the distribution of the PMMA spheres and 3DIO WO₃ center-to-center distance between the spheres and macropores, respectively. Beyond that, the 3DIO gas sensing properties were investigated systematically and the sensing mechanism of 3DIO WO₃ was proposed. The results indicated that the response of the 3DIO sensor possessed excellent sensitivity to acetone gas, especially at trace levels. The 3DIO gas sensor response was ~7 to 5 ppm of acetone and could detect acetone low to 0.2 ppm effectively, which was in close proximity to the theoretical low detection limit of 0.14 ppm when R_a/R_g ≥ 1.2 was used as the criterion for reliable gas sensing. All in all, the obvious satisfaction of the gas-sensing properties was ascribed to the structure of the 3DIO, and the sensor could be a promising novel device in the future. Full article