Aluminosilicate Glass Phase Research Articles

Rare earth element recovery and aluminum-rich residue production from high alumina fly ash by alkali pre-desilication enhance the mechanochemical extraction process

Recycling rare earth elements (REEs) from coal fly ash by conventional acid leaching may generate large amounts of harmful waste acid, and the obtained acid residues have low utilization value. Accordingly, a green non-acid leaching approach for REE recovery and Al-rich residue production from high alumina fly ash (HAFA) was developed based on the alkali pre-desilication enhanced mechanochemical extraction process. REEs mainly existed in the chemically stable amorphous aluminosilicate glass phase of HAFA, which hindered the leaching of REEs. Thus, NaOH solution (150 g/L) was used to dissolve the glass phase and release REEs from the glass phase under a mild condition (80 °C, 2 h). The released REEs could be easily extracted by green EDTA during the mechanochemical extraction process. The extraction rates of La, Ce, Pr and Nd were about 82.03–88.71%, and valuable Al-rich residue (68.23% Al2O3 content and 2.51 Al/Si ratio) can be obtained when the mass ratio of the pre-desilication residue to EDTA, milling time, rotary speed, and liquid-solid ratio were 2:3, 2 h, 500 rpm, and 0.8, respectively. The used EDTA and REEs in the extracted solutions can be recycled by pH adjusting and organic solvent extraction. Thus, this study provides a new idea for green and high-value utilization of HAFA.

Effect of SiC whiskers on the anti‐oxidation properties of Yb2Si2O7/mullite/SiC tri‐layer environmental barrier coating for CMCs

AbstractThe mullite and ytterbium disilicate (β‐Yb2Si2O7) powders as starting materials for the Yb2Si2O7/mullite/SiC tri‐layer coating are synthesized by a sol–gel method. The effect of SiC whiskers on the anti‐oxidation properties of Yb2Si2O7/mullite/SiC tri‐layer coating for C/SiC composites in the air environment is deeply studied. Results show that the formation temperature and complete transition temperature of mullite were 800–1000 and 1300°C, respectively. Yb2SiO5, α‐Yb2Si2O7, and β‐Yb2Si2O7 were gradually formed between 800 and 1000°C, and Yb2SiO5 and α‐Yb2Si2O7 were completely transformed into β‐Yb2Si2O7 at a temperature above 1200°C. The weight loss of Yb2Si2O7/(SiCw–mullite)/SiC tri‐layer coating coated specimens was 0.15 × 10−3 g cm−2 after 200 h oxidation at 1400°C, which is lower than that of Yb2Si2O7/mullite/SiC tri‐layer coating (2.84 × 10−3 g cm−2). The SiC whiskers in mullite middle coating can not only alleviate the coefficient of thermal expansion difference between mullite middle coating and β‐Yb2Si2O7 outer coating, but also improve the self‐healing performance of the mullite middle coating owing to the self‐healing aluminosilicate glass phase formed by the reaction between SiO2 (oxidation of SiC whiskers) and mullite particles.

Effect of Temperature on Morphology, Phase Transformations and Thermal Expansions of Coal Fly Ash Cenospheres

Cenospheres are small, hard-shelled hollow spheres with high silica and alumina content. These micron-sized aluminosilicate hollow spheres constitute only a small percentage of the coal fly ash (CFA) obtained from the coal combustion processes. In this study, cenospheres were separated from CFA obtained from a coal-fired power plant located in Kapar, Malaysia. The cenospheres were heated at 1000, 1200 and 1400 °C to study the changes in morphologies, compositions, phase formation and thermal expansion. The sintering temperatures were selected based on the differential scanning calorimetry (DSC) curve results. X-ray diffractometry (XRD), field emission electron microscopy (FESEM), energy dispersive spectroscopy (EDS) as well as thermogravimetry and differential scanning calorimetry analysis (TG–DSC) were used for the characterization study. The study found that the cenospheres have excellent weight stability but are thermally unstable as a result of crystallization and melting. The phase ordering process and devitrification of the amorphous aluminosilicate glass phase at high temperatures lead to the increment and enrichment of the mullite phase in cenospheres. It is suggested that a preheating treatment at 1100 °C could be used to enhance the mechanical properties and thermally stabilize the cenospheres, which make it more suitable for use as a pore-forming agent in ceramics.

Preliminary investigation of chlorine speciation in zirconolite glass-ceramics for plutonium residues by analysis of Cl K-edge XANES

ABSTRACTA zirconolite glass-ceramic material is a candidate wasteform for immobilisation of chlorine contaminated plutonium residues, in which plutonium and chlorine are partitioned to the zirconolite and aluminosilicate glass phase, respectively. A preliminary investigation of chlorine speciation was undertaken by analysis of Cl K-edge X-ray Absorption Near Edge Spectroscopy (XANES), to understand the incorporation mechanism. Cl was found to be speciated as the Cl- anion within the glass phase, according to the characteristic chemical shift of the X-ray absorption edge. By comparison with Cl K-edge XANES data acquired from reference compounds, the local environment of the Cl- anion is most closely approximated by the mineral marialite, in which Cl is co-ordinate to 4 x Na and/or Ca atoms.

Distribution and speciation of rare earth elements in coal combustion by-products via synchrotron microscopy and spectroscopy

Rare earth element (REE) binding environments and associated mineral phases in coal and coal combustion byproducts (CCBs) are not well understood. Advanced characterization of REEs' binding environments before, during, and after production can be vital in establishing an economically viable recovery method for REEs from these materials. This study characterized different REE distributions, cerium (Ce) oxidation states, and Ce binding environments in 11 CCBs collected from various locales via X-ray fluorescence (XRF) elemental mapping and X-ray absorption near edge structure (XANES) at Stanford Synchrotron Radiation Lightsource (SSRL). Bulk Ce LIII edge XANES identified Ce(III) (>85%) as the predominant Ce oxidation state in all studied samples. Ce speciation and XRF elemental map analyses further reveal the REEs in studied samples can be (1) dispersed throughout the aluminosilicate glass phase, (2) as micro-particles in large glass grain, or (3) as independent trace phases in studied samples. Both fly ash and bottom ash contain mainly Ce(III) micro-hotspots (<40 μm), co-localization with a range of elements, including, but not limited to: P, S, Ca, Ti, and Mn. At microscale, Ce can change from Ce(III) to mixture of Ce(III) and Ce(IV) and to 100% Ce(IV) oxide, revealing potential REE-phase decomposition and oxidation in coal and coal combustion process. Our study also observed intraparticle and interparticle distribution difference between light REEs and heavy REEs. The synchrotron-based characterization confirmed the presence of these trace, previously only theorized, phases during the combustion process and ultimately may inform the simplification of separation and extraction processes for REE from different CCBs.

Magnetic affine sorbents for the isolation of recombinant proteins

An affine sorbent of the “core–shell” type was prepared on the basis of fly ash ferrospheres with the aluminosilicate glass phase content of 41 wt % as a magnetic core. The synthesis of the functional coating included four steps: the steam treatment of the fraction, the synthesis of a mesoporous silica coating on the ferrospheres surface, the surface activation by boiling in an alkaline medium, and the subsequent immobilization of Ni2+ ions on the surface of modified ferrospheres by impregnation. The modified ferrospheres were characterized by the methods of simultaneous thermal analysis, infrared spectroscopy, X-ray diffraction, and low-temperature nitrogen adsorption. It was found that the obtained sorbent was characterized by an acceptable strength of binding of the functional shell and the sorption capacity with respect to the green fluorescent protein of up to 6.7 mg/cm3. The stability of the sorbent in repeated use was studied. It was established that the sorption capacity after the seventh cycle was stabilized at the level of 70% of the initial capacity.

Interface microstructure and compressive behavior of fly ash/phosphate geopolymer hollow sphere structures

In the present work, novel fly ash/phosphate geopolymer hollow sphere structures (FPGHSS) were prepared by the pre-bonding and curing technology. Moreover, the interface microstructure and its role on the compressive behavior were studied. The compressive properties of FPGHSS demonstrated a typical character of cellular materials, with three well defined stages in stress–strain curve. The compressive strength was approximately 5.8MPa, and the failure of FPGHSS was mainly due to the evolution of the multi-collapsed layers of fly ashes and large macro-cracks during the compression. The microstructure of fly ash was composed of aluminosilicate glass phase, crystalline quartz and mullite. The phosphate geopolymer comprised of aluminum-phosphate phase and α-Al2O3, and the nano-pore structure was observed. Moreover, the chemical reaction interface of FPGHSS was generated. It should be noted that the horizontal cracks were mainly produced in fly ashes, and the evolution of horizontal cracks leaded to the clasped layers. Moreover, the large macro-cracks propagated preferentially in the phosphate geopolymer and along the interface region of FPGHSS, due to the dense glass phase of fly ash and the chemical reaction interface. Further, the interface structure between α-Al2O3 and amorphous phase could increase the propagation path of cracks in phosphate geopolymer.

Microstructural features of lime-stabilised pyroclastic soils

Some results of an experimental investigation on a lime-stabilised Italian pyroclastic soil are presented and discussed. This material is very reactive with calcium oxide (CaO) provided by the lime, the effectiveness of the stabilisation mainly depending on the amount of low alkali alumino-silicate glass phase. However, there is a lack of systematic studies on the suitability of such pyroclastic soils to lime stabilisation and the effects of treatment parameters on the engineering properties of the stabilised material. The results of laboratory tests (chemical, physical and mechanical) on untreated and lime-stabilised samples allow an overview of the microstructural features of the material and, as a consequence, on the suitability and effectiveness of lime stabilisation.

Chromium Speciation in Coal and Biomass Co-Combustion Products

Chromium speciation is vital for the toxicity of products resulting from co-combustion of coal and biomass. Therefore, understanding of formation processes has been studied using a combination of X-ray absorption fine structure (XAFS) spectroscopy and thermodynamic equilibrium calculations. The influence of cofiring on Cr speciation is very dependent on the type of fuel. Cr(VI) contents in the investigated fly ash samples from coal and cofiring average around 7% of the total chromium. An exception is cofiring 7-28% wood for which ashes exhibited Cr(VI) concentrations of 12-16% of the total chromium. Measurements are in line with thermodynamic predictions: RE factors of Cr around 1 are in line with volatile Cr only above 1400 °C; lower Cr(VI) concentrations with lower oxygen content and Cr(III) dissolved in aluminosilicate glass. Stability of Cr(VI) below 700 °C does not correlate with Cr(VI) concentrations found in the combustion products. It is indicated that Cr(VI) formation is a high-temperature process dependent on Cr evaporation (mode of occurrence in fuel, promoted by organic association), oxidation (local oxygen content), and formation of solid chromates (promoted by presence of free lime (CaO) in the ash). CaCrO(4)(s) is a probable chemical form but, given different leachable fractions (varying from 25 to 100%), different forms of Cr(VI) must be present. Clay-bound Cr is likely to dissolve in the aluminosilicate glass phase during melting of the clay.

A model for predicting the carbonation depth of concrete containing low-calcium fly ash

Low-calcium fly ash (FL) is a general product from the combustion of anthracite and bituminous coals and has been widely used as a mineral admixture to produce high strength and high performance concrete. Carbonation of cement blended with fly ash is much more complex than ordinary Portland cement because of the pozzolanic activity in an aluminosilicate glass phase of fly ash. In this paper, based on multi-component concept, a numerical model that can predict carbonation of low-calcium fly ash contained concrete was built. This numerical model includes two parts: hydration and carbonation models. The hydration model starts with a mix proportion of concrete and considers both Portland cement hydration and pozzolanic activity. By applying a hydration model, the amount of hydration product that is susceptible to carbonate as well as porosity was obtained as a function of curing age. Furthermore, the diffusivity of CO 2 in concrete was determined and the carbonation depth of concrete was also predicted. The prediction results showed good agreement for the results of the experiment performed in this study.

Oxidation behaviour of SiC-platelets and particulates-reinforced Al2O3/ZrO2 matrix composites

The oxidation behavior of hot-pressed SiC-platelets and particulates-reinforced Al2O3/ZrO2 composites has been studied in an electric furnace at atmospheric pressure at different temperatures. The mass gain as a result of transformation of SiC into SiO2 is described as a function of oxidation temperature, time and type of SiC. The mass gain up to 1100°C was low, but increased strongly at 1350°C. The oxidation process follows a parabolic rate at all oxidation temperatures. Oxidation of composites containing SiC-particulates is higher than the corresponding one containing SiC-platelets. The activation energy, obtained in the present investigation, was 297–333 kj/mol. Diffusion of oxygen and carbon monoxide through the matrix and oxide products appeared to be the rate controlling process. The reaction products were aluminosilicate glass phase and mullite as indicated by SEM and EDX.

29Si, 27Al and 23Na solid-state nuclear magnetic resonance studies of combustion-generated ash

Combustion-generated ash was synthesized, heated and analyzed by solid-state nuclear magnetic resonance spectroscopy with magic angle spinning (NMR–MAS). The structural information obtained from this study indicates that the ash contains different magnetic resonance environments for Al, Si and Na after combustion. The final ash material contains a major amorphous aluminosilicate glass phase with mullite and corundum crystals. 29Si NMR spectra showed a single peak at −110.5 ppm ( δ) for the initial powder sample due to SiO 2. When the sample was heated to 1500°C the peak shifts to lower field ( δ=−99 ppm) due to the formation of aluminosilicate amorphous glass containing alkali or alkaline earth metals. The 27Al NMR spectra of the initial sample gives a single peak at 10.1 ppm due to the AlO 6 (octahedral group) present in the Al 2O 3. When the sample is heated to 1500°C, a new peak appeared at 50 ppm due to the formation of AlO 4 tetrahedral represent the mullite and aluminosilicate compounds. The 23Na NMR spectra of both the initial and the heated samples (at 1500°C) give rise to a single peak at −14.3 and −28.7 ppm, respectively. The sharp peak at −14.3 ppm is contributed from the crystalline Na 2O and the broad peak around −28.7 ppm is due to the Na + present to compensate SiO − present in the amorphous aluminosilicate glass material. The presence of low amounts of heavy metals Cr (2800 ppm), Cd (2700 ppm) and Pb (2800 ppm) in the ash generally does not show any influence on the 27Al, 29Si and 23Na solid-state NMR spectra. On the other hand, the ash sample containing higher amounts of heavy metals (Cd=14210 ppm, Cr=13120 ppm and Pb=13140 ppm) showed significant effect on the 27Al NMR spectra. The replacement of Al 3+ from its octahedral sites by Cr 3+ leads to the decrease in the intensity of the AlO 6 octahedral peak. The presence of Cr 3+, Cd 2+ and Pb 2+ in the aluminosilicate matrix exerts a deshielding effect on 29Si resonance and thus shifts δ to low-field. The results clearly indicate an interaction between the heavy metals and the ash matrix compounds.

Mössbauer Spectroscopy Indicates That Iron in an Aluminosilicate Glass Phase Is the Source of the Bioavailable Iron from Coal Fly Ash

Iron speciation by Mössbauer spectroscopy indicates that ferric iron in an aluminosilicate glass phase is the source of the bioavailable iron in coal fly ash and that this iron species is associated with combustion particles, but not with crustal dust derived from soil minerals. Urban particulate has been shown to be a source of bioavailable iron and has been shown to be able to induce the formation of reactive species in cell culture experiments. Crustal dust and laboratory-generated coal fly ash have been studied as surrogates for two sources of metal-bearing particles in ambient air. As much as a 60-fold difference in the amount of iron mobilized by the chelator citrate was observed between fly ash and crustal dust samples with similar total iron contents. The extent of iron mobilization by citrate in vitro has been shown to correlate with indirect measures of excess iron in cultured cells and with assays for reactive oxygen species generation in vitro. Mössbauer spectroscopy of coal fly ash, before and after treatment with the chelator desferrioxamine B, showed that the iron in an aluminosilicate glass phase was preferentially removed. The removal of the glass-phase iron greatly reduced the amount of iron that could be mobilized by citrate and prevented the particles from inducing interleukin-8 in cultured human lung epithelial (A549) cells. Ferric iron in aluminosilicate glass is associated with particles formed at high temperatures followed by rapid cooling. The observation that ferric iron in aluminosilicate glass is the source of bioavailable iron in coal fly ash suggests that particles from ambient sources and other specific combustion sources should be examined for the presence of this potential source of bioavailable iron.

Devitrification of the Grain Boundary Glassy Phase in a High‐Alumina Ceramic Substrate

Devitrification of the (Na, Mg, Ca) aluminosilicate glass phase in a high‐alumina (&gt;99%) ceramic substrate has been studied by TEM. MgAl2O4 (spinel) and a Ca‐rich plagio‐clase (a solid solution between CaAl2Si208 (anorthite) and NaAlSi3O8 (albite)) are the dominant crystalline phases. Significant spatial variability exists both in the extent of crystallization and in the glass composition.

Investigation of atomic structures of calcium in ash and deposits produced during the combustion of lignite and bituminous coal

X-ray absorption fine structure spectroscopy was used to investigate the molecular structure of calcium in ash and deposit samples generated in combustion tests on a Texas lignite and an Illinois No. 6 coal. The tests were carried out in a 50 lb/hr −1 coal combustion facility. For samples extracted from the flame region and rapidly cooled, the calcium is contained in an aluminosilicate glass phase. In both waterwall and superheater deposits, the calcium tends to be predominantly in the form of CaSO 4 in the initial layers of the deposits, and in aluminosilicate glass in the outer layer. Approximate percentages of calcium in the two phases are derived by computer analysis of the near-edge spectra. A simple model for the combustion reactions of calcium is suggested on the basis of this data.

Dielectric properties of SrTio3glass-ceramics

Abstract Strontium titanate glass-ceramics, prepared by the crystallization of strontium titanate-aluminosilicate glasses have been investigated for possible application as low-loss dielectric materials at microwave and millimeter wave frequencies. Under suitable crystallization conditions, these glass-ceramics exhibit temperature compensated dielectric permittivity, a consequence of the balance between the negative temperature coefficient of the SrTiO3 crystalline phase and the positive temperature coefficient of the residual aluminosilicate glass phase. Temperature coefficients of less than 10 ppm/°C with dielectric constants in the range of 16–18 have been achieved over a temperature range of -55 to 125°C. Dielectric losses at frequencies up to 4MHz are extremely low (tan δ < 0·0005). However, dielectric measurements in the range of 100–1000MHz have shown that the losses increase as the frequency approaches the millimeter wave region, indicating that these glass-ceramics might not be acceptable as mill...

Dielectric properties of SrTiO3glass-ceramics

Abstract Strontium titanate glass-ceramics, prepared by the crystallization of strontium titanate-aluminosilicate glasses have been investigated for possible application as low-loss dielectric materials at microwave and millimeter wave frequencies. Under suitable crystallization conditions, these glass-ceramics exhibit temperature compensated dielectric permittivity, a consequence of the balance between the negative temperature coefficient of the SrTiO3 crystalline phase and the positive temperature coefficient of the residual aluminosilicate glass phase. Temperature coefficients of less than 10 ppm/°C with dielectric constants in the range of 16–18 have been achieved over a temperature range of -55 to 125°C. Dielectric losses at frequencies up to 4MHz are extremely low (tan δ < 0·0005). However, dielectric measurements in the range of 100–1000MHz have shown that the losses increase as the frequency approaches the millimeter wave region, indicating that these glass-ceramics might not be acceptable as mill...