Calcium Hexaluminate Research Articles

Corrosion resistance of calcium hexaaluminate insulating firebrick for synthesising ternary lithium-ion battery cathode materials

The widespread use of ternary lithium-ion battery cathode Li(NixCoyMn1−x−y)O2 (LNCM) materials in energy storage equipment has resulted in an increase in the demand for insulating refractory of roller kiln. However, the severe corrosion during LNCM calcination will result in desquamate and damage of the insulating refractory. In this study, the corrosion resistance of bubble alumina brick (bubble alumina), mullite and CA6 (calcium hexaaluminate) insulating refractories was thoroughly investigated. This study combined the laboratory scale experiments on the interfacial reaction and after life cycle analysis on industrial insulating refractory to investigate the interactions between the insulating refractories and LNCM precursor mixture powders after the calcination temperature of 950 °C for 10 h and the corrosion behaviour of the LNCM precursor to different insulating fire bricks after the heat treatment temperature of 950 °C for 10 h every time and repeated 10 times according to the actual LNCM preparation system. The phase composition and microstructure of the three insulating refractories before and after corrosion were characterised to obtain a comprehensive understanding of the corrosion mechanism and behaviour. Results showed that the CA6 has excellent corrosion resistance and has the potential to be used in the lithium battery industry.

Improving the steel corrosion resistance of calcium hexaaluminate with addition of M2O5 (M = Nb, Ta)

Calcium hexaaluminate (CA6) materials have garnered attention for their potential use as lining materials in metallurgical furnaces during the steel-making process. The present work explored the effects of M2O5 (M = Nb, Ta) on the steel corrosion behaviour of calcium hexaaluminate combined with the evolution of physical properties. The introduction of M2O5 (M = Nb, Ta) was proved to be advantageous in mitigating the emulsification of CA6 material in the molten steel, and the increase in the densification of emulsion layer could prevent CA6 materials from further steel corrosion and penetration. The improved steel corrosion resistance of CA6 materials was attributed to the optimization of the physical properties facilitated by the formation of defects and the in situ phase. In addition, the reduction in the wettability between the CA6 materials and molten steel served to diminish the reactivity of the interface reaction. This work determined the corrosion behaviour and corrosion mechanism of the CA6 materials in molten steel and provided a new research route for designing CA6 materials with improved corrosion resistance to molten steel.

Exploring the limits of Fe3+ solid solution in calcium hexaluminate

Calcium hexaluminate (CA6) forms a broad range of solid solutions by substitution of Al3+ with other cations such as, for example, Fe, Co, Mn, etc. Solid solutions of CA6 with magnetic ions have attracted interest because they allow designing the magnetic behavior depending on the composition of each solid solution. In this paper, we have studied materials pertaining to an area of the CaO–Al2O3–Fe2O3 ternary phase diagram that is potentially interesting, but which has not been too much explored so far. We report on various synthesis methods of calcium hexaluminate with iron in solid solution. The percentages of iron introduced have been varied in order to determine the maximum percentage of iron that can enter into solid solution in the calcium hexaluminate such that the materials obtained exhibit magnetic behavior.Graphical Exploring the limits of Fe3+ solid solution in calcium hexaluminate.

Low-temperature fabrication and properties of porous calcium hexaluminate (CA6) ceramics with low thermal conductivity by one-step method

In this study, one-step molten salt method followed by low-temperature sintering is employed for fabricating porous calcium hexaluminate (CA6) ceramics with ideal plate-like structure and low thermal conductivity via the assistance of CaCl2. The porous CA6 ceramic containing 15 wt% CaCl2 sintered at 1400 °C for 3 h exhibits desirable compressive strength (7.14 MPa), low bulk density (1.47 g/cm3), high porosity (61.12%), and low conductivity (0.439 W/(m·K)). The results imply that CaCl2, as a pore-forming agent and an additional source of CaO, provides a large number of liquid phases and resulting in quantities of closed pores. Moreover, the reactions between molten Al2O3 and CaCl2 alter the mass transfer modes of Al2O3 and accelerate its phase transition to α-Al2O3 in addition to contributing to the conversion of CaCl2 to CaO and the subsequent formation of CA6, which results in the entire synthesis of low-temperature CA6 at around 1400 °C. Therefore, there is no need for extra salt washing treatment owing to the consumption and evaporation of CaCl2, which makes this one-step molten salt method much more advantageous in terms of its economical and highly efficient.

Phase evolution and lightweight application of dolomite@Al2O3 spherical particle with core–shell structure

AbstractDolomite@Al2O3 spherical particles with core–shell structure were fabricated by using the dolomite powders as the core and wrapping corundum powders on their surface. The phase evolution and microstructure of the dolomite@Al2O3 spherical particles with different firing temperatures were studied. As the spherical particles fired at different temperatures, the dolomite in the spherical core decomposed into MgO and CaO. Due to the Kirkendall effect, MgO and CaO diffused into the shell of the spherical particle and left Kirkendall holes in the core. The holes produced by the Kirkendall effect and the decomposition of dolomite act together to form a hollow structure in the center. And during the diffusion process, MgO and CaO reacted with corundum to generate spinel and calcium hexaaluminate. A small amount of spinel and calcium hexaaluminate can form a ternary compound Ca2Mg2Al28O46. Moreover, simulation software Factsage and Abaqus were used separately to help prove the phase evolution and the introduction of spherical particles to the properties of refractories. To test the lightweight effect of dolomite@Al2O3 spherical particles on refractories, it was introduced into spinel–corundum refractories as medium particles in different content and fired at different temperatures. The results show that after firing at 1650°C for 3 h with introducing 30% dolomite@Al2O3 spherical particles, the samples can reach a high compressive strength (128 MPa), high refractoriness under load (1683°C), and low thermal conductivity (1.79 W (m K)−1).

Physical properties and corrosion resistance of magnesia-calcium hexaaluminate refractories to molten steel

The continuous casting tundish serves as the final vessel for molten steel in the smelting process. Resistance to steel corrosion and penetration is critical for clean steel smelting and the durability of working linings. In this study, calcium hexaaluminate (CA6) was selected to gradually replace the commonly used MgO refractory. We analyzed the phase composition, cold modulus of fracture, cold crushing strength, bulk density, porosity, and resistance to molten steel corrosion and penetration of MgO–CA6 refractories. It was observed that the mechanical properties of the material were reduced by the addition of CA6; however, a certain percentage of CA6 and MgO formed a small amount of MgO·Al2O3 at high temperatures to retain its high performance. The materials are less likely to react with [Mn], [Si] and other alloying elements in steel after the addition of CA6 and it exhibits excellent resistance to steel corrosion, which is important for clean steel production and the durability of working linings.

Preparation and characterization of calcium hexaaluminate (CA6) porous ceramic for application in high-temperature flue gas filtration

Preparation and characterization of calcium hexaaluminate (CA6) porous ceramic for application in high-temperature flue gas filtration

Anti-corrosion effect of insulating firebrick coated with CA6 in the calcination of lithium-ion cathode materials

The anti-corrosion properties of calcium hexaluminate (CA6) during the roasting of Li(NixCoyMnz)O2 (LNCM) materials have been investigated by means of specific experimental setups and test methods to verify its feasibility as an adiabatic insulation material for the roasting kiln of LNCM materials. Mullite insulating firebrick currently in industrial use was coated with CA6 (labelled M-CA6) and corrosion tests were carried out. The corrosion tests were carried out at different additions of corrosion sources, roasting temperatures, number of cycles of corrosion experiments. And the materials before and after corrosion characterized by the X-ray diffraction and scanning electron microscopy to determine the influence of different corrosion conditions on the interactions between M-CA6 and LNCM materials. In addition, The corrosion resistance of calcium CA6 and the feasibility of its application in practical industry was identified by characterising and comparing the high-temperature creep properties of the materials. The results of the laboratory scale experiments confirmed that M-CA6 was more resistant to LNCM than mullite insulating firebrick, and confirmed the feasibility and superiority of using CA6 as a lining insulation for LNCN kilns.

Alumina-based ceramics reinforced with calcium hexaaluminate

In this study, alumina ceramics containing calcium hexaaluminate are investigated. The specimens are obtained by axial pressing of granulated powders and pressureless sintering. Fracture toughness is tested using the indentation method. Microhardness is determined by the Vickers method. XRD and microstructural studies using a scanning electron microscope are carried out. It was found that the use of Ca(OH)2 leads to the formation of CaAl12O19 in sintered alumina ceramics. As the platelet content increases, the average length of the platelets increases from 2 to 3 µm. The width of the platelets is about 0.4 µm. Increasing the CaAl12O19 content decreases the size of Al2O3 grains. The average grain size of the alumina ceramics is 1.65 ± 0.02 µm. For the material containing 6 wt.% CaAl12O19 the average grain size is 0.95 ± 0.05 μm. An increase in the critical stress intensity factor for the formation of 6 wt.% CaAl12O19 in a material compared to alumina ceramics without additives has been established. The relative density of such a material is 95.3 ± 0.5 %, microhardness is 1800 ± 50 HV, fracture toughness is 5.2 ± 0.4 MPa ∙ m1 / 2. The increase in fracture toughness of composites containing CaAl12O19 platelets is due to the fracture of platelets, crack bridging and crack deflection.

Preparation of calcium hexaluminate porous ceramics by gel-casting method with polymethyl methacrylate as pore-forming agent

Calcium hexaluminate (CA6) porous ceramics were prepared by gel-casting method, with α-Al2O3 and CaCO3 as raw materials and polymethyl methacrylate (PMMA) microspheres as pore-forming agent. The effects of the amount of pore-forming agent PMMA microspheres on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, thermal shock stability and thermal conductivity of CA6 porous ceramics were systematically studied. The pores of CA6 porous ceramics are mainly formed by the burning loss of PMMA microspheres and the decomposition of organic matter. Adding an appropriate amount of PMMA microspheres as pore-forming agent has a positive effect on the thermal shock stability of CA6 porous ceramics. When the amount of pore-forming agent is 15 wt%, the volume density of CA6 porous ceramics is 1.33 g/cm3, the porosity is 63%, the flexural strength is 13.9 MPa, the thermal shock times can reach 9 times, and the thermal conductivity is 0.293 W/(m·K), which can meet the application in refractory, ceramics or high temperature cement industries.

Preparation and characterization of low-cost refractories for aluminum-melting-furnace linings from ferrotitanium slag

Management and utilization of solid waste are necessary for sustainable development in the metallurgical industry. This study focuses on the use of ferrotitanium slag (FTS) as a low-cost source of hibonite in the refractory lining of an aluminum-melting furnace. FTS enhances the resistance of castable samples to cryolite melt penetration but reduces their bulk density and cold mechanical strength. To explain this enhancement mechanism, we present a model in which the Al2O3 and SiO2 components in the castable samples react with the cryolite melt to form a high-viscosity barrier layer against the molten cryolite. Moreover, the well-developed calcium hexaaluminate crystals in FTS, which show excellent resistance against Na+ and F−, prevent the penetration of cryolite melt through the FTS aggregate area, and reduce the penetration speed. By promoting high-efficiency resource utilization, this study contributes to the development of a circular economy in the metallurgical industry.

Corrosion of Alumina-Spinel Refractory by Secondary Metallurgical Slag Using Coating Corrosion Test.

In this paper, the corrosion mechanism of commercial alumina-spinel refractory was investigated at 1350 and 1450 °C. Disc samples were coated with shells of two different slags containing 4 and 10 wt.% SiO2. The after-corrosion refractory was investigated in view of changes in its microstructure and phase composition by SEM/EDS and XRD techniques, respectively. At 1350 °C slags slightly infiltrated the microstructure, whereas at 1450 °C slags infiltrated the alumina-spinel refractory causing its significant corrosion. As a result of corrosion, new phases were formed, including calcium dialuminate (Ca2Al4O7), calcium hexaluminate (CaAl12O19), and gehlenite (Ca2AlSi2O7). Formation of calcium aluminate layers in the microstructure of the refractory inhibited further dissolution of alumina aggregates; however, expansive behavior of CaAl12O19 raised the microstructure porosity. The additional SiO2 in the slag doubled the amount of low melting gehlenite in the matrix, accelerating the corrosion process of alumina-spinel brick at high temperatures.

Erosion mechanism of tabular alumina of various microstructures under different basicity of blast furnace slag

In this study, three types of commercial tabular alumina aggregates with different microstructural characteristics were selected for corrosion tests on blast furnace slags with varying basicities. A corrosion mechanism with different microstructural characteristics is proposed, and the dominant microstructures in high- and low-basicity environments are evaluated. The pore structure was found to have a crucial influence on the dissolution of tabular alumina aggregates in the blast furnace slag. Under low-basicity (0.5–0.7) conditions, a small amount of clustered pore structure was beneficial for obtaining the maximum thickness (40–60 μm) of the isolation layer between the tabular alumina aggregate and slag. Under high-basicity (0.9–1.1) conditions, the circular pore structure tended to form a uniform and stable double-layer isolation structure of calcium hexaaluminate and MgAl2O4 spinel.

The properties characterization and strengthening-toughening mechanism of Al2O3-CA6-MA-Ni multi-phase composites prepared by adding calcined dolomite

Basing on the systematical research of the powder properties of calcined dolomite mineral (CM) obtained from raw dolomite mineral (RDM), a novel Al2O3-CA6-MA/Ni multi-phase composite was developed using the in-situ generated strengthening-toughening phases of calcium hexaluminate (CA6) and magnesium aluminate spinel (MA) being introduced into an Al2O3-Ni system by regulating the reaction between CM and Al2O3. Under the anisotropic growth behaviour of CA6, the composite obtained a unique “ring” microstructure, while the mechanical properties were improved. In addition, the strengthening-toughening mechanisms related to this modification and improvement were studied in depth, that is, the pull out and fracture behaviours of CA6 and MA grains promoted crack deflection, bridging and branching, and the nano precipitation behaviour of metal Ni particles at the boundaries between CA6 and Al2O3 grains realized the bonding effect. This finding has important practical significance and economic value for the deep processing and application of RDM.

Ceramic composites based on in situ calcium hexaaluminate/aluminum titanate prepared from ferrotitanate slag

Ferrotitanium slag has attracted attention owing to its high aluminum content, which can partly replace the alumina in the traditional alumina/silicon carbide multiphase refractory materials. Alumina/calcium hexaaluminate ceramic composites have been prepared using ferrotitanium slag. However, previous studies neglected the utilization of valuable titanium resources in ferrotitanium slag. Therefore, in this study, calcium hexaaluminate/aluminum titanate composite ceramics were prepared using ferrotitanium slag and aluminum hydroxide as the main raw materials and titanium dioxide (TiO2) as an additive. The influence of TiO2 concentration on the phase composition, microstructure, and mechanical and thermal expansion properties of the ceramics was studied. The results showed that calcium hexaaluminate and aluminum titanate were generated in situ and the composite was reinforced by calcium hexaaluminate, providing flexural strength to the material. The Mg2+ and Fe3+ impurities present in the ferrotitanium slag formed a solid solution with aluminum titanate, accelerating the production of aluminum titanate while inhibiting its thermal decomposition. With the addition of TiO2 (2–20 wt%), the aluminum titanate formed in the ceramic effectively reduced the thermal expansion coefficient of the material. However, at concentrations > 5 wt%, the rutile phase and excessive liquid phase in the sample resulted in a significant reduction in the flexural strength at room temperature. At 5 wt% TiO2, the main crystal phases of the ceramic composite were calcium hexaaluminate and aluminum titanate. The composite exhibited a flexural strength of 19 MPa at room temperature(≈25 ℃), and a thermal expansion coefficient lower than 4 × 10−6 °C−1 at 1100 °C.

Corrosion mechanism of cement-bonded Al2O3–MgAl2O4 pre-cast castables in contact with molten steel and slag

In this work, a cement-bonded corundum-spinel (Al 2 O 3 –MgAl 2 O 4 ) pre-cast refractory brick with two typical Al 2 O 3 aggregates was designed as the refractory lining. Corroded microstructure of the used corundum-spinel bricks after industrial trials in a commercial RH refining ladle was analyzed. Degradation processes of two types of alumina aggregates in the same corroded interface also were discussed. Typical corroded microstructure revealed that needle-like calcium hexaaluminate (CA 6 ) was observed in the matrix of the original layer. The formation of CA 6 was attributed to the reactions with pure calcium aluminate cement and matrix components under a high-temperature gradient during refining. Furthermore, the corrosion process of the used Al 2 O 3 –MgAl 2 O 4 bricks would be discussed based on a post-mortem microstructural characterization, and the corrosion mechanism of the two types of aggregates was also elucidated.

Preparation of CaCO3 coated corundum aggregates by dip-coating and heat treatment and its effects on the properties and microstructures of Al2O3–MgO castables

The CaCO3 coated corundum aggregates were prepared by impregnating tabular corundum aggregates with sizes of 1–5 mm in calcium hydrogen citrate solution and heat treatment at 430 °C, which were also used in Al2O3–MgO castables. The effects of Ca2+/Cit3− mole ratio in precursor solution on coating characteristics of CaCO3 coated corundum aggregates as well as the effects of CaCO3 coatings on properties and microstructure of castables were investigated. It is found that the thickness and continuity of CaCO3 coating is increased and the size of CaCO3 particles in coatings decreases first and then increases as Ca2+/Cit3− mole ratio is decreased. High-temperature properties of castables are improved by in-situ formation of calcium hexaaluminate (CA6) layer at aggregate/matrix interface after sintering at 1600 °C. The Al2O3–MgO castables exhibit the best thermal shock resistance when Ca2+/Cit3− mole ratio is 1/3. It is contributed by deflections of cracks and consumptions of fracture energy in a continuous platelet CA6 layer with thickness of 10 μm, which is in-situ formed through reaction between Al2O3 and CaO derived from CaCO3 coatings. The present investigation provides a novel approach to enhance thermal shock resistance of the Al2O3–MgO castables.

Corrosion Resistance of Calcium Hexaaluminate Insulating Firebrick for Synthesising Ternary Lithium-Ion Battery Cathode Materials

Corrosion Resistance of Calcium Hexaaluminate Insulating Firebrick for Synthesising Ternary Lithium-Ion Battery Cathode Materials

Effect of nickel oxide and titanium oxide on the microstructural, optical, and mechanical properties of calcium hexaaluminate ceramics

The high porosity and poor mechanical properties limit the application of calcium hexaaluminate (CA6) ceramics in the field of colored ceramics. In order to synthesize a new blue ceramic material, Ni2+ and Ti4+ were introduced into CA6 as an effective system to replace cobalt-aluminum spinel. Blue ceramic material with low porosity and good mechanical properties was successfully prepared by doping nickel oxide and titanium oxide into CA6 using a high-temperature solid-phase method. The results show that the sintering, densification, mechanical and optical properties of the ceramic material are affected by the amount of NiO and TiO2 doping. The color for the sample is related to the energy change produced by the substitution of Ni2+ and Ti4+ for Al3+, producing yellow light absorption and blue light reflection in the visible range. Due to its good mechanical and color properties, it can be used as a new type of environmentally friendly ceramic with great industrial potential for a wide range of high-tech applications such as mobile phone backplanes.

Metal-Ceramic Beads Based on Niobium and Alumina Produced by Alginate Gelation.

Full metal-ceramic composite beads containing different amounts of niobium and alumina, particularly 100 vol% alumina, 100 vol% niobium, and 95/5 vol% niobium/alumina, were produced by the alginate gelation process. The suspension for bead fabrication contained sodium alginate as gelling agent and was added dropwise into a calcium chloride solution to trigger the consolidation process. After debinding in air, sintering of the composite beads was performed under inert atmosphere. Samples in green and sintered state were analyzed by digital light microscopy and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Investigations by mercury intrusion porosimetry revealed that pure alumina beads featured smaller pores compared to composite beads, although the open porosities were comparable. The fracture strength was evaluated on single beads. Contrary to the pure alumina, the composite beads showed a clear plastic deformation. Pure niobium beads showed a ductile behavior with very large deformations. XRD analyses revealed the presence of calcium hexaluminate and beta-alumina as minor phases in the alumina beads, while the composite ones contained about 25 wt% of impurities. The impurities comprised NbO arising from the oxidation, and β-Nb2C, from the reaction with the residual sodium alginate.