CaO Al2O3 SiO2 Research Articles

In situ observation of the dynamics in the middle stage of spinodal decomposition of a silicate glass via scanning transmission electron microscopy

Real-space observations in the middle stage of spinodal decomposition have not been achieved because of the small sizes and unclear interfaces of the phases. Here, we performed in situ real-space experiments with heating to quantitatively reveal the local dynamics and structures of a calcium aluminosilicate glass in the middle stage of spinodal decomposition by scanning transmission electron microscopy. This glass separate into CaO–Al2O3–SiO2 phase and SiO2 phases. The observations revealed that the CaO–Al2O3–SiO2 phases with low Ca concentrations behaved as if they were in the middle stage, whereas the CaO–Al2O3–SiO2 phases with high Ca concentrations coarsened as if they were in the final stage. This coexistence of stages suggested that the behaviors of the phases depend not only on time, but also on their local relative concentrations. Differing from the well-known final stage of spinodal decomposition, the phases with relatively high Ca concentrations in the middle stage showed peculiar directional migration guided by the phases with relatively low Ca concentrations. These findings have not been observed before and thus have the potential to provide a new way to control the phase-separated structure.

Evolution of Nonmetallic Inclusions with Varied Argon Stirring Condition during Vacuum Degassing Refining of a Bearing Steel

In the current study, industrial trials are performed to investigate the effect of argon stirring conditions on nonmetallic inclusions of a bearing steel during vacuum degassing (VD). Three argon stirring modes, including strong stirring, moderate stirring, and weak stirring during the last 10 min of the high‐vacuum period, are compared. The removal fraction of >1 μm inclusions and >5 μm inclusions is the highest with the moderate stirring mode, reaching 15.7% and 42.2%, respectively. Inclusions of MgO·Al2O3 are easier to remove than CaO–Al2O3 and entrained CaO–Al2O3–SiO2 ones as MgO·Al2O3 inclusions have a higher coagulation coefficient, a higher contact angle, and a lower adhesion work between the inclusion and the molten steel. Meanwhile, the removal efficiency of inclusions in the bearing steel is in the order of Al2O3 > MgO·Al2O3 > solid CaO–Al2O3 > liquid CaO–Al2O3 and entrained CaO–Al2O3–SiO2. Liquid CaO–Al2O3 inclusions with >5 μm size originate from slag entrainment and can be efficiently removed from the molten steel by the moderate stirring mode.

Structure–property relationship amphoteric oxide systems via phase stability and ionic structural analysis

AbstractThe effects of basicity and amphoteric oxides (Al2O3 and FetO) on the structure–property relationships of CaO–SiO2–(Al2O3 and FetO) and CaO–SiO2–Al2O3–FetO slags were investigated to determine the constitutional effects on the structure of high‐temperature ionic melts. The proportion of Qn species, which is determined by Raman spectroscopy, and the viscosity measured by the rotating cylinder method are both correlated and shown together with the slag structure index (NBO/T) concept. The NBO/T of CaO‐SiO2 binary slags showed a linear relationship with basicity (CaO/SiO2), including an inflection point at CaO/SiO2 = 1.0 resulting from the stability and Qn‐dominant unit of the melt, which changes close to the wollastonite (CaSiO3) congruent point. This inflection point changes with the increasing amphoteric oxide content (Al2O3 and FetO) because of the change in the dominant polymeric unit (Si4+–O–Si4+→M4+–O–Si4+; M: Al and Fe), in accordance with the equilibrated primary phases. As the Al2O3 content increased, the viscosity and activation energy of slags both drastically increased owing to the change in the flow unit (Si–O–Si, Al–O–Si, and Al–O–Al). In contrast, as FetO increased, the viscosity and activation energy (Eη) of slags decreased because of the change in the flow unit (Si–O–Si, Fe–O–Si, and Fe–O–Fe). Ultimately, the flow unit (T–O–T; T = Si, Al, and Fe) and activation energy of the slags were found to be closely related to the solid primary phase on the phase diagram, and the physical‐property–structure relationship was determined from the phase stability.

Oxidizability characterization of slag system on the thermodynamic model of superalloy desulfurization

AbstractThe oxidizability characterization method of slag system has a decisive influence on the accuracy of the desulfurization model. By carrying out the balance experiments of melting system composed of CaF2–CaO–Al2O3–SiO2–TiO2–MgO–FeO slag system and Inconel 718 superalloy, the influence of Al2O3 and FeO on desulfurization distribution ratio at 1,773 K was studied. Based on the ion and molecule coexistence theory in slag, the effects of three oxidizability characterization methods, [Fe]–[O] balance, [Al]–[O] balance, and [Fe]–[Al]–[O] balance, on the accuracy of desulfurization thermodynamic model were studied. The results show that the effect of FeO on desulfurization distribution ratio is more significant than that of Al2O3 when the FeO content in slag is greater than 0.014%. When the FeO content in the slag is less than 0.014%, the effect of Al2O3 on the slag oxidizability will be greater than that of FeO. The calculated value of [Fe]–[Al]–[O] balance model is closer to the experimental-measured value compared to [Fe]–[O] balance model and [Al]–[O] balance model alone at 1,923 K.

Behaviour of oxide inclusions and sulphur in ‘two-stage basicity control’ refining method of Si-killed spring steel

ABSTRACT A ‘two-stage basicity control’ refining method was carried out that high basicity (R > 1.5) top slag was used in the LF refining and then low basicity (R = 0.8–1.0, Al2O3 < 6%) top slag was used in VD refining. This method can obtain inclusions with better deformability and make the top slag have good desulphurization ability. The average sulphur removal percentage of the refining is 47.22%. There were mainly three types of inclusions classified as Al2O3–SiO2, CaO–Al2O3–SiO2, and MgO–CaO–Al2O3–SiO2 inclusions in the LF refining. In the VD refining, the number of Al2O3–SiO2 and CaO–Al2O3–SiO2 inclusions reduced obviously. The main type of inclusions was MgO–CaO–Al2O3–SiO2 and their melting points reduced from about 1873 K to less than 1673 K during the VD refining. The results of thermodynamic calculations show that the mass fraction of [Al] should be controlled less than 20×10−6 to obtain low-melting point inclusions (<1673 K).

Influence of TiO2 on the viscosity of the molten slag and the confirmation of the acid–base property on TiO2

ABSTRACT The viscosities of high titanium containing slag with TiO2 ranging from 23 to 43 mass%, Al2O3 9 mass% and CaO/SiO2 ranging from 0.3 to 0.7 were experimentally determined by the rotating cylinder method. Experiments were conducted in the temperature range of 1573–1873 K. The effects of TiO2, basicity, and temperature on viscosity were studied. The experimental results indicated that above the liquidus temperatures, the viscosities decrease with TiO2 and basicity increasing. And increasing on CaO content should be more effective in depolymerizing slag structure and lowering viscosity compared to TiO2 addition. Moreover TiO2 behaved as a weak acid oxide in the experiment slags by XPS analysis. Based on the experimental data of CaO–SiO2–Al2O3–TiO2 quaternary system, the activation energy of viscous flow was calculated between 115 and 141 kJ mol−1 depending on the slag composition.

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

AbstractPhase equilibria and liquidus temperatures in the CaO–SiO2–Al2O3–MgO system at a CaO/SiO2 weight ratio of 0.9 in the liquid phase have been experimentally determined employing high‐temperature equilibration and quenching technique followed by electron probe X‐ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.

Preparation and characterization of synthetic tobermorite (CaO–Al2O3–SiO2–H2O) using bio and municipal solid wastes as precursors by solid state reaction

In this present study, synthetic tobermorites are prepared using bio-waste (snail shell) and municipal waste (container glasses) as lime and silica precursors respectively. Six batch compositions were formulated with varying combination of soda-lime glass and snail shell ash. The bodies were sintered at 950°C for a holding period of 2h in an electric muffle furnace. Analyses such as scanning electron microscopy (SEM/EDS), Fourier Transform Infra-red Spectroscopy (FT-IR), X-ray diffractometry (XRD) were used to assess the microstructure, functional groups and the phase composition of the prepared tobermorites respectively. The results of the morphology shows that the tobermorites possess irregular but spherical shaped grain with coated water films while the EDS shows the presence of Ca and Si with small amount of Al confirming tobermorite. The FT-IR indicates Ca–O–Si and Si–O–Si as main functional groups while the phase composition investigated by XRD indicate low intensity peaks of calcium silicate (CaSiO3).

Enhancing Pb Removal and Synthesizing Glass–Ceramics from Waste CRTs Funnel Glass by Red Mud

In this paper, a novel and effective process for removing lead and synthesizing glass–ceramics from the hazardous waste CRTs funnel glass and red mud was developed. Glass–ceramics was produced through an appropriate sintering treatment of the residual glass after lead from funnel glass was removed. Vacuum smelting results showed that the addition of red mud had contributed to enhance Pb removal from funnel glass as compared to the same process without red mud. When the red mud mass was added 60 wt%, more than 99.93% of Pb was successfully removed and the Pb content was reduced to 0.0093 wt%. Metallic lead with 99% purity was recycled. The sintering process showed that the CaO–Al2O3–SiO2 type glass–ceramics were synthesized with different microstructure crystallinity and morphology. Overall, the results demonstrated that red mud can not only enhance PbO removal from waste CRT funnel glass, but also fabricate glass–ceramics combining residual glass. Route of removing Pb and synthesizing glass–ceramics from waste CRTs funnel glass by red mud.

Influence of Sodium Oxide on the Fusion of Solid Municipal Waste Ash

The model composition of municipal solid waste (MSW) and its ash chemical composition are characterized. Mineral compounds and melting points are studied using a variety of analytical methods, including XRD, FTIR, and AFT. The main parameters of slag formation are calculated: base-to-acid ratio, slag viscosity index, and fouling coefficient. Solid municipal waste ash contains significant amounts of CaO, SiO2 and Al2O3. Using the Terra program, thermodynamic calculations of the chemical equilibria of the CaO–SiO2–Al2O3–Na2O systems containing from 5 to 25% sodium oxide are performed. It is experimentally shown that each 5 wt % increase in the Na2O content reduces the characteristic melting temperatures of MSW ash by 30–50°С.

Kinetics and fusion characteristics of municipal solid waste incineration fly ash during thermal treatment

During the thermal treatment of municipal solid waste incinerators (MSWI) fly ashes (FA), the fusion characteristics, kinetics, and phase transition are the theoretical basis on heavy metals solidification improvement and energy consumption reduction. In this study, the thermal behavior of MSWI FA from two typical furnaces in China, i.e., grate furnaces (GF) and circulating fluidized bed (CFB), are systematically investigated. The results show that FA melting can be divided into five stages but with different positions and areas of weightlessness peaks between two types of FA. We identify four kinetic models from which to obtain the apparent activation energy and pre-exponential factor during the melting transition. The apparent activation energy of GF FA ranges from 466 to 1016.2 kJ/mol, which is generally higher than that of CFB FA (380.1–636.6 kJ/mol). The weightlessness rate of GF FA is approximately three times that of CFB FA because of its high chloride load. Silicon dioxide (SiO2) in FA is prone to react with other compounds to form low-melting-temperature minerals at 1100 °C, such as Ca2MgSi2O7. Therefore, the fusion temperatures of CFB FA with a higher content of SiO2 are about 150 °C lower than that of GF FA, which is consistent with the theory that the fusion temperature of FA is at its lowest when the ratio of basic to acid oxides approaches 1. Finally, SiO2–CaO–Al2O3 ternary phase diagrams, ash fusion temperatures, and several fouling and slagging indices are verified to predict the ash fusion characteristics of MSWI FA.

The crystallization and structure features of glass within the K2O–MgO–CaO–Al2O3–SiO2-(BaO) system

The effect of the BaO addition on the microstructure and structure of the glasses from the K2O–MgO–CaO–Al2O3–SiO2-(BaO) system was investigated in this study. The results were obtained by testing the crystallization capability of the glasses during the fast-firing cycle. Thermal analysis proved a high tendency to crystallize the tested glasses/frits. X-ray analysis showed the presence of diopside and anorthite as the main crystalline phases in all glazes obtained from the tested glasses. Microscopic observations showed that barium oxide introduced in the range of 0.4–2.1 mol% into the examined system strongly influences the number and size of crystalline phases. The introduction of 2.1 mol% BaO into the composition also resulted in crystallization of celsian. The thermal characteristic of the glasses was carried out using differential scanning calorimetry (DSC) and the crystalline phases were determined by X-ray diffractometry. The glaze microstructure was investigated by scanning (SEM) and transmission electron (TEM)microscopy. Additional information on the structure of frits was derived from mid-infrared (MIR) and Raman spectroscopy.

Influence of refining process and utilization of different slags on inclusions, titanium yield and total oxygen content of Ti-stabilized 321 stainless steel

Ti-stabilized 321 stainless steel was prepared using an electric arc furnace, argon oxygen decarburization (AOD) furnace, ladle furnace (LF), and continuous casting processes. In addition, the effect of refining process and utilization of different slags on the evolution of inclusions, titanium yield, and oxygen content was systematically investigated by experimental and thermodynamic analysis. The results reveal that the total oxygen content (TO) and inclusion density decreased during the refining process. The spherical CaO–SiO2–Al2O3–MgO inclusions existed in the 321 stainless steel after the AOD process. Moreover, prior to the Ti addition, the spherical CaO–Al2O3–MgO–SiO2 inclusions were observed during LF refining process. However, Ti addition resulted in multilayer CaO–Al2O3–MgO–TiOx inclusions. Two different samples were prepared by conventional CaO–Al2O3-based slag (Heat-1) and TiO2-rich CaO–Al2O3-based slag (Heat-2). The statistical analysis revealed that the density of inclusions and the TiOx content in CaO–Al2O3–MgO–TiOx inclusions found in Heat-2 sample are much lower than those in the Heat-1 sample. Furthermore, the TO content and Ti yield during the LF refining process were controlled by using TiO2-rich calcium aluminate synthetic slag. These results were consistent with the ion–molecule coexistence theory and FactSage™7.2 software calculations. When TiO2-rich CaO–Al2O3-based slag was used, the TiO2 activity of the slag increased, and the equilibrium oxygen content significantly decreased from the AOD to LF processes. Therefore, the higher TiO2 activity of slag and lower equilibrium oxygen content suppressed the undesirable reactions between Ti and O.

The impact of nano-quartz on the structure of glass-ceramic glazes from the SiO2–Al2O3–CaO–MgO–Na2O–K2O system

The present study reports the impact of the introduction of nano-grained quartz (SA = 325 m2/g) into the composition on the structure and properties of the ceramic glaze of the CMAS-Na2O–K2O system. The results were compared to the glaze which had an identical oxide composition, with a difference that quartz was introduced in the form of quartz powder with a much smaller specific surface area SA = 1.41 m2/g. Both glazes are characterized by a large part of the glassy phase, above 90% by volume. The results obtained show a higher arrangement of the continuous glassy phase structure in the glaze with the addition of nano-quartz. This glaze also shows significantly higher values for all measured mechanical properties. It seems that if, in the near future, new cheaper methods for the production of nano-quartz are developed, it will be a new interesting direction of research aimed at improving the parameters of glazes and glass-ceramic materials of CMAS type.

ZCAS-assisting low-temperature hot-press sintering of SiC ceramic for immobilizing simulated radioactive graphite

The safe treatment and disposal of high-level radioactive graphite is an essential challenge in the governance of irradiated graphite. SiC has an insurmountable defect in densification sintering at low temperatures, although it is an ideal host material for immobilizing high-level radioactive graphite. To solve this issue, we employ ZCAS (short for ZnO–CaO–Al2O3–SiO2 glass) as a sintering aid to prepare SiC-ZCAS composite ceramic with the relative density up to 98% by vacuum hot-press sintering. For investigating the optimum formula and technological conditions of (1-x) SiC-x ZCAS composite ceramic, the effects of synthesis and sintering process on preparation (1-x) SiC-x ZCAS (x = 25-40 wt%) composites were investigated in detail. The results show that the SiC-ZCAS composite powder with x = 25-40 wt% can be synthesized at 1350 °C for 2 h when the Si/C mole ratio is 1.05:1. The relative density, Vickers hardness, and thermal conductivity of SiC-ZCAS composites ceramic increase rapidly by increasing sintering temperature and pressure. However, these properties will display different effects from the increase in the content of ZCAS, such as continuous elevation of the relative density and dramatic depression of thermal conductivity. the Vickers hardness starts to decline when the content of ZCAS is 30 wt%. Considering the above mentioned, we can conclude that the 0.7SiC-0.3ZCAS composite ceramic has choiceness comprehensive properties when sintered at 1550 °C and 60 MPa for 1 h, with the relative density, Vickers hardness and thermal conductivity of 95.5%, 1084 (HV 10) and 7.104 W/m·k, respectively.

Viscosity Estimation of Multicomponent Slags of the CaO–SiO2–Al2O3–FexO System Based on Microstructure Analysis

Slag viscosity is key factor of liquid steel refining and the smooth slag tapping of entrained flow coal gasifiers, and the accurate estimation of slag viscosity can provide a guidance for the selection of industrial raw material and the control of industrial process. In this study, a structure-related viscosity model about the multicomponent slags of the CaO–SiO₂–Al₂O₃–FeₓO system is developed based on the flow mechanism of silicate melt to improve slag viscosity behavior. The present estimation model improved the expression of activation energy equation on the basis of the model proposed by Nakamoto and expanded the application from the ternary CaO–SiO₂–Al₂O₃ system to the quaternary CaO–SiO₂–Al₂O₃–FeₓO system. Then, the structural roles of all the components were clarified, and the types of oxygen bonds were defined by Raman spectroscopy. The fractions of various oxygen bonds were obtained, and the database of oxygen bonds in any composition within the range of experimental slags was generated by utilizing the Lagrange interpolation method. The result showed that the present model had an advantage in estimating slag viscosity within the compositional range of this experiment slag, and the mean deviations between the viscosities calculated by the present model and measured or published viscosities were within 20%. The model will offer a reference for the estimation and adjustment of slag behavior in the metallurgical slag or coal slag, especially for the iron-rich system.

Transformation of Inclusions in a Complicated‐Deoxidized Heavy Rail Steels During Heating

The transformation of inclusions in a heavy rail steel during heating is investigated in laboratory experiments. When the steel is heated at 1000–1300 °C, inclusions of CaO–SiO2–Al2O3–MgO are transformed into CaS–MgO·Al2O3 ones and the contents of CaS and Al2O3 in inclusions increase, whereas those of CaO and SiO2 decrease. When the steel is heated at 1400 °C, the MgO·Al2O3 phase in inclusions precipitates, whereas CaS, CaO, and SiO2 still exist, which is caused by the lower thermodynamic driving force at high temperature. With the holding time increasing from 0 to 7 h, the composition of inclusions tends to the equilibrium value which is predicted using Factsage. The transformation phenomenon is validated using thermodynamic analysis and a kinetic model is established to predict the composition of inclusions during heating. The MgO·Al2O3 phase formed in inclusions in the steel during heating results in an exposure of MgO·Al2O3 spinel inclusions after rolling, which are rigid and harmful to the performance of steel rails.

Crush and Acid Resistance of Microsphere Narrow Fractions from Fly Ash as the Basis of Composite Materials

The crush test and acid resistance of microsphere narrow fractions with average diameter dav of 5, 8 and 25 μm of the SiO2–Al2O3–FeO system and with dav of 4 and 10 μm the CaO–SiO2–Al2O3– FeO system separated from fly ash from pulverized combustion of Ekibastuz and Irsha-Borodinsky coals were studied. It has been established that all investigated ash fractions of both raw materials are characterized by high strength: microspheres of a larger narrow fraction with dav = 25 μ m a re not destroyed by compressive loading at pressures up to 51.7 MPa, dispersed narrow fractions of microspheres with dav ≤ 10 μm – up to 68.9 MPa. Microsphere narrow fractions with aluminosilicate composition have satisfactory acid resistance; the weight loss after treatment with 15 % hydrochloric acid at 65 °C for 30 minutes was 10–15 wt. %

Effect of TiO2 on reduction behavior of Cr2O3 in CaO–SiO2–Al2O3–MgO–TiO2–Cr2O3 by carbon from Fe–C melt

The chromium-bearing titanomagnetite ore will turn to be the important raw material for blast furnace process in Panxi area, China. The reduction behavior of Cr2O3 between CaO–SiO2–Al2O3–TiO2–Cr2O3 and Fe–C systems was investigated. The effect of TiO2 content in the slag system on the reduction of Cr2O3, TiO2, and SiO2 and the consumption of C in hot metal was investigated by both theoretical calculation and physical experiment. The theoretical calculation results reveal that higher reduction temperature promotes the reduction of Cr and Ti, while high basicity and TiO2 content have little influence on the reduction of chromium but significantly influence the reduction of Ti. The smelting reduction experiment results show that the content of Cr in hot metal significantly increases with extending the reduction time and decreases with the increase in TiO2 content. However, the content of Ti in hot metal significantly increases as the TiO2 increases, reaching 0.073, 0.085, 0.107, and 0.121 wt.% for 5, 10, 15, and 20 wt.% of TiO2 input, respectively. Kinetic studies proved that the reduction of Cr2O3 was a first-order reaction. The addition of TiO2 inhibited the reduction of Cr2O3 and resulted in the decrease in reaction rate constant.

Effect of particle size distribution on the sinter-crystallisation kinetics of a SiO2–Al2O3–CaO–MgO–SrO glass-ceramic glaze

Sinter-crystallisation kinetics of a glaze with an average particle size of 9.6µm, 4.0µm and 0.65µm, which devitrified anorthoclase and diopside, were studied by non-isothermal techniques. It was confirmed that the crystallisation kinetics could be described by the Johnson–Mehl–Avrami–Kolmogorov model, assuming the Avrami index and activation energy to be independent of particle size. To describe the sintering kinetics, a kinetic model was developed, based on the Johnson–Mehl–Avrami–Kolmogorov model, assuming the effect of temperature on the process rate to be the same as its effect on the inverse viscosity of the glass matrix, and introducing a correction that took into account the difference between the heating rate of the kiln and that of the test piece. The pre-exponential factor of the developed model was confirmed to be proportional to the inverse of glaze average particle size. Glaze sinterability was observed to increase as the heating rate rose and/or average particle size decreased.