Alumina Phase Research Articles

Multi-stage Electrostatic Separation for Recovering of Aluminum from Fine Granules of Black Dross

Separation of aluminum from fine granules of black dross, which is a waste by-product in secondary aluminum production, was investigated. The separation was performed by a multi-stage electrostatic separation method. There are three stages to complete the separation, including preliminary separation, pulse charging enhancement and secondary concentration. Chemical and mineralogical compositions of collection products were analyzed and determined by X-ray diffraction (XRD) and X-ray Fluorescence (XRF). After multistage electrostatic separation, the Al2O3 content of the collection products increases from 50.74% to 69.77%. The mineralogical phase analysis indicates that the final recovery of metallic aluminum phase increases from 8% to 37%, and the aluminum oxide phase increases from 20% to 26%. The research results show the multi-stage electrostatic separation method is effective for recovering of aluminum from fine granules of black dross, and upgrades the black dross to a recoverable material.

Examination of ye’elimite formation mechanisms

Ye’elimite is the main constituent of calcium sulfoaluminate (CSA) cement and one of the major constituents of belite sulfoaluminate or belite sulfoaluminate ferrite cements. The main objective of this work is to describe precisely the formation mechanisms of ye’elimite by solid-state reaction. Mineralogical composition development was monitored using XRD analysis, while microstructural monitoring was conducted using BSE-SEM coupled to EDS analysis. The results show that CaAl2O4 and CaAl2O7 are the main intermediate products during ye’elimite formation. At the microstructural scale, ye’elimite forms on calcium aluminate phases. Finally, Avrami’s model was suggested to discuss the ye’elimite formation rate according to sintering temperature and duration.

Root-driven weathering impacts on mineral-organic associations in deep soils over pedogenic time scales

Abstract Plant roots are critical weathering agents in deep soils, yet the impact of resulting mineral transformations on the vast deep soil carbon (C) reservoir are largely unknown. Root-driven weathering of primary minerals may cause the formation of reactive secondary minerals, which protect mineral-organic associations (MOAs) for centuries or millennia. Conversely, root-driven weathering may also transform secondary minerals, potentially enhancing the bioavailability of C previously protected in MOAs. Here we examined the impact of root-driven weathering on MOAs and their capacity to store C over pedogenic time scales. To accomplish this, we examined deep horizons (100–160 cm) that experienced root-driven weathering in four soils of increasing ages (65–226 kyr) of the Santa Cruz marine terrace chronosequence. Specifically, we compared discrete rhizosphere zones subject to root-driven weathering, with adjacent zones that experienced no root growth. Using a combination of radiocarbon, mass spectrometry, 57Fe Mossbauer spectroscopy, high-resolution mass spectrometry, and X-ray spectromicroscopy approaches, we characterized transformations of MOAs in relation to changes in C content, Δ14C values, and chemistry across the chronosequence. We found that the onset of root-driven weathering (65–90 kyr) increased the amount of C associated with poorly crystalline iron (Fe) and aluminum (Al) phases, particularly highly disordered nano-particulate goethite (np-goethite). This increase coincided with greater C concentrations, lower Δ14C values, and greater abundance of what is likely microbially-derived C. Continued root-driven weathering (137–226 kyr) did not significantly change the amount of C associated with crystalline Fe and Al phases, but resulted in a decline in the amount of C associated with poorly crystalline Fe and Al phases. This decline coincided with a decrease in C concentrations, an increase in Δ14C values, and a shift toward plant-derived C. In contrast, soil not affected by root-driven weathering showed comparatively low amounts of C bound to poorly crystalline Fe and Al phases regardless of soil age and, correspondingly, lower C concentrations. Our results demonstrate that root-driven formation and disruption of MOAs are direct controls on both C accrual and loss in deep soil. This finding suggests that root impacts on soil C storage are dependent on soil weathering stage, a consideration that is critical for future predictions of the vulnerability of deep soil C to global change.

Phase assemblage of cement pastes with SCM at different ages

For economical and ecological reasons, supplementary cementitious materials (SCM) are increasingly used in concrete. Although having a lot of advantages, concretes with SCM have different cement matrices and do not have the same evolution as a function of time as CEM I concretes. In this paper, the microstructure of various cement pastes with/without SCM (slag GGBS, fly ash FA and metakaolin MK), has been investigated as a function of the curing time.The microstructure was characterized not only by usual techniques such as XRD and TGA-DTA, but also by 29Si and 27Al NMR spectroscopy. The use of a combination of techniques for microstructural characterization allowed to quantify the proportion of each cementitious phase. This quantification showed the evolution of the cementitious matrices as a function of the binder content and of the curing time. For example, the alumina content in the C-S-H and the average length of C-S-H chains are higher for cement pastes with SCM than for OPC ones. Hydrated alumina phases proportions (AFt, AFm and TAH) are also higher in SCM cement paste. For MK and FA cement pastes, in addition to pozzolanic reactions, it seems that a small part of calcium from clinker hydration is used to form these aluminate phases.

An experimental investigation on implications of traverse speed in joining of dissimilar Al–Cu by friction stir welding

Al–Cu joints are used in the electrical industry because of its unique mechanical, thermal and corrosion resistance properties. The present investigation concentrates on understanding the influence of traverse speed on friction stir welded butt joints of 3-mm-thick AA 6061-T6 and pure copper. The welds were produced by varying the traverse speed (30 mm/min, 90 mm/min and 150 mm/min) at a constant rotational speed of 800 rpm. Changes in microstructure and mechanical properties with the change in forces generated during the welding were observed and analysed. Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses revealed the formation of AlCu, Al2Cu and Al4Cu9 phases at low and moderate speeds (30 mm/min and 90 mm/min), whereas at higher traverse speed (150 mm/min), iron aluminide phases (Fe2Al5 and FeAl3) formed along with Al–Cu phases. These iron aluminium phases act as initial promoters for fracture. The hardness values for all welding parameters were mapped which show hardness variation due to the inhomogeneous distribution of precipitates within the NZ. This is due to different thermal cycles for aluminium and copper as the tool passes the joint line. The tensile strength was measured and found to be maximum at a traverse speed 90 mm/min.

Anomalous size effect in thermal residual stresses in pressure sintered alumina-chromium composites

This paper explores an anomalous size effect in thermal residual stresses occurring in the alumina matrix of Al2O3/Cr sintered composite upon varying the particle size of the chromium reinforcement. When a coarse chromium powder (45 μm mean particle size) is used the average residual stress in the alumina phase after cooling is compressive in accordance with the classical Eshelby solution. However, in the case of a fine chromium (5 μm mean particle size) it switches to tension. This effect, detected by photoluminescence piezospectroscopy, is also confirmed by X-ray and neutron diffraction experiments. As the classical micromechanics models are incapable to capture it, a finite element model is developed with the actual composite microstructure being reconstructed from the microtomography images. It is shown by numerical simulations that the anomalous size effect is associated with the complex microstructure of the composite fabricated with the fine chromium powder. It is also pointed out that the temperature dependence of the coefficients of thermal expansion of the matrix and the reinforcement affects the residual stress levels.

The effect of milling time on the structure properties and morphology of aluminium powder from construction industry waste milled using high energy milling

The effect of milling time on the structure and morphology of aluminum frame powder has been investigated. In this study, aluminum was made in aluminum powder, as a raw material for making bipolar plates in the Proton Exchange Membrane Fuel Cell (PEMFC) technology. Aluminum powder is synthesized using high-energy milling (HEM) with variations in milling time for 10, 20, 30, 40, and 50 minutes. Aluminum powder that has been milled, is characterized using X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM). From the results of XRD characterization, an aluminum frame that is milled for 10-20 minutes has an aluminum phosphide phase. Whereas for an aluminum frame that is milled for 30, 40 and 50 minutes has an aluminum phase Rudashevskyite. From the results of XRD also obtained, as aluminum frame has a cubic crystal structure with lattice parameters α = β = γ = 900, and aluminum powder rang each has a crystal size of 258.31, 865, 218.2, 560, and 255.9 Ǻ for 10 milling times, 20, 30, 40, and 50 minutes, so it can be concluded that the size of the aluminum frame powder crystal fluctuates. While from the results of SEM, aluminum nanoparticle powder obtained from the nanosheet structure, where the gray one is aluminum particles and the black one shows the presence of other elements. The composition of order 94% aluminum nanoparticles is pure aluminum and 6% other elements, this is reinforced from the SEM image which looks more dominant gray.

Formation of a Double-Layer Ultrafine Crystal Structure for High-Current Pulsed Electron Beam-Treated Al–20Si–5Mg Alloy

In this paper, the effect of high-current pulsed electron beam (HCPEB) on the microstructure refinement of an Al–20Si–5Mg alloy in the cross-section modified zone was studied, and a double-layer ultrafine crystal structure of the Al–20Si–5Mg alloy was formed. It was found that the cross-section modified zone was divided into three zones, namely, the remelted layer, the heat-affected zone, and the thermal stress wave-affected zone after HCPEB treatment. For the remelted layer, metastable structures were formed due to the rapid heating and cooling rates. For the heat-affected zone, the grain of the aluminum phase was refined due to the cooperative effects of shock wave (formed during an eruption event of the brittle phase), thermal-stress wave (formed during thermal expansion of the alloy surface), and quasi-static thermal stress (formed as a result of an unevenly distributed temperature gradient in the inner material) at high temperatures. For the thermal stress wave-affected zone, the grain refinement was not obvious due to the decreasing energy of the shock wave and the thermal-stress wave at low temperatures. In addition, firm evidence for the tracing of shock waves in the heat-affected zone was demonstrated for the first time and verified for the founding of the broken acicular eutectic silicon. Through this experiment, the mechanical properties of Al–20Si–5Mg alloy materials in both the remelted layer and heat-affected zone were significantly improved after HCPEB treatment.

Expansion of CEM I and slag-blended cement mortars exposed to combined chloride-sulphate environments

This study investigates the effects of specimen curing duration, temperature, and slag composition on expansion of CEM I and composite slag-cement mortars exposed to a combined NaCl and Na2SO4 solution for up to 664 days. Test prisms prepared at 0.5 w/b ratio, were wet-cured for either 7 or 28 days prior to submersion in a combined salt solution at temperatures of 20 or 38 °C, to simulate temperate or warm tropical climates respectively. Equivalent reference specimens were stored in saturated limewater at 20 °C and tested in parallel. Mortar samples were used to investigate expansion and sorptivity, while corresponding paste specimens were prepared, cured and exposed under similar conditions for chemical and microstructural investigation. Such characterisation was performed on specimens immediately prior to exposure to salt solution and after the onset of expansion. The results show significant resistance to sulphate-induced expansion for specimens cured and exposed at 38 °C. For slag blends, the influence of exposure temperature was found to be more pronounced than curing duration. Differences in slag composition and curing duration also played key roles on the expansion resistance of mortar specimens. Expansion was attributed to the formation of ettringite crystals due to the reaction of aluminate phases of the binders with sulphate ions, although Friedel's salt and Kuzel's salt were also formed. The presence of chloride mitigated sulphate expansion of CEM I. For slag blends, it was shown that sulphate expansion was significantly reduced with increasing slag contents.

A Novel Fabrication of Spherical Fe50Ni50 Alloy Powders via in-Situ De-Wetting of Liquid Solid Interface

Spherical Fe50Ni50 alloy powders were fabricated via a novel route based on in-situ interface de-wetting between liquid Fe-Ni alloy and alumina. The obtained Fe50Ni50 alloy particles exhibit very good spherical shape according to SEM images. Furthermore, the cross-sectional SEM images show that there are no pores and bulk inclusions in the internal region of the spherical particles. The XRD results show a trace amount of the impurity alumina phase appearing in taenite phase. The size distribution agreed well with the SEM observation confirms that the alumina powders successfully segregated pre-alloy powders. As an incidental benefit, the surface alumina particles were treated as the electrical insulation coatings. The magnetic character shows that spherical Fe50Ni50 powders exhibit a good soft magnetic property even though with a slightly decreasing of saturation magnetization due to non-magnetic coatings. Our strategies provide a method to in-situ fabricate insulation coated Fe-Ni spherical alloy powders as magnetic powder core.

Three-dimensional structure of high-performance heat insulator produced with micro and nano particle alumina

The internal structure of heat insulator materials, which were built by mixing micrometer-size α-alumina with nanometer-size fumed alumina though changing their mixture ratios, was inspected using transmission electron microscopy (TEM) tomography and synchrotron radiation nano-tomography. The heat insulator samples formed by two kinds of alumina phase have high porosity that consist of hierarchical structures of nanometer and micrometer size. From TEM observation, it is found that the porosity was 90.7% in the aggregate of fumed alumina with an average particle diameter of 35 nm whilst the average pore size was 70–80 nm. From observation of synchrotron radiation nano-tomography, it is found that α-alumina particles distribute homogeneously in samples that have fumed alumina phase of more than 50 mass%. The effect of internal microstructure on heat insulator properties were considered by performing a simple heat transfer simulation using image based models prepared from synchrotron radiation nano-tomography images. Similar changes in heat transfer with the experiment were obtained by the simulation. Therefore, it is suggested that the change of heat transfer depending on a mixture ratio of α-alumina and fumed alumina is mainly caused by the internal microstructure, that is, the distribution and connectivity of α-alumina phase within fumed alumina phase.

Comparative roles between aragonite and calcite calcium carbonate whiskers in the hydration and strength of cement paste

Aragonite CaCO3 whisker (CW) could improve the mechanical properties of cementitious composites. The microstructure of calcite is different from that of aragonite, but only a few studies reported different effects in cementitious composites between incorporating aragonite and calcite. In order to fully use CW as high-performance, low-cost microfiber and clearly understand different mechanisms between aragonite and calcite CW, in this study, the heat treatment method was used to convert aragonite CW to calcite CW, and the effect of calcite and aragonite on the hydration and strength of CW reinforced cement paste (CWRC) was studied to bridge the gaps in this area. The aragonite and calcite CWs exhibited the overall physical dilution effect in Portland cement, significantly decreasing the hydration heat of cement and the total amount of non-evaporable water and Ca(OH)2. Calcite CW presents a more significant nucleation effect than aragonite CW. Compared to aragonite CWRC, the hydration of silicon phase in calcite CWRC is faster and the hydration heat release is higher; however, there is no significant difference in the hydration of aluminum phase. The Ca(OH)2 content in calcite CWRC was more than that in aragonite CWRC. More hydration products formed on the surface of calcite CW than that on aragonite CW, improving the bonding strength between calcite CW and cement matrix. Hence, both the compressive strength and flexural strength of calcite CWRC are higher than those of aragonite CWRC. The rheological test of fresh aragonite and calcite CWRC proved that both aragonite and calcite CWs significantly decreased the flow of cement paste, presenting viscosity effect. Compared to aragonite CW, calcite CW improved the flowability of fresh cement paste, indicating the lubricant effect of calcite CW. Aragonite and calcite CW present chemical effect similar to limestone powders, i.e., aluminum phase in Portland cement interacted with CO32- dissolved from CW to form mono- and semi- carbonate. The Ca(OH)2 orientation indexes of cement decreased by the introduction of CWs. Aragonite and calcite CWs can also be used as microfiber and filler in cementitious composites. These effects improved the micro-structure, thus enhanced the flexural and compressive strength of cement paste.

On the in situ synthesis of (Fe,Cr)Al and (Fe,Cr)Al–Al2O3 nanostructured materials

In the present work, FeAl, (Fe,Cr)Al and (Fe,Cr)Al–Al2O3 intermetallic compounds were synthesized by mechanical alloying (MA), in which the intermetallic compounds were formed in one step at room temperature, and the nanocrystalline structure led to significant improvements of strength and ductility. Specifically, MA of a Fe50Al50 powder mixture led to the formation of the FeAl compound in 30 h with a grain size of about 80 nm. Increasing milling time to 100 h decreased the grain size to about 50 nm. In the next step, a Fe40Cr10Al50 power mixture was converted into a solid solution and then an intermetallic compound with a grain size of about 30 nm after 100 h of MA. Finally, the synthesis of nanocomposites with (Fe,Cr)Al intermetallic compound containing Al2O3 as the reinforcing phase was investigated. For this purpose, two different methods, the reduction of chromium oxide (Cr2O3) and hematite (Fe2O3), were examined. In the first method, a mixture of Fe–Al–Cr2O3 powder aluminum was milled and alumina phase was formed, but after 100 h of MA, the Cr2O3 phase was still observed. In the second method, a Cr–Al–Fe2O3 mixture was milled, and the heat of reaction immediately resulted in the formation of (Fe,Cr)Al nanoparticles with a grain size of about 29 nm, along with Al2O3 crystalline particles. The results of nanoindentation tests on these particles revealed that their hardness increases by chromium addition as well as Al2O3 reinforcing nanoparticles.

Early stage in situ detection of polynuclear aluminum phases in aqueous solution

Polynuclear cationic aluminum hydroxide phases are known intermediates in the formation of aluminum oxides or (oxide)hydroxides upon hydrolysis of aluminum salt solutions. In the presence of sulfate anions, these aluminum polyoxocations (Al13) can form crystalline Al13 sulfates with varying chemical composition. The formation of these Al13 sulfates in aqueous solution has been poorly understood. Here, we investigate the early stage crystallization of Al13 clusters in a sulfate-containing solution, in situ and in real time. Dynamics associated with Al13 sulfate formation have been obtained for the first time, using synchrotron X-ray diffraction (XRD) of solutions suspended by acoustic levitation. Time-resolved in situ data show that the cubic phase, Na[(AlO4)Al12(OH)24(H2O)12](SO4)4·10H2O, forms after only minutes. The formation mechanism of Al13 sulfates was found to depend on the sulfate:aluminum (SO4:Al) ratio. Ex situ XRD of the product Al13 sulfates in solution shows that for SO4:Al ratio ≤1.5 two other crystalline phases form, and convert to the cubic phase upon washing and drying. In situ XRD for the same ratio shows transient formation of an intermediate during the crystallization process.

Synthesis and characterization of alumina fibers using solution blow spinning

Abstract This work shows the successful production of alumina nanofibers after thermal treatment of solution blow spun hybrid fibers. These nanofibers were converted into γ-Al2O3 and α-Al2O3 after the thermal treatment in air between 500 to 1200 °C. The X-ray diffraction patterns presented all the characteristics of the γ and α phases of alumina. In addition, the scanning electron micrographs showed alumina nanofiber diameters varying between 200 and 270 nm for different temperatures. The results demonstrated that the solution blowing spinning method is efficient to produce alumina nanofibers.

Physical, mechanical properties and antimicrobial analysis of a novel CaO·Al2O3 compound reinforced with Al or Ag particles

Ceramic-metal (CaO·Al2O3–Al and CaO·Al2O3–Ag) compounds were prepared by mechanical milling and consolidated through an in-situ sintering process. The aim of this work is to study the effects of the Al and Ag particles to ceramic-base compound, primarily in the microstructure, and its mechanical and antimicrobial properties. Chemical systems with a 1:1 M ratio between CaCO3 and Al2O3 powder were formed, with the addition of 10 wt% Al or 10 wt% Ag, respectively. The compound material that consolidated were microstructurally characterized through X-ray diffraction, scanning electron microscopy, optic microscopy, and X-ray computed tomography. In addition, the hardness, the fracture toughness, the transversal elastic modulus, and the antimicrobial property were evaluated. The results of X-ray diffraction identified the formation of the calcium aluminate phases, such as CaO·6Al2O3 (hibonite:CA6), CaO·2Al2O3 (grossite:CA2), and CaO·Al2O3 (krotite:CA); as well as Al and Ag were identified in its respective system. In addition, the mechanical properties show changes compared to the reference material that was synthesized under the same conditions and, finally, these materials also have an antimicrobial effect, against the Staphylococcus bacterium that is common in the oral cavity, when studied in synthetic saliva.

Heterophase interface-mediated formation of nanotwins and 9R phase in aluminum: Underlying mechanisms and strengthening effect

Nanostructured crystalline Al/amorphous AlN multilayer films with a wide layer thickness (h) range from ∼10 nm up to ∼200 nm were prepared by using magnetron sputtering. Nanotwins and 9R phase were substantially observed in the Al layers, showing a strong thickness dependence. The 9R phase predominantly penetrated through the Al layer in the II regime of h ≤ ∼20 nm, while mainly terminated within the layer interior in the I regime of h > ∼20 nm. On the contrary, the coherent nanotwins were boosted when h > ∼20 nm and the percentage of twinned Al grains was greatly increased. The formation mechanisms of 9R phase and coherent nanotwins were discussed in terms of the interfacial chemistry/physics modulated by the amorphous AlN layers, which displayed gradient characteristics and hence was sensitive to the layer thickness. A significant thickness dependence of hardness was also evident that the hardness monotonically increased with reducing h in the I regime, while reached a peak value and hold almost unchanged in the II regime. The hardness in the II regime is about 1 GPa greater than the predictions from an interfacial barrier crossing model. This discrepancy is mainly contributed by the layer-penetrating 9R phase rather than the nanotwins. This study provides a new perspective on fabricating nanotwinned Al by utilizing heterophase interfaces.

Removal of radioactive cesium and europium from aqueous solutions using activated Al2O3 prepared by solution combustion

Surface-protected activated aluminium oxide was synthesized using solution combustion technique and characterized, as inorganic sorbent material, for the removal of 152+154Eu and 134Cs radionuclides from wastewater. The synthesis aluminium oxide showed construction of protective organic species at the aluminium oxide surface. The structural and thermal analysis investigations of the prepared material revealed presence of oxygen in interstitial position with a stable amorphous phase of aluminium oxide. Different experiments were conducted to investigate the sorption ability of the material to radioactive europium and cesium ions including the effect of pH, initial concentration of radionuclides and temperature. Various kinetic models have been applied and also different isotherm models were studied. The results show that the pseudo-second-order model was applicable for the adsorption process and the monolayer capacity equal 133.16 and 109.78 mg/g for 152+154Eu and 134Cs, respectively. Activated amorphous alumina was found to be cost effective and of high uptake capacity for cesium and europium from aqueous solutions.

Effects of ground wollastonite on cement hydration kinetics and strength development

Wollastonite is a naturally occurring calcium silicate mineral that can be used to improve the chemomechanical performances of cementitious materials. This article presents the influences of ground wollastonite on Portland cement hydration kinetics, phase formation and compressive strength. Ground wollastonite with 3.5 μm and 9.0 μm mean particle sizes were used in this study to replace 10%–50% by weight of cement in the paste and mortar mixtures. The effect of ground wollastonite was compared with that of ground limestone. The performance of cementitious mixture containing wollastonite and limestone were monitored using isothermal calorimeter, thermogravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscope (SEM). Ground wollastonite was observed to accelerate the cement hydration due to the filler effect. The hydration acceleration effect of wollastonite was similar to that of limestone. Ground wollastonite accelerated both C3S hydration and the reaction of aluminate phases. Wollastonite with the mean particle size of 9.0 μm remained chemically inert up to 28 days of curing as observed from XRD and TGA results. Whereas wollastonite with the mean particle size of 3.5 μm consumed some Ca(OH)2 due to a slow pozzolanic reaction. Combining all of these effects, using wollastonite-3.5 μm to replace 30% (by weight) of cement in a mortar mixture reduced the compressive strengths after 7 days and 28 days curing only by 3% and 10%, respectively.

Application of microwave irradiation for fabrication of sulfated ZrO2\u2013Al2O3 nanocomposite via combustion method for esterification reaction: process condition evaluation

The zirconia–alumina nanocatalyst was prepared using microwave combustion method, and the effects of alumina/zirconia ratio, fuel-to-oxidizer ratio, and calcination temperature on its structural and chemical performance were examined. Furthermore, the most suitable synthesized nanocatalyst was impregnated using different dosages of sulfate group to improve the activity of the sample for the esterification reaction. The results revealed that there was a significant improvement in the crystalline structure of alumina and zirconia for the samples having the alumina/zirconia ratio of 2. It was also noted that some amorphous structures were observed for low fuel dosages, while the activity of the sample was reduced with higher fuel consumption than the stoichiometric ratio, which can be attributed to the increase in the crystallite size and formation of different alumina phases. Based on calcination results, non-considerable changes were observed in the morphology and activity of the catalyst. It can be concluded that the solution combustion method is suitable for nanocatalyst preparation without any requirement of further thermal treatment. In addition, the activity of the sample was enhanced by impregnation with H2SO4 where the conversion of 91.6% was achieved. The catalyst showed high reusability at least for seven times as well as high stability of zirconia–alumina as a support.Graphical abstract