Diopside Phase Research Articles

The network structure and properties of a unique mining solid waste-based glass-ceramics

To alleviate the environmental pollution caused by the stockpile of mining-based, such as coal-based solid waste, mining solid waste-based CaO-MgO-Al2O3-SiO2 (CMAS) glass-ceramics with a diopside phase as the main crystalline phase was developed by one-step sintering method, and the effect of different solid waste content on the network structure and properties were investigated. The results show that, with the diversification of solid waste content, the crystallization temperature (Tp), the amount of Fe3+ concentration, and diopside crystalline, the main structure of glass Q2 changed also. Accordingly, significant changes have occurred in properties such as the bending strength, Vickers hardness, and density. Optimizing the raw material ratio, crystallization mechanism, a mining solid waste-based glass-ceramic with unique network structure is well developed and the bending strength, Vickers hardness, fracture toughness, and density reaches maximum values of 175.19 MPa, 10.47 GPa, 2.03 MPa⋅m1/2, and 3.00 g/cm3, respectively. The findings contribute significantly to the development of durable and reliable materials for the utilization of mining solid waste resources.

Thermodynamic and Experimental Studies of Selective Decomposition of Diopside in Ti-Bearing Blast Furnace Slag.

Titanium-bearing blast furnace slag (TBFS) is formed during the smelting of vanadium-titanium magnetite ore, containing more than 10 wt % Ti. The metal resource in TBFS has not yet been utilized because of the difficulty of extracting the objective phase from the complex eutectic. In this work, thermodynamic and experimental studies of selective leaching of diopside phase in TBFS in 20 wt % H2SO4 were conducted. The Gibbs free energy of reaction with H2SO4 calculated by applying a mechanical mixture model was in the order of Ti-bearing diopside < MgAl2O4 < Ti-rich diopside < CaTO3. The extraction rate of Ti ions was controlled by the leaching temperature and leaching time in 20 wt % H2SO4. The extraction rate of Ti reached as high as 0.50 under optimal conditions. The diopside phases in TBFS were completely decomposed, while the perovskite (CaTiO3) was almost unaffected by 20 wt % H2SO4. The experimental results agreed with the trend of thermodynamic calculations, supporting their validity. The hydrolysis of TiOSO4 showed the possibility of obtaining high-purity TiO2. This work shows the selective leaching of the diopside phase in TBFS and suggests an environmentally friendly method for utilizing Ti-bearing blast furnace slag and waste acid.

OPTIMIZING VANADIUM CONVERTER SLAG UTILIZATION: TARGETED ENRICHMENT AND STABILIZATION OF VANADIUM THROUGH NON-EQUILIBRIUM SOLIDIFICATION

The objective of this research was to optimize the comprehensive utilization of vanadium converter slag through targeted enrichment and stabilization of heavy metal vanadium. Employing the non-equilibrium solidification theory and FactSage software, we investigated the potential of modifying vanadium converter slag. When the original slag failed to generate vanadium-rich spinel with usable V elements, introducing modifying agents Fe and Al proved effective. Fe facilitated the enrichment of Cr within spinel, while Al significantly promoted the V enrichment. Expanding on this, we systematically examined the influence of Fe2O3, Al2O3 and MgO contents on spinel phase precipitation during vanadium slag solidification. The addition of Al resulted in the precipitation of corundum, hematite, spinel, olivine and diopside phases. With an increase in the Fe2O3 content, the precipitation of FeV2O4 and MgV2O4 initially increased, peaking at 9.67 % before subsequently decreasing. Maintaining the Fe2O3 content within a range of 25–30 % proved optimal for enhancing vanadium precipitation and enrichment. In contrast, variations in the Al2O3 content had minor impacts on SP-V phase precipitation, with slight effects on FeV2O4 reaching 10.34 %. Furthermore, the incorporation of MgO facilitated the precipitation of MgV2O4 while concurrently suppressing the FeV2O4 precipitation. By judiciously controlling the MgO content at approximately 20 %, vanadium enrichment in the form of FeV2O4 and MgV2O4 spinel phases reached a remarkable 94.46 %.

Influence of Solid Waste Material Content on the Properties of Steel Slag-Waste Clay Brick Ceramic Bricks

Steel slag and waste clay brick are two common solid wastes in industrial production, and their complex chemical compositions pose challenges to the production of traditional alumina silicate ceramics. To investigate the influence of steel slag and waste clay brick on the performance of CaO–SiO2–MgO ceramic materials, this study examined their effects on the mechanical properties, crystal composition, and microstructure of the ceramics through single-factor experiments. The results demonstrate that when keeping the dosage of waste clay brick and talcum powder constant, a 43% dosage of steel slag yields optimal performance for the ceramic materials with a modulus of rupture of 73.01 MPa and a water absorption rate as low as 0.037%. Similarly, when maintaining a constant dosage of steel slag and talcum powder, a 41% dosage of waste clay brick leads to superior performance of the ceramic materials, with a modulus of rupture reaching 82.17 MPa and a water absorption rate only at 0.071%. Furthermore, when keeping the dosage of steel slag and waste clay brick constant, employing a talcum powder dosage of 24% results in excellent performance for the ceramic materials with a modulus of rupture measuring 73.01 MPa while maintaining an extremely low water absorption rate at only 0.037%. It is noteworthy that steel slag contributes to akermanite phase formation while talcum powder and waste clay brick contribute to diopside phase formation.

Analysis and Mechanism Study of Residual Stress during the Spontaneous Crystallisation Process of Molten Titanium-Containing Blast Furnace Slag

Molten titanium-containing blast furnace slag can be used to obtain cast stone materials by controlling a reasonable heat treatment system. The material acquired during this process showcases residual stress, which additionally impacts the macroscopic characteristics of the material. This article simulates the process of manufacturing microcrystalline cast stones based on the self-crystallisation ability of titanium-containing products. This research employs X-ray diffraction to precisely and conveniently assess the residual stress of microcrystalline cast stones and investigates how viscosity and the thermal expansion coefficient influence the residual stress level. The study provides a theoretical foundation for explaining titanium-containing blast furnace slag and combines characterisation methods such as XRD (X-ray diffraction), SEM (Scanning electron microscope), DTA (Differential thermal analysis), and theoretical calculations such as Factpage and Fullprop to study the effect of the TiO2 content on the microstructure of self-crystallised mechanical characteristics of microcrystalline cast stones through residual stress. The results of the experiment indicate that as the TiO2 content in the system increases, the glass phase is reduced, the crystallinity improves, and the main crystal phase changes from a feldspar phase to a diopside phase. Furthermore, its viscosity, thermal expansion coefficient, and residual stress decrease while its corresponding compressive strength and bending strength increase.

Machinable diopside-lanthanum phosphate composite ceramics for fabricating load bearing bone implants

Bioceramic-based composites are extensively being used in orthopedics for decades due to their excellent biocompatibility and osteoconductivity. However, the inherent brittleness and high hardness of ceramics heavily impact their machinability, thus restricting their use as bone fixation devices. In this study, Mg-doped calcium silicate ceramic diopside (DI) has been employed as a ceramic matrix material owing to its superior mechanical properties in comparison to hydroxyapatite. To impart machinability in the diopside, rare earth lanthanum phosphate (LP) was introduced as a reinforcement. The indigenously synthesized and 800 °C calcined DI and LP powders were employed for composite fabrication by varying LP content (0–50% w/w). The composites were sintered at different temperatures (1000 °C, and 1200 °C) and sintering conditions (single sintered, and double sintered), and the 1200 °C double sintered (ds) composites were optimized due to their highest densification and mechanical properties. Among the optimized composites, ds-DI-LP (50:50, DL50) displayed the highest compressive strength (140 ± 5.21 MPa) compared to DL5 (121.16 ± 14.82 MPa) and DL10 (122.78 ± 28.74 MPa) composites. It was observed that the weak and layered LP interface in between the DI phase promoted machinability in the composites. Apart from being machinable, these ceramic composites were found to be bioactive, resulting in the formation of an apatite layer when immersed in simulated body fluid. Further, the optimized ceramic composites were found to be biocompatible and osteoconductive in nature on their functional assessment using primary rat calvarial osteoblast cells. Based on the results obtained, the ds-DL10 composites displayed considerable bioactivity and the highest ALP activity compared to ds-DL5 (p ≤ 0.01) and ds-DL50 (p ≤ 0.05) composites.

Microstructure and mechanical properties of SiO2-Al2O3-CaO-ZrO2 based glazes containing zircon and diopside phases

ABSTRACT MgO was added to the SiO2-Al2O3-CaO-ZrO2-based glaze to produce a high-hardness glaze by forming a composite crystal phase. The effects of MgO addition on the surface properties of the glaze were analyzed through crystal phase and microstructural investigations, and the whiteness, roughness, gloss, wear rate, and hardness were measured. When the amount of added MgO increased by 2.20 wt% or more, the diopside (MgCaSi2O6) crystal phase was observed via X-ray diffraction (XRD) and scanning electron microscopy (SEM), in addition to the zircon (ZrSiO4) crystal phase. All samples exhibited high whiteness and the surface roughness increased with increasing MgO content. At the same time, the gloss decreased significantly, resulting in a matte. The Vickers hardness test results of the glazed surface indicated high hardness values of 6.23–7.07 GPa with increased amounts of MgO.

Site preference-based luminescence studies in Eu doped calcium magnesium silicate phosphor: A combined experimental and DFT approach

The luminescence of Eu3+ doped calcium magnesium silicate (CMS: Eu3+) phosphor is studied experimentally by optimizing the single diopside structure in the solid state reaction method and theoretically verified using the first principle density functional theory (DFT) calculation. Field Emission Scanning Electron Microscopy (FESEM) analysis shows a transition from agglomerated morphology to particle formation at 900 °C. A combination of diopside and akermanite structures are formed at 800 °C, whereas a single diopside phase is optimized from 900 °C onwards. DFT calculation has been carried out to identify the lattice parameters of the diopside structure of calcium magnesium silicate (CMS) and Eu3+ doped CMS by replacing Eu3+ in the lattice site of Ca and Mg to identify the stable structure. The lattice expansion is minimal when Eu3+ replaces the Ca2+ position. Lattice parameters of the diopside phase calculated using Rietveld refinement of the XRD pattern are in agreement with DFT findings. FTIR analysis confirms a redshift corresponding to Ca–O and a blueshift corresponding to Mg–O stretching wavenumbers due to the structural modification of CMS due to Eu addition in the lattice. The stability and site preference of Eu3+ ion in CMS is determined from the mixing energy (ΔHm) of Eu doped in the Ca site and Mg site of CMS separately via DFT calculation and confirm that the mixing energy is minimum when Eu3+ replaces Ca lattice position. UV–visible, DRS spectroscopy confirms a bandgap of ∼6 eV for CMS and CMS: Eu3+. Luminescence studies confirm constant peak splitting in the 5D0→7FJ (J = 1,2,4) transitions due to the crystal field effect caused due to the incorporation of Eu3+ in the diopside structure with dodecahedral coordination. Eight coordinated site preference of Eu3+ in diopside is confirmed via DFT calculations. Modification in asymmetry ratio (R-ratio) with temperature also corroborates with modification in the intensity of photoluminescence spectra dictated by the reduction in inversion symmetry. CIE color chromaticity analysis shows a coordinate of (0.61, 0.39) in the red region of visible spectra with a color purity of 83.49% irrespective of annealing temperature.

Cobalt-containing diopside pigments based on granulated blast furnace slag

PurposeThe purpose of this study was to the low-temperature synthesis of cobalt-containing diopside pigments based on granulated blast furnace slag and to study the characteristics of the mineral formation processes, changes in the structure and colour indices.Design/methodology/approachSynthesis of cobalt-containing diopside pigments based was carried out by the directional formation of the mineralogical composition with the introduction of part of the components using granulated blast-furnace slag.FindingsIt has been established that the formation of the diopside phase in pigments containing blast-furnace slag as the main component proceeds at low temperatures (1,100°C–1,150 °C). The colour of diopside pigments is formed because of the isomorphic substitution of Si4+ ions for Al3+ ions and Mg2+ ions for Co2+ ions. It is expedient to add CoO in an amount of 0.9 mol (18 Wt.%) into the composition of diopside pigments based on blast-furnace slag to obtain defect-free violet glazes.Practical implicationsThe developed diopside pigments enable obtaining of high-quality violet glazes for ceramics. The application of the obtained results can significantly reduce the consumption of traditional raw materials in the composition of silicate ceramic pigments, as well as reduce their firing temperature.Originality/valueCalcium, magnesium and silicon oxides are the main components of blast-furnace slag. In addition, granulated blast furnace slag is mainly represented by the glassy phase, which determines its high activity during the firing process. These factors are prerequisites for using the blast-furnace slag as a valuable substitute for chemically pure or natural raw materials in silicate pigments and reducing their firing temperature.

One‐Step Preparation of Glass‐Ceramics from a Ferrochrome Slag and an Electrolytic Manganese Slag

Electrolytic manganese slag materials can be used in the preparation of glass‐ceramics. Herein, glass‐ceramics with excellent properties are obtained by melting electrolytic manganese slag and ferrochrome slag as raw materials. The effects of the addition of ferrochrome slag on the recrystallization of the base glass and the physicochemical properties of the glass‐ceramic are especially investigated. With the addition of ferrochrome slag, the introduced Cr2O3 may play a role in the deconstruction of the glass network and is more likely to produce a spinel phase as a way to promote the heterogeneous nucleation of the diopside phase. The sample with a ferrochrome slag addition of 12 wt% shows the optimal recrystallization ability. Furthermore, the ratio of nonbridging oxygen atoms (NBO) to the total amount of oxygen atoms (T) is the highest (NBO/T = 1.90). Also, the bending strength and the density of the samples reach 96.08 MPa and 3.05 g cm−3, respectively, and the acid and alkali resistance become 98.16% and 99.48%, respectively. Moreover, the glass‐ceramic structure did effectively stabilize and solidify the harmful heavy metal elements. This study has provided innovative ideas for an efficient, harmless, and low‐cost treatment of electrolytic manganese slag in the preparation of glass‐ceramics.

Microstructural characterization of AP40 apatite-wollastonite glass-ceramic

The microstructure of an apatite-wollastonite (code name AP40) glass-ceramic is analyzed in this study by combining 2D microscopy, phase analysis, X-ray absorption and synchrotron X-ray refraction computed tomography (XCT and SXRCT, respectively). It is shown that this combination provides a useful toolbox to characterize the global microstructure in a wide scale range, from sub-micrometer to millimeter. The material displays a complex microstructure comprising a glassy matrix with embedded fluorapatite and wollastonite small crystals. In this matrix, large (up to 200 μm) spike-shaped structures are distributed. Such microstructural features are oriented around a central sphere, thereby forming a structure resembling a sea urchin. A unique feature of SXRCT, in contrast to XCT, is that internal interfaces are visualized; this allows one to show the 3D distribution of these urchins with exceptionally good contrast. Furthermore, it is revealed that the spike-shaped structures are not single crystals, but rather composed of sub-micrometric crystals, which are identified as fluorapatite and diopside phases by SEM-EDX analysis.

Effect of Cr2O3 on Crystallization of Diopside Glass–Ceramics

CaO–MgO–Al2O3–SiO2–Cr2O3 diopside glass–ceramics were prepared from blast furnace slag, low-carbon ferrochromium alloy slag, and quartz sand by the melting method. The prepared glass–ceramics were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD),scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS). The effect of Cr2O3, a nucleating agent, in the crystallization process of diopside glass–ceramics was studied. The results show that chromium is present in glass–ceramics as Cr3+ and Cr6+, and Cr3+ accounts for more than 80% of the chromium contents. When the mass percentage of Cr2O3 in glass–ceramics is less than 1.60%, a small amount of diopside phase is precipitated during heat treatment, and Cr3+ is dispersed in the diopside phase. When the mass percentage of Cr2O3 reaches or exceeds 1.60%, Cr3+ preferentially forms the magnesia chrome spinel phase, which further induces the in situ growth of diopside. The leaching concentration of chromium meets the Chinese national standard, indicating that diopside glass–ceramics can effectively solidify the heavy metal chromium, and this fact makes the application of glass–ceramics feasible.

Thermodynamic analysis of the reactions of diopside phase formation during synthesis of ceramic pigments from granulated blast-furnace slag

The purpose of this study is to perform the thermodynamic analysis of the reactions of diopside phase (CaOMgO2SiO2) formation during the synthesis of ceramic pigments based on the granulated blast-furnace slag. Thermodynamic analysis is of great practical importance in studying solid-phase interactions, which are involved in the pigment technology. At the same time, the scope of energy-intensive experimental studies of ceramic pigments is significantly reduced. When performing thermodynamic calculations, we assessed the fundamental possibility and direction of occurrence of chemical reactions by determining the changes in the Gibbs energy. In order to verify the calculation data, the change in the mineralogy of the diopside compositions was evaluated with the use of X-ray phase analysis at different firing temperatures. It was found that the diopside phase in the CaO–MgO–Al2O3–SiO2 system is formed in several stages. The merwinite (3CaOMgO2SiO2) mineral is formed first. Further, merwinite, along with blast-furnace slag minerals, is involved in the diopside formation. The formation of diopside is completed at the temperature of 12000С. The findings of the study will provide a reasonable approach to the choice of compositions of ceramic pigments using complex raw materials, including the wastes of various industries.

A study of the solidification and stability mechanisms of heavy metals in electrolytic manganese slag-based glass-ceramics.

To better solve the waste pollution problem generated by the electrolytic manganese industry, electrolytic manganese slag as the main raw material, chromium iron slag, and pure chemical reagents containing heavy metal elements mixed with electrolytic manganese slag doping. A parent glass was formed by melting the slag mixture at 1,250°C, which was, thereafter, heat-treated at 900°C to obtain the glass-ceramic. The results from characterizations showed that the heavy metal elements in the glass-ceramic system were well solidified and isolated, with a leakage concentration at a relatively low level. After crystallization, the curing rates of harmful heavy metals all exceed 99.9%. The mechanisms of heavy metal migration, transformation, and solidification/isolation in glass-ceramic curing bodies were investigated by using characterization methods such as chemical elemental morphological analysis, transmission electron microscopy, and electron microprobe. The most toxic Cr and Mn elements were found to be mainly kept in their residual state in the glass-ceramic system. It was concluded that the curing mechanism of the heavy metals in a glass-ceramic can either be explained by the chemical curing induced by bonding (or interaction) during phase formation, or by physical encapsulation. Characterization by using both Transmission electron microscopy and EPMA confirmed that Cr and Mn were mainly present in the newly formed spinel phase, while the diopside phase contained a small amount of Mn. Zn, Cd, and Pb are not found to be concentrated and uniformly dispersed in the system, which is speculated to be physical coating and curing.

The effect of titanium slag on the properties of glass-ceramic composites from South African coal fly ash

In this study, the impact of titanomagnetite slag on the structure of glass-ceramic composites was investigated. The slag was added as a source of titania which promotes nucleation within the parent glass matrix and magnesium oxide which facilitates crystallisation. A mixture of fly ash and varying quantities of waste beverage glass and titanomagnetite slag were melted. The melts were then annealed at lower temperatures to reduce the internal stress and, subsequently, cooled to room temperature to produce parent glasses. The mass of fly ash was held constant at 60%, while the mass of the waste beverage glass and titanomagnetite slag was varied between 10-30% and 0-30% respectively. Afterwards, the parent glasses were converted to glass-ceramic composites through a controlled double-staged thermal treatment. Characterisation of these composites showed that an increase in titanomagnetite slag resulted in many crystal phases forming within the glass-ceramic matrix and an increased porosity. The desired diopside phase was not detected in the composites same as when pure magnesium oxide was added instead of the titanomagnetite slag. The compressive strengths, the bulk densities and melting temperatures of the glass-ceramic composites were found to be lower as compared to glass-ceramic composites produced from a mixture containing pure magnesium oxide. However, they were highly resistant to attack by nitric acid and sodium hydroxide.

Effect of calcination temperature on use of high-boron-content waste for low-temperature wall tile production

The effects of the calcination temperature on raw-colemanite-waste properties and calcined waste content on wall tile production were investigated. Waste containing 11.24% B2O3 calcined between 500 and 800°C was added to wall tile granules in various ratios (0–100 wt.%) to produce a low-temperature-sintered wall tile by adding the maximum content of boron waste, as determined through optimal calcination. The low-temperature (850–1000°C) sinterability of the samples and the effect of the calcined colemanite-waste content on the wall tile properties were investigated. The samples were characterised using X-ray fluorescence, X-ray powder diffraction, differential thermal analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, and colourimetry. The waste calcined at 800°C exhibited a substantially different phase distribution, bond structure, morphology, and colour. The wall tile produced using 40 wt.% colemanite waste calcined at 800°C and subsequently sintered at 950°C exhibited the optimal properties. The linear firing shrinkage, water absorption, and flexural strength of the optimised wall tile were 0.88%, 16.04%, and 36.07 MPa, respectively. The optimised wall tile exhibited major albite, quartz, and diopside phases and 64% higher strength. The sample calcined at 800°C showed that high colemanite-waste content could be incorporated into ceramic bodies.

Mixed phase bioceramics in the CaMgSi2O6 – MoO3 system: Mechanical properties and in-vitro bioactivity

Mixed phase materials in the quasi binary diopside (CaMgSi2O6) – molybdite (MoO3) system were synthesized by a precipitation method. Materials were fabricated with diopside to molybdite ratios of 1:0, 10:1, 5:1, 2:1 and 1:1. XRD, SEM and EDS results show that alongside the initial diopside phase, phases such as calcium molybdate CaMoO4, rod-like enstatite MgSiO3 and cristobalite SiO2 formed as the molybdite content increased, and diopside was entirely absent at the highest molybdite content. At lower Mo content, mixed phase materials showed higher hardness and slower biodegradation in SBF relative to pristine diopside, while maintaining reasonable hydroxyapatite (HAp) formation capability. In contrast, materials with higher molybdite content exhibited lower hardness and bioactivity. The variation in the mechanical and bioactive performance could be attributed to the presence of bulk CaMoO4, acting as a reinforcement, and rod-like MgSiO3 with a highly porous and fragile structure. The trend of hardness is not consistent to the proportion of the component phases could be attributed to morphologies, interfaces, and densities of the samples. Both of secondary phases had poorer HAp deposition compared to pure diopside, indicating the MoO3 addition lowered mixed phase CaMgSi2O6 – MoO3 bioceramics’ ability to form Hap. The results suggest that moderate addition of molybdite to diopside would be an effective pathway towards crystalline bioceramics with enhanced performance.

Catalytic mechanism of iron oxide doping on the crystallization process of Cr2O3-containg glass ceramics

The influence of Fe2O3 addition to the nucleation process of Cr2O3-containing CaO-MgO-Al2O3-SiO2 (CAMS) based glass ceramic was investigated by Raman spectroscopy, differential scanning calorimetry (DSC), X-Ray diffraction (XRD), electron back-scattered diffraction (SEM-EBSD) and transmission electron microscope (TEM). The experimental data illustrated that the single Cr2O3-doped specimens were insufficient in achieving the desired nucleation rate for high degree of polymerization. However, with the introduction of Fe2O3 to the Cr2O3-containing samples, the newly generated [FeO4] mended the glass network by inserting [Fe3+O4-SiO4] unit into the original Si-O network, which was helpful for the migration of the coordination polyhedron in the glass matrix. Additionally, [FeO4] unit and [MgO4] are equivalent when generating spinel crystal. Hence, the generated [FeO4] benefited the formation of spinel and diopside phases. This resulted in massive fine spinel crystal distribution in the glass matrix, which promoted the precipitation of diopside. Crystallographic examination confirmed the formation of partial diopsides on the spinel {111} lattice plane with epitaxial growth. The orientation relationships were 〈112〉spinel//〈001〉diopside and (111)spinel//(002)diopside. However, when higher Fe2O3 (that is, 2 wt% Cr2O3+15 wt% Fe2O3) was added to the glass system, abnormal grain growth of spinel occurred, which resultantly reduced the nucleation rate.

Migration and transformation of heavy metals in glass-ceramics and the mechanism of stabilization

There are many kinds of heavy metals in tailings and other solid waste, and the content is high. Glass-ceramic as a promising final product not only has excellent physical properties, but also can effectively solidify heavy metals. However, the study on the stabilization of single heavy metal in glass-ceramics is still limited. Therefore, zinc and copper, two common heavy metals in tailings, were used as representatives to conduct this research to find the phase transformation and the stabilization mechanism. X-ray diffraction (XRD), Scanning electron microscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and Inductively coupled plasma source mass spectrometer (ICP-MS) were used as the characterization methods. The results showed that the main crystallization phase was diopside (CaMgSi2O6). With the increase of zinc and copper content, the integrity of glass network structure got worse, and the viscosity of glass was decreased. So, the volume density and shrinkage of glass-ceramics were also increased. The shrinkage for glass-ceramics with different content of zinc was from 16.1% to 20.2%, and the volume density was increased from 2.3 g/cm3 to 2.54 g/cm3. The glass-ceramics with different content of copper shows similar trend. The shrinkage ranged from 16.1% to 20.4%, and the volume density was increased from 2.32 g/cm3 to 2.56 g/cm3. For samples with different zinc content, the bending strength was increased from 135 MPa to 161 MPa. On the contrary, for samples with different content of copper first increased to 148 MPa and then decreased to 133 MPa. The curing process of zinc and copper in glass-ceramic shows different trends. When the content of heavy metal is less than 1.5 wt%, zinc and copper all remain in the diopside phase in the form of solid solution. With increased of the heavy metal content, zinc is stably cured in the glass-ceramic as the petedunnite phase (CaZnSi2O6), while copper is remained in the glass-ceramic in the phase of CuAlO2. Heavy metals are well solidified in glass ceramics, so the leaching concentration is much lower than the standard limit, and the leaching rate tends to be stable when the heavy metal content up to 1.5 wt%. The results showed that the conversion of Zn-rich and Cu-rich tailings and other solid waste into environmentally friendly glass-ceramics had great potential and reliability.

Migration, transformation and solidification/stabilization mechanisms of heavy metals in glass-ceramics made from MSWI fly ash and pickling sludge

In consideration of recycling solid waste to achieve high value-added products, glass-ceramics have been fabricated from municipal solid waste incineration (MSWI) fly ash, pickling sludge (PS), and waste glass (WG) by melting at 1450 °C firstly to achieve parent glass and then crystallizing at 850 °C. Results demonstrated that heavy metals have been well solidified in the prepared glass-ceramics, and relatively/extremely low leaching concentrations of heavy metals have been detected. The synthetic toxicity index of heavy metals has been greatly reduced from 7-18 to <3.2 after crystallization treatment, and the leaching concentrations of Cr, Ni, Zn, Cu, and Pb are 0.15, 0.05, 0.26, 0.12, 0.19 mg L-1 respectively. Chemical morphology analysis, principal component analysis, TEM and EPMA were utilized to clarify the migration, transformation, and solidification mechanism of heavy metals from the as-received solid wastes. The major heavy metals, Cr and Ni which is responsible for the most toxicity, mainly exist in form of the oxidation state and residual state in parent glass, while the residual state in the glass-ceramics. The solidification performance was mostly positively correlated with the form of residue state, which the stability of heavy metals in glass-ceramics is improved. The solidification mechanism of heavy metals in glass-ceramics could be explained by the combination of chemical solidification/stabilization and physical coating. The TEM and EPMA confirmed that Cr and Ni mainly exist in the spinel crystalline (NiCr2O4, Fe0.99Ni0.01Fe1.97Cr0.03O4) by solid solution or chemical substitution, and a small amount of Cr in the diopside phase. Pb, Cu, and Zn are homogenously dispersed in the glass-ceramics, which is considered as physical coating solidification.