Silicate Phases Research Articles

Phase separation of alkaline earth fluorosilicate glass: A strategy to suppress fluoride evaporation

Fluorosilicate glasses are widely applied as optical and luminescent materials to host a variety of lanthanides. However, fluorides evaporation causes notable difference between the actual composition and the nominal composition of fluorosilicate glass. In this work, a series of SiO2 - Al2O3 - MF2 (M = Ca, Sr, Ba) glasses are fabricated, and quantitative element analysis is performed to determine the actual composition of the fluorosilicate glasses through an inductively coupled plasma atomic emission spectrometer (ICP-AES) and a gas chromatography analysis. A series of molecular dynamics (MD) simulations are carried out to study the glass network microstructures. The phase separation with silicate and fluoride phases is clearly observed in the MD simulated structures and agrees well with the observed self-crystallization of fluorides in the experiments. SiF4 as well as CaF2 / SrF2 / BaF2 are confirmed as the major evaporated substances from the glass melts. The phase separation may well inhibit the evaporation of SiF4 through partitioning Si4+ and F− ions into silicate phase and fluoride phase, respectively. The phase separation may enrich the alkaline earth cations into fluoride phase thus aggravate the evaporated loss of alkaline earth elements. This study is then help to understand the mechanism of fluoride evaporation from glass melts, and quantitatively predict actual composition of the fluorosilicate glasses.

Phase evolution in epitaxial Gd2O3 due to anneal temperature for silicon on insulator application

Epitaxial growth of Si on rare-earth oxides on Si wafer using sputter technology promises cheaper and high-volume manufacturing of Silicon-on-insulator (SOI) wafers over costly solutions like the smart-cut method. Further, rapid thermal annealing (RTA) in standard complementary metal–oxide–semiconductor (CMOS) processing affects the performance of SOI substrate through chemical and crystal phase change. Herein, we present a systematic study on the rich physical and chemical phase evolution of epitaxial Gd2O3 grown on Si (111) using RF sputtering subjected to RTA temperatures (850−1050 °C). First, X-ray diffraction analysis shows a significant enhancement of the epitaxial cubic phase in as-deposited Gd2O3 at an optimum RTA temperature of 850 °C, which is partially explained by epitaxial-regrowth of an amorphous layer at Gd2O3/Si interface as observed in Transmission Electron Microscopy. Secondly, the degradation of crystallinity beyond 900 °C is correlated with temperature-dependent Si diffusion into Gd2O3 that becomes observable by X-ray photoelectron spectroscopy. The Gd2O3 crystallinity is completely quenched into the amorphous gadolinium silicate phase at 1050 °C. Eventually, we employed X-ray reflectivity modeling to evaluate the film thickness variation with RTA. This study provides a detailed insight into the structural and chemical dynamics occurring in epitaxial-Gd2O3 film for CMOS-relevant temperatures.

Nature and origin of an undetected seismic phase in waveforms from Southern Tyrrhenian (Italy) intermediate-depth and deep earthquakes: first evidence for the phase-A in the subducted uppermost lithospheric mantle?

We have found a previously unreported later seismic phase in seismograms of European seismic stations from intermediate-depth and deep earthquakes of the Southern Tyrrhenian subduction zone. We observe this phase at stations from 6 to 9° from the epicentre, towards north. Only seismograms of earthquakes located in a well-defined region of the slab, in the depth range of 215–320 km, show the later x-phase. In this work, we describe the nature and possible origin of this phase, and we provide a simple 2D model to explain the observed arrival times. Our analyses reveal that the x-phase propagates downward in a high velocity layer, possibly located within the deepest part of the slab. We suggest that this layer reveals the presence of the dense hydrous magnesium silicate phase A, introduced from petrological laboratory experiments, inferred to carry water in the upper mantle and predicted to be found in cold subduction zones.

High-pressure order-disorder transition in Mg2SiO4 : Implications for super-Earth mineralogy

(Mg, Fe)SiO$_3$ post-perovskite is the highest pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond pPv. In this work we have explored the high-pressure phase relations in Mg$_2$SiO$_4$ using computations based on density functional theory. We find that a partially disordered I-42d type structure would be stable in the interiors of these super-Earth planets. The discovery of a structure where two very dissimilar cations, Mg$^{2+}$ and Si$^{4+}$ occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy. We have explored the mechanism of the phase transition from the ordered ground state and the effect of the disordering on electronic properties of the silicate phase.

A novel hydrogeochemical approach to delineate the origin of potentially toxic elements in groundwater: Sophisticated molar ratios as environmental tracers.

The occurrence, mobilization, and origin of Potentially Toxic Eelements (PTEs) in the environment is always a difficult research question that has not been fully addressed to date; solving this problem would be a major achievement for environmental science and pollution research, a significant scientific breakthrough, and an important contribution to environmental analysis and monitoring. The lack of a holistic methodology that uses chemical analysis to determine the origin of each PTE in the environment is the main motivation for this project. Therefore, the hypothesis tested here is to develop a scientific approach applied to each PTE to determine whether its origin is geogenic (i.e., water-rock interaction with dominance of silicate or carbonate mineral phases) or anthropogenic (i.e., agricultural practices, wastewater, industrial activities). A total of 47 groundwater samples from the Psachna Basin in central Euboea, Greece, were used and plotted on geochemical mole ratio diagrams (i.e., Si/NO3 vs. Cl/HCO3) and used to perform a robust geochemical modeling analysis. The proposed method showed that elevated groundwater concentrations of various PTEs in groundwater were mainly related to intensive fertilization (e.g., Cr, U), water-rock interaction (e.g., Ni), and saltwater intrusion. (i.e., As, Se). This work highlights that a comprehensive framework with sophisticated molar ratios combined with modern statistical methods, multi-isotope signatures, and geochemical modeling could provide answers to unresolved scientific questions about the origin of PTEs in water resources and improve environmental resilience.

Luminescence and structural characteristics of Tb3+-embedded Ca2SiO4 phosphors synthesized using agro-food waste as solid state lightning devices

Ca2SiO4 phosphors doped with Tb3+ ions have been synthesized by the solid-state reaction method using rice husk and egg shell as the starting raw materials. The prepared Tb3+-doped Ca2SiO4 phosphors exhibited monoclinic structure and the diffraction patterns reveal the single phase of calcium silicate. The emission spectra were measured under the excitation of 369 nm. The spectra show six emission peaks centred at 624, 584, 543, 489, 436 and 414 nm, assigned to the 5D4→7F3, 5D4→7F4, 5D4→7F5, 5D4→7F6, 5D3→7F4 and 5D3→7F5 transitions, correspondingly. With increase of the Tb3+ content from 0.1 to 1.0 mol %, the emission intensities are found to be increased and then decreased after 1.0 mol %. The shortening of lifetime from 2.70 to 2.31 ms was observed with increase in Tb3+ doping content from 0.1 to 2.0 mol % which is ascribed to the cross-relaxation between the 5D3 and 5D4 states in Tb3+. The chromaticity co-ordinates were calculated from the emission spectra and found to be (0.13,0.47), which falls in the exact green region. The results indicate that the low cost Tb3+-doped Ca2SiO4 phosphors are useful for the green component of white light emitting devices.

Advanced argillic alteration at Cave di Caolino, Lipari, Aeolian Islands (Italy): Implications for the mitigation of volcanic risks and the exploitation of geothermal resources

Four sites in the western sector of Lipari Island with still active hydrothermal activity are here considered. The petrography (mesoscopic observations and XRPD) and geochemistry (major, minor and trace elements chemistry) of ten representative and extremely altered volcanic rocks were characterized. Two types of parageneses of altered rocks are discriminable, one rich in silicate phases (opal/cristobalite, montmorillonite, kaolinite, alunite and hematite) and one in sulphates (gypsum, plus minor amounts of anhydrite or bassanite). The altered silicate-rich rocks are rich in SiO2, Al2O3, Fe2O3 and H2O, and depleted in CaO, MgO, K2O and Na2O, while the sulphate-rich ones are extremely enriched in CaO and SO4 in comparison with local unaltered volcanic rocks. The content of many incompatible elements is similar in altered silicate-rich rocks and lower in sulphate-rich ones with respect to the pristine volcanic rocks; conversely, almost all REEs are markedly enriched in silicate-rich rocks and heavy REEs are enriched in sulphate-rich altered rocks compared to unaltered volcanic rocks.Reaction path modelling of basaltic andesite dissolution in local steam condensate predicts the production of amorphous-silica, anhydrite, goethite, and kaolinite (or smectites and saponites) as stable secondary minerals and alunite, jarosite, and jurbanite as ephemeral minerals. Considering possible post-depositional reactions and admitting that the presence of two distinct parageneses is apparent, since gypsum is prone to form large crystals, it can be concluded that there is an excellent agreement between the alteration minerals occurring in nature and those predicted by geochemical modelling. Consequently, the modelled process is the main responsible for the production of the advanced argillic alteration assemblage of “Cave di Caolino” on Lipari Island. Since rock alteration is sustained by the H2SO4 solution produced by hydrothermal steam condensation, there is no need to invoke the involvement of SO2-HCl-HF-bearing magmatic fluids, in line with the absence of fluoride minerals.

Thermodynamic Analysis of Mineral Phase Composition of Steel Slag System

In order to transform the crystalline form of Ca2SiO4 (C2S) in phosphorus-containing slag from monoclinic β-polycrystalline to square γ-polycrystalline, a volume expansion of about 11% was generated, which caused the phosphorus-containing slag to undergo self-powdering. The CaO-SiO2-Al2O3-MgO-MnO-P2O5-FeO slag system was analyzed using FactSage7.1 thermodynamic software, and the effects of different P2O5, FeO and basicity on the mineral phase composition of slag system were analyzed in the range of 1300~1700 °C. It was shown that P2O5, FeO and basicity all have an effect on the composition of the mineral phases. When the mass fraction of P2O5 in the slag was lower than 0.25%, it had less effect on the transformation of C2S crystalline structure. When the P2O5 content was higher than 0.25%, it was favorable to the generation of low-melting-point substances, but the P2O5 in the slag reacted with C2S in the silicate phase, making P5+ solidly soluble in C2S, inhibiting the transformation of β-C2S to γ-C2S and hindering the self-powdering of the slag. The FeO content in the slag system ranged from 20% to 28%, and as the FeO content increased, the C2S content in the silicate phase decreased from 33.3% to 25.9%, while the temperature at which the silicate was completely dissolved into the liquid phase decreased from 1600 °C to 1500 °C and the complete melting temperature of the slag decreased. The low FeO content facilitates the self-powdering of slag. In the high-phosphorus slag, at temperatures below 1450 °C, with the increase of basicity, the proportion of C2S in the silicate phase first increased and then decreased. With basicity at 1.8; the highest content of silicate phase, accounting for 33.7%; and the temperature exceeding 1450 °C, the silicate phase dissolved into the liquid phase, which is conducive to the removal of phosphorus from the slag, achieving the self-powdering of high-phosphorus slag.

Catalytic Oxidation of VOC over Cobalt-Loaded Hierarchical MFI Zeolite

The zeolites ZSM-5 with different Si/Al ratios were modified with a buffer solution of HF and NH4F. This acidic treatment led to the obtaining of a material with secondary mesoporosity. The deposition of cobalt from an aqueous solution of cobalt acetate on the surface of treated samples generates the formation of different cobalt oxide species: bulk-like Co3O4 phases strongly interacting with the support, Co2+ in ion exchange positions (γ and β sites) and silicate-like phases. The mechanism of cobalt silicate phase formation is proposed here and includes a replacement of silanol groups and the bridging hydrogens by Co and the inclusion of the Co atoms in tetrahedral framework positions. The catalysts, obtained through use of the treated ZSM-5, exhibit higher activity in the reactions of propane, and n-hexane completes oxidation compared with the catalyst samples containing un-treated zeolite. Both the finer dispersion of metal oxide particles on the hierarchical sample and the presence of secondary mesoporosity improve the effectiveness of the active phase utilization via access to larger number of active sites.

Rapid quench piston cylinder apparatus: An improved design for the recovery of volatile-rich geological glasses from experiments at 0.5-2.5 GPa.

The accurate and precise determination of the compositions of silicate glasses formed from melts containing volatile components H2O and CO2 recovered from high-pressure, high-temperature experiments is essential to our understanding of geodynamic processes taking place within the planet. Silicate melts are often difficult to analyze chemically because the formation of quench crystals and overgrowths on silicate phases is rapid and widespread upon quenching of experiments, preventing the formation of glasses in low-SiO2 and volatile-rich compositions. Here, we present experiments conducted in a novel rapid quench piston cylinder apparatus on a series of partially molten low-silica alkaline rock compositions (lamproite, basanite, and calk-alkaline basalt) with a range of water contents between 3.5 and 10 wt%. Quench modification of the volatile-bearing silicate glasses is significantly reduced compared to those produced in older piston cylinder apparatuses. The recovered glasses are almost completely free of quench modification and facilitate the determination of precise chemical compositions. We illustrate significantly improved quench textures and provide an analytical protocol that recovers accurate chemical compositions from both poorly quenched and well-quenched silicate glasses.

Role of Mg Gangue Minerals in Natural Analogue CO2 Sequestration

Abstract Mineral carbonation reactions consume CO2 and thus have the potential for the long-term fixation of atmospheric CO2. This paper explores the possibility of integrating industrial-scale carbon storage into mining operations. Ultramafic rocks are typically considered to be one the most promising rocks for carbon capture and storage owing to their high content of Mg-bearing silicate minerals, such as forsterite (Mg2SiO4) and serpentine (Mg3Si2O5(OH)4). Beyond the thermodynamic considerations showing that magnesite (MgCO3) and quartz (SiO2) form from forsterite and serpentine alteration, the degree to which CO2 is carbonated depends on the kinetics of the reaction. For industrial carbon capture and storage to be viable, reasonable carbonation efficiency has to be achieved. To this effect, the reaction rates have to be increased, which can be achieved either by increasing the reactive surface area, increasing the reaction temperature, or using reagents to drive the reactions. However, these approaches are usually energy demanding or not efficient enough. As part of its activities, the mining industry excavates tens or hundreds of million metric tons of rock per mine and in certain areas these mafic rock groups can represent a significant percentage of the waste material left on the surface. This could represent a locally important source of readily available material for carbon capture and storage if the conversion process is sufficiently efficient. To test and quantify the carbonation potential of mine waste, a sample of serpentine skarn waste rock obtained from an iron ore mine in Sweden was reacted for 60 weeks in a laboratory humidity-cell test (HCT) at 20°C. The results show the dissolution of olivine, the precipitation of serpentine, an increase in the neutralization potential of the sample, and the appearance of inorganic carbon during the 60 weeks of testing. At ambient temperatures the sluggish precipitation kinetics of secondary phases will favor the formation of more hydrous Mg silicate phases, such as serpentine (Mg3Si2O5(OH)4), in place of Mg-bearing carbonates. This reaction lowers considerably the efficiency of forsterite carbonation, as only 25% of the Mg released from forsterite dissolution to form carbonate minerals. This study aims to model the carbonation efficiency of Mg silicates through the use of models supported by laboratory testwork and taking the example of a mine site in northern Sweden. This study evaluates the reaction of CO2 with Mg-bearing silicate rocks and the demonstration that carbonation reactions occur with Mg silicate wastes consuming CO2. Consequently, weathering of waste rock may well represent an important sink for carbon in the environment.

Eco-Friendly Geopolymer Composites Prepared from Agro-Industrial Wastes: A State-of-the-Art Review

Portland cement (PC) is a common material used in civil infrastructure engineering. Cement production emits roughly 2.2 billion tons of CO2 per year, contributing 8% of global emissions in 2016. This contributes to almost half of the calcination process, and together with thermal combustion, clinker generation could be responsible for 90% of the sector’s emissions. One effective technique for dealing with these industrial by-product wastes is to employ them to make cement replacements such as concrete and mortar, which can be used in a variety of applications. As a result, the purpose of this research is to review the current advancements, challenges, and future perspectives on the utilization of agro-industrial waste (AIW) produced around the world in cement-based products. Geopolymers (GPs), on the other hand, reduce carbon dioxide emissions and have the potential to be a complete or partial replacement for PC in the construction sector. The GP technology enables the use of AIW in combination with an alumina–silicate (A–S) phase with minimal environmental impact. GP-cement is mostly produced by activating alkali silicates or alkali sols with secondary raw materials such as calcined clays, fly ash (FA), zeolite, metakaolin, etc. Mixing various resource materials, including additives, A–S, and alkali sols, alkali concentrations, optimizing the curing temperature, the SiO2/Na2O ratio, microstructural behavior, and other factors, results in GP-cement with outstanding mechanical and durability characteristics. The review concludes that AIW-based geopolymer composites have shown promising results in terms of their mechanical properties, durability, and environmental sustainability, which makes them emerge as promising future building materials with applications in a wide range of industries.

Infrared and Raman vibrational modelling of β-C2S and C3S compounds

Density Functional Theory was applied to obtain a refined IR and Raman theoretical vibrational spectra of the major silicate phases of Portland cement, i.e. dicalcium silicate (β-C2S) and tricalcium silicate (C3S). We investigate the crystal structure and vibrational frequency values in the electronic ground state using a plane wave density functional method with new norm-conserving pseudopotentials. The calculated results show that the optimized geometry can reproduce the crystal structure well, and the theoretical vibrational frequency values show good agreement with the experimental values and allow us to describe some uncertainties in the experimental assignments previously described. The assignment performed for 100–1200 cm−1 region shows that the vibrations of C2S and C3S are presented in the IR spectra, while the vibrations with a minor contribution from the silica groups are observed in the Raman spectra. Satellite bands from C3S are observed in the experimental spectra but there not in the calculated one.

A novel method of enhancing valuable element recovery for ultra-high-titanium magnetite

The metallization reduction-magnetic separation process was adopted to carry out the experiments to promote the recovery ratio of valuable elements by adding diboron trioxide additive in this work. It is demonstrated that diboron trioxide has the function of destroying Si–O–Si bond for the reduction of ultra-high-titanium magnetite. The optimum experimental condition is found to be the reduction temperature of 1125 °C, the reduction time of 30min, and the diboron trioxide percent of 8 wt% for which the metallization ratio is 92%, and the recovery ratio of ferrum, vanadium, titanium is 90.9%, 78.5%, 76.1%, respectively. V and Ti are synchronously concentrated in non-magnetic materials. The phase composition transformation rule of the valuable components was Fe3O4 (FeTiO3) → FeO → Fe; FeTiO3 (→ FeTi2O5) → (Fe,Mg)Ti2O5; FeTiO3 → TiO2 → Ti3O5 → Ti2O3; (Fe3O4 + TiO2) → Fe3Ti3O10 → Fe. Increasing the temperature can promote the aggregation and growth of iron particles, but too high temperature is not conducive to the separation of Fe, V and Ti. It is evidently observed that when the temperature exceeds 1150 °C, the iron particles are closely connected with the gray titanium oxide, while at 1125 °C, the iron particles are relatively independent. Meanwhile, part of Ti is observed in silicate phase as solid-dissolved form because of high temperature and cause loss. Overall speaking, the metallization reduction-magnetic separation process with diboron trioxide as an additive has achieved the better separation of valuable components, with a higher degree of metallization, and large-scale lowered reaction temperature and time.

Incorporation and solidification mechanism of manganese doped cement clinker.

Using municipal and industrial solid waste as a substitute raw material and fuel in cement rotary kiln co-processing is considered an economic and environmentally friendly alternative to the use of traditional fuels. However, the presence of heavy metals in solid waste is a growing concern in the cement rotary kiln co-processing technique. The solidification mechanism of heavy metals in cement clinker is directly related to their stabilization. Cement clinkers doped with manganese oxide (MnO2: 0.0%-5.0% wt%) were prepared in a laboratory to investigate the impacts of extrinsic Mn on cement clinker calcination. The insignificant changes in X-ray diffractometer patterns indicated that the fixed Mn had little influence on the mineral lattice structure. Raman spectra and X-ray photoelectron spectroscopy revealed the transformation of the silicate phase when the Mn dose was increased. Moreover, the satisfactory solidification ratio confirmed the incorporation of Mn in the cement clinker. These results provided evidence of the influence rule of Mn in the cement clinker calcination process. Furthermore, Raman spectroscopy showed great potential for the qualitative and semi-quantitative analysis of the cementitious materials derived from cement rotary kiln co-processing. These results will be important for the further development of green cement manufacturing technology.

Effect of acidic amino acids on wet pre-carbonation of β-C2S in steel slag

Wet pre-carbonation of steel slag is an important solution to the steel slag soundness problem. This method, however, can consume a significant amount of silicate phase in steel slag. To address this issue, it is proposed that the consumption of calcium silicate can be reduced while the hydration performance of steel slag can be improved by adding acidic amino acids into the wet carbonation of steel slag. β-C2S is the main cementitious component in steel slag. Synthetized β-C2S was used to simulate the reaction mechanism of steel slag during carbonation and to study the role of acidic amino acids in carbonation. The carbonation products were characterized by XRD, FT-IR, TG-DTG, and SEM. The results demonstrated that without the addition of acidic amino acids, the carbonation degree of β-C2S reaching the maximum at 1 h under the experimental conditions. Calcite and amorphous silica was formed as a result of the carbonation of β-C2S. The final carbonation degree was lower after adding higher concentrations (0.1 mol/L) of acidic amino acids than before. Higher concentrations of acidic amino acids increased the content of metastable CaCO3, primarily vaterite. Acidic amino acids were added in the pre-carbonation of steel slag, then the steel slag was mixed with cement to prepare cementitious materials. The cementitious materials’ 28-day compressive strength was higher than without amino acids, and the maximum of strength increased by 9.45 % compared to that without amino acids. Whether amino acids were added or not, the boiling expansion value decreased to 0 after 1 h of carbonation, indicating that the carbonation reaction improved the soundness of cementitious materials.

Using ensemble model to predict isothermal hydration heat of fly ash cement paste considering fly ash content, water to binder ratio and curing temperature

It is widely accepted that fly ash (FA) addition in cement impairs the reaction of silicate phase and promotes the reaction of aluminate phase. Meanwhile, FA also retards early-age hydration heat release of cement. No generalized hydration prediction model is available for accurate description of the isothermal heat release of FA cement paste. In this study, we try to adopt an ensemble model, one of the machine learning approaches, to predict the early-age isothermal hydration heat flow of FA cement paste. Effect of curing temperature, water to binder ratio (w/b), FA content and curing time on heat release is thoroughly considered in the ensemble model, and the hyperparameters of the model are tuned by Bayesian optimization with 5-fold cross validation. The results indicate that whether for the training data or the testing data, the root-mean-square error (RMSE) and mean-absolute-error (MAE) have been well controlled at approximately 0.1 and 0.06 mW/g paste, respectively. The optimized model is also applied to predicting the time-dependent isothermal hydration heat flows of three different FA cement systems cured under different temperatures. The coefficient of determination (R2) values for both the predicted heat flow rates and the calculated cumulative heat have attained 0.98, proving the applicability of the trained model on isothermal heat prediction for FA cement paste. Sensitivity analysis is also conducted for the optimized model. Curing temperature and time are revealed to be the two most important input variables for isothermal heat prediction using machine learning, followed by FA content, and the least important one is w/b. Neglecting any of them reduces the prediction accuracy of the model.

Investigation of mineral commodity residues based on alkalinity, solubility and other physicochemical aspects aiming the management of amazonian acidic soils

The large amounts of mineral residues generated by the bauxite-alumina industries in the Amazon — with a production scale in the same order of magnitude as their source commodities — have been seen as new sources of raw materials (secondary mines) and/or an inseparable part of a sustainable production system, i.e., a source for the generation of co-products within a circular economy system. In the present study, two alkaline residues from this mining-metallurgical industry were assessed for their potential to amend productive acidic Amazonian soils, namely, (1) insoluble solid residue from the Bayer process (bauxite residue, BR) and (2) ash from energy production from coal (coal combustion residues, CCRs: fly ash, FA, and bottom ash, BA). A physicochemical investigation was performed to evaluate the possible benefits that these residues can offer to the soil‒plant system. The alkalinity of the residues was adjusted “to a value of pH 8–10” by leaching with H3PO4 using a central composite experimental design. The chemical analyses indicated high levels (total and soluble) of essential elements such as Ca and S in the CCRs. All residues showed a high cation exchange capacity (CEC). Regarding the water holding capacity (WHC), FA showed a higher value than the other residues (68.6%). After pH adjustment, available P increased significantly for all residues, and the Ca and S contents remained high for the CCRs, while in BR, there was a decrease in available Na, and aluminum (Al3+) was not available because the potential acidity (H + Al) < 0.6. Finally, complementary analyses showed that with respect to mineralogy, BR is composed mainly of iron oxyhydroxides and aluminosilicate phases, while carbonate, sulfide and silicate phases dominate the CCRs. The neutralizing character, the presence of nutrients in the CCRs and the unavailability of Al3+ in BR are positive physicochemical characteristics for the management of Amazonian acid soils; the use of this residues would add to the circular economy and sustainability of the Amazon.

Modification of low-melting phase monticellite (CMS) by La2O3

Monticellite (CMS) refers to a silicate phase that exhibits the worst refractoriness, whereas it exists widely in magnesia refractories, such that the high temperature performance of magnesia refractories is seriously reduced. Given the above-described problem, CaLa4(SiO4)3O was fabricated in this study, and the effect of La2O3 addition on modification and microstructure evolution of low-melting phase monticellite (CMS) was investigated. As indicated by the results, La2O3 reacted with SiO2 and CaO to directly synthesize CaLa4(SiO4)3O while modifying and transforming the existing CMS into the high-melting-point phase CaLa4(SiO4)3O for its higher activity, such that the content of CMS in the system was significantly reduced. Besides, CaLa4(SiO4)3O grains exhibited a hollow tubular to club-shaped morphology by controlling the addition of La2O3. On that basis, contributing to the microstructure of form CaLa4(SiO4)3O grains interlocking and enveloping the low-melting phases inside after heating at 1400 °C. It is noteworthy that the CaLa4(SiO4)3O test cone had no softening and bending during the refractoriness test (the highest test temperature was 1800 °C), thus confirming CaLa4(SiO4)3O as a high-melting-point phase.

Research on the ring formation mechanism of magnesian flux pellets in rotary kiln

The forming of rings was a key problem of magnesian flux pellets in rotary kiln, which seriously limited production efficiency. Pellet powder and flux were the raw materials of the ring. Based on this, the bonding strength, melting behavior and microstructure of pellet powder and its mixed powder with flux were investigated. The influence of basicity (R = CaO/SiO2) on ring behavior of pellet powder was analyzed, and ring formation mechanism of magnesian flux pellets was clarified. The results showed that acid pellet powder was not easy to form rings due to lower bonding strength of briquettes. Due to changes in bonding process after mixing of flux, magnesian flux pellet powder produced ferrite and silicate liquid phase with lower melting point, which promoted diffusion and recrystallization of hematite and enhanced compressive strength of briquettes, then finally caused rings to form. Moreover, it is required to control roasting temperature below 1200 °C, which is a necessary condition for magnesian flux pellet powder to form an initial ring which was easy to be destroyed.