Magnesia Aggregates Research Articles

Effects of NiO addition on the microstructural evolution and performance of sintered magnesia aggregates

This study introduces a novel approach to enhance the properties of sintered magnesium oxide (MgO) aggregates by incorporating nickel oxide (NiO) powder as a solid-solution modification additive. The addition of NiO improves the densification of the specimens and promotes grain growth through the activation of sintering via the solid-solution reaction of MgO–NiO at high temperatures. The formation of the (Mg, Ni)O solid-solution structure enhances the strength of the specimens, while gradually reducing thermal conductivity. However, excessive addition of NiO (>6 mol%) adversely affects the sintering properties of the specimens due to limited diffusive mass transfer rate during sintering and the Zener effect, resulting in diminished grain size and strength. Although thermal conductivity is not significantly affected, at 1000 °C, one specimen (MN-6) demonstrates a 61.03 % decrease in thermal conductivity compared to a specimen without NiO addition. These findings suggest that the incorporation of a moderate amount of NiO enhances the corrosion resistance of the cement clinker of the specimens primarily by increasing densification, grain size, and high-temperature contact angle.

Evaluation of fracture behavior of lightweight magnesia‐based refractories via wedge splitting test along with acoustic emission detection

AbstractThe effect of spinel powder on the fracture behavior and mechanical properties of lightweight magnesia‐based refractories containing microporous magnesia aggregates with high apparent porosity (37.4%) were investigated by the wedge splitting test (WST) with the digital image correlation and acoustic emission. With the addition of spinel powder, lightweight magnesia spinel refractories showed a higher cold compressive strength compared with lightweight pure magnesia refractories. From the WST, the addition of spinel powder increased the specific fracture energy and characteristic length of lightweight magnesia spinel refractories, which improved the crack propagation resistance. The increased tortuosity of main crack and a higher ratio of crack propagation along the aggregates/matrix interface were main reasons for reducing the brittleness of lightweight magnesia spinel refractories. Besides, acoustic emission (AE) signal activity indicated that the propagation of pregenerated micro‐cracks by the thermal mismatch and the development of fracture progress zone were primary ways to consume energy in lightweight magnesia spinel refractories. The reduced proportion of crack propagation within aggregates was also detected by the peak frequency of AE signals in lightweight magnesia spinel refractories. For microporous magnesia aggregates with high apparent porosity (37.4%), lightweight magnesia spinel refractories also showed reduced brittleness fracture behavior than lightweight pure magnesia refractories.

Role of nano-Al2O3 particles in improving the properties of MgO–C slide plate materials

The unfired Al–Si incorporated MgO–C slide plate materials were manufactured using magnesia aggregates and fines, SiC aggregates, Al, Si and B4C powders as the raw materials, trace nano-Al2O3 particles as additive, and phenolic resin as binder. The contributions of nano-Al2O3 in the properties, phase composition and microstructure of MgO–C materials were studied. The reseult indicates that nano-Al2O3 is beneficial to improve mechanical properties of the materials, which is attributed to the good filling and pinning effects of nanoparticles. Highly reactive nano-Al2O3 can promote the densification of MgO–C materials and in-situ formation of ceramic phases at high temperatures, and a large number of non-oxide whiskers contributes to the formation of a continuous interlocking networks, which improves the high-temperature strength and TSR of MgO–C materials. Moreover, nano-Al2O3 promotes the formation of fine-grained pinel and forsterite protective layer in the high-temperature oxidizing atmosphere and prevents the diffusion of oxygen into the interior of the material, thus improving the oxidation resistance of MgO–C refractories.

High temperature confocal scanning laser microscopy analysis of dead‐burned magnesia aggregates

AbstractDead‐burned magnesia is a commonly used material in the manufacturing of refractories for the steelmaking industry. Aggregates of dead‐burned magnesia contain secondary phases due to the impurities within the magnesite rock used in its production. While these phases can aid in sintering magnesia, they may have some impact on the high‐temperature performance of the refractory product. High‐temperature confocal scanning laser microscopy was utilized to observe the behavior of dead‐burned magnesia aggregates at elevated temperatures (up to 1550°C). Liquid formation was detected even at temperatures below 1350°C. In some cases, this liquid quickly exuded from the aggregate surface. This liquid phase was characterized through microscopy and chemical analysis to determine the impact of impurity content on the formation and behavior of this liquid phase, and conclusions are drawn on its detrimental impact on the material's refractoriness in a steelmaking environment.

Role of Mg(OH)2 in pore evolution and properties of lightweight brine magnesia aggregates

Lightweight magnesia aggregates were fabricated using high-purity MgO agglomerates with the addition of Mg(OH)2 as a pore former. The pore evolution and its relationship to the resulting properties were investigated. Mg(OH)2 decomposition increased the number of inter-agglomerate pores, which subsequently affected the porosity and pore structure. When Mg(OH)2 was 0–20 wt%, the inter-agglomerate pores were converted to both open and closed small pores, which effectively reduced the thermal conductivity and improved the thermal shock resistance (TSR) by accommodating thermal stress and inducing crack deflection. Small pores also favored the formation of a dense (Mg, Fe)O corrosion layer, preventing further slag penetration. However, large open pores occurred with further increasing Mg(OH)2 content, which dramatically deteriorated the TSR and slag resistance. The specimen with 20 wt% Mg(OH)2 exhibited the best overall performance, with a thermal conductivity of 16.6 W/(m·K) at 500 °C, and a residual flexural strength ratio of 32.3%; its slag resistance was comparable with that of dense magnesia.

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg2SiO4–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg2SiO4 (M-M2S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M2S aggregates increased SixOyz− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO2–Al2O3–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.

Wetting and interfacial interaction of molten iron on MgO substrate under different oxygen partial pressures

The interaction between molten iron and refractory is of great significance to understand the damage of refractory in steelmaking processes. In this work, the interactions between the molten iron and three kinds of magnesia aggregates: No. 1 containing CaZrO3-ZrO2 intergranular phase (mainly CaZrO3), No. 2 containing ZrO2-MgAl2O4 intergranular phase (mainly ZrO2) and No. 3 fused magnesia, were studied by the sessile drop method at 1823 K under two kinds of oxygen partial pressures of PO2 ≈ 1.87 × 10−1 Pa and PO2 ≈ 1.87 × 10−3 Pa. It was found that the interaction mechanism between molten iron and substrate was almost the same under the two oxygen partial pressures. No. 2 substrate had a larger interacted area with molten iron due to its physical properties and chemical composition and more amount of slag was formed between them. The oxygen was easy to be released from the closed pores in No. 2 substrate so that the molten iron on No. 2 substrate had higher oxygen content, resulting in the decreases of the contact angle and the surface tension of molten iron. Accordingly, No. 2 substrate was penetrated most seriously. The intergranular phase of No. 1 substrate was more resistant to FeO corrosion than that of No. 2 substrate. The FeO penetration into No. 3 substrate was mainly along the grain boundary. The larger bulk density and grain size of No. 3 substrate could reduce the FeO penetration.

Post-mortem analysis of MgO–Al2O3–C bricks containing bauxite used in steel ladle walls

A serious problem with integrated masonry linings ladle was that the ladle wall bricks were difficult to be partially replaced. Thus, the service life of the ladle was determined by the properties of ladle wall bricks. In this study, the new generation of MgO–Al2O3–C bricks with bauxite aggregates was tested in a ladle lining of an integrated steel ladle for 103 cycles, and the corroded microstructure of the used brick was investigated. A multilayered structure of bauxite aggregates could be observed in the used brick, which inhibited the slag penetration along the boundary of the magnesia aggregates and reduced the degree of the aggregates fall off from the used bricks. Besides, during the process of slag penetration, bauxite aggregates could melt into the slag, which increased the viscosity of the slag and weakened the penetration ability of the slag. The MgO–Al2O3–C bricks with bauxite addition could improve the service life of the ladle to a certain extent.

Pore evolution of microporous magnesia aggregates with the introduction of nano-sized MgO

Microporous refractories applied in the working-lining of metallurgical furnaces have been rapidly developed in recent years owing to the outstanding mechanical properties, thermal insulation performance and slag resistance, the pore structure of which plays a critical role in the variation of service performance. Meanwhile, the microporous magnesia aggregates were prepared in our previous research with the introduction of nano-sized particles to overcome the shortcomings of high thermal conductivity, poor thermal shock resistance and slag penetration resistance, however, the pore evolution during sintering still remains to be investigated. Hence, in this study, the pore evolution of microporous magnesia aggregates is explored specifically and the effect of nano-sized MgO on pore structure and sintering is simultaneously discussed. The sintering model of microporous magnesia was built for analyzing the pore structure evolution process. The results revealed that a micro-nano double-scale sintering model developed by the introduction of nano-sized MgO dramatically promoted the sintering kinetic force and boundary migration velocity. The sintering pressure discrepancy and free energy change per unit mole of specimens were respectively increased by ∼43 times and ∼48 times, which effectively improved the closed porosity and pore distribution homogeneity, while reduced the pore size. Meanwhile, the high sintering diving force lead to the significant improvement of direct bonding degree and grain size of microporous magnesia. With the addition of 3 wt% nano-sized MgO, the optimal sintering properties with closed porosity of 6.4%, bulk density of 3.23 g/cm3 and median equivalent pores diameters of 4.07 μm were achieved. The exploration of pore evolution in microporous magnesia aggregates contributed to the fabrication and industrialization development of microporous refractories.

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al 4 C 3 , AlN and octahedral MgAl 2 O 4 were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al 4 C 3 and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.

Comparison study on effect of nano-sized Al2O3 addition on the corrosion resistance of microporous magnesia aggregates against tundish slag

The microporous magnesia aggregates show a promising application prospect as tundish lining, due to the excellent thermal insulation. In this study, the effect of nano-sized Al 2 O 3 addition on the corrosion resistance of microporous magnesia aggregates against tundish slag is explored. The results show that the addition of nano-sized Al 2 O 3 deteriorates the slag resistance of microporous magnesia aggregates, which is mainly because that the apparent porosity of aggregates increases with the addition of nano-sized Al 2 O 3 . Furthermore, MgO·Al 2 O 3 spinel is formed in situ at the grain boundaries of Al 2 O 3 -bearing aggregates and the dissolution of MgO·Al 2 O 3 spinel into molten slag damages the structure of aggregates. For the Al 2 O 3 -free microporous magnesia aggregates, as expected, the penetration of high basicity slag (CaO/SiO 2 = 9, mass ratio) into refractory is slighter than that of low basicity slag (CaO/SiO 2 = 4, mass ratio). But, for the Al 2 O 3 -bearing microporous magnesia aggregates, the corrosion of refractory by high basicity slag is severer. This is mainly because that MgO·Al 2 O 3 spinel is more unstable in high basicity slag. Therefore, it is not suitable to add nano-sized Al 2 O 3 for the preparation of microporous magnesia as tundish lining.

A novel lightweight periclase-composite (Mg8-xFex+yAl16-yO32) spinel refractory material for cement rotary kilns

This study presents novel lightweight periclase-composite (Mg8-xFex + yAl16-yO32) spinel refractories (LPSR) for the high temperature zone of cement rotary kilns. The LPSR was prepared by using microporous magnesia aggregates instead of sintered magnesia aggregates in traditional periclase-composite spinel refractories (TPSR). Hercynite-corundum composite aggregates, as well as microporous magnesia aggregates with a median pore size of 3.50 μm and a 20.1% lower bulk density than those of the sintered magnesia aggregates were used as raw materials. The microstructures, fracture behavior and strength of the LPSR in contrast with those of the TPSR were determined by SEM and three-point bending tests. After substituting the microporous magnesia aggregates for the sintered magnesia aggregates, a rougher surface of the microporous aggregates and wider transition-layer containing a solid solution spinel phase at the microporous magnesia aggregate/composite spinel aggregate interfaces were observed. Thus, a better bonding at the microporous magnesia aggregate/matrix interfaces as well as of the microporous magnesia aggregate/composite spinel aggregate interfaces was achieved. The wider transition-layer and better interfaces impeded crack propagation along the aggregate/matrix interface and increased the percentage of crack propagation within the aggregates. Thus, the mechanical strength of the LPSR was significantly enhanced. Compared with the TPSR, the LPSR had a lower bulk density of 2.56 g/cm3, but also a higher apparent porosity of 27.8% and a higher compressive strength of 46.4 MPa.

Adsorption mechanism of oxide inclusions by microporous magnesia aggregates in tundish

The microporous refractory with low thermal conductivity shows a promising application prospect as tundish lining. In this study, the interactions between microporous magnesia aggregates and oxide inclusions in steel were explored. The experimental results and thermodynamic calculation show that the interaction process of microporous magnesia aggregates and oxide inclusions at high temperature can be divided into dissolution, reaction and post-reaction. The microporous magnesia aggregates can absorb the inclusions in steel by reaction and liquid phase penetration. The microporous magnesia aggregates have a strong adsorption for Al 2 O 3 and TiO 2 . Moreover, the microporous magnesia aggregates mainly absorb SiO 2 by penetration, but excessive SiO 2 will lead to the serious corrosion of microporous magnesia aggregates.

Improved thermal shock resistance of MgO–C refractories with addition of calcium magnesium aluminate (CMA) aggregates

Thermal shock resistance is of great importance for MgO–C refractories used in steelmaking process. In this study, calcium magnesium aluminate (CMA) aggregates were selected to replace partially fused magnesia aggregates in MgO–C refractories, and their effects on phase compositions, microstructure, thermal shock resistance and fracture behavior of MgO–C refractories were investigated. The results showed CMA aggregates partly disintegrated, and spinel particles and whiskers were observed together with Al 4 C 3 phase in MgO–C specimens coked at 1400 °C. With the increase in coking temperature up to 1600 °C, CMA aggregates completely disintegrated to generate much liquid phase, which promote the formation of spinel whiskers apart from spinel particles and hexagonal AlN in MgO–C refractories. CMA aggregates containing MgO–C refractories coked at 1400 °C possessed higher thermal shock resistance because CMA aggregates had abundant micro-sized pores and lower thermal expansion coefficient. When reheating up to 1600 °C in coke bed after thermal shocks, they had higher recovery strength index than CMA-free refractories. Furthermore, MgO–C refractories containing CMA showed lower brittleness and better bearing capacity via wedge splitting test.

Effects of Disodium Hydrogen Phosphate Addition and Heat Treatment on the Formation of Magnesium Silicate Hydrate Gel

The formation of magnesium silicate hydrate gel is crucial in preventing magnesia aggregates from over hydrated during the construction of refractory castables since the presence of magnesium hydroxide diminish the mechanical properties of the material. This work aimed to investigate the accelerating effects of sodium hydrogen phosphate and heat treatment on the formation of magnesium silicate hydrate gel. Time-dependent pH of magnesia - silica fume slurries with and without sodium hydrogen phosphate addition and heat treatment was measured to verify the dissolution of MgO and magnesium silicate hydrate formation. The effects of sodium hydrogen phosphate were differentiable only at small added amounts, whereas heat treatment at 50 degrees Celsius performed noticeable acceleration. This observation could be applicable in molding to maintain the stability of basic refractory castables.

Fabrication and thermal shock behavior of ZrO2 toughened magnesia aggregates

MgO-based refractories have been regarded as ideal linings for various furnaces owing to high refractoriness and excellent corrosion resistance to basic slag. However, thermal shock damage resulting from poor thermal shock resistance (TSR) of magnesia is the common failing. The present work aimed at improving TSR of magnesia aggregates by introducing microscale monoclinic ZrO2. The results showed that the ZrO2 added could increase cation vacancy concentration and inhibit abnormal growth of MgO grains, which therefore enhanced densification of the specimens. However, when the ZrO2 amount exceeded 15 wt%, densification decreased slightly due to agglomeration of ZrO2 and formation of more microcracks at the MgO grain boundaries. With increasing ZrO2 content, although the flexural strength degraded, the fracture toughness increased constantly because of the toughening effects of crack deflection and crack branching. Besides, although the addition of ZrO2 decreased the thermal conductivity, TSR of the specimens was significantly improved with increasing ZrO2 content due to the increase in toughness and decrease in strength, thermal expansion coefficient as well as Young's modulus. After thermal shock tests, 15 wt% ZrO2 containing specimen exhibited the highest TSR, whose residual strength ratio was about triple of that of the pure magnesia specimen. However, further increasing ZrO2 content to 20 wt% reduced the TSR attributing to the spalling of intergranular ZrO2 agglomerations.

The phase composition and microstructural evolution of a novel MgO-C-Al-Si refractory used in bottom-blowing elements at high temperatures in flowing nitrogen

ABSTRACT A novel MgO-C-Al-Si refractory with low carbon content used in bottom-blowing elements was prepared, and its phase composition and microstructural evolution were investigated by XRD, SEM and EDS after being treated at 1200–1600°C in flowing nitrogen. Results show that the samples are composed of MgO, Al4C3, SiC, Al4SiC4, MgAl2O4, Mg3Al2N4 and Al2O3 at 1200–1400°C, while the samples are composed of MgO, Al4C3, SiC, Al4SiC4, Al4Si2C5, MgAl2O4, Mg3Al2N4 and Al2O3 at 1500–1600°C. Al and Si present gradient reactivity at high temperatures and Al has priority over Si to react with MgO, forming MgAl2O4. Based on the microstructure analysis, we find that the higher the temperatures are, the more abundant the products. Al(g), Al2O(g), SiO(g), Mg(g) and CO(g), as the gas–gas reaction substances, react to form many of Al4SiC4 flakes containing Mg, O and N within pores or gaps of the refractory, along with trace amounts of SiC whiskers and MgAl2O4 spinel whiskers. In addition, the surface morphology of the magnesia aggregates is modified by the MgAl2O4 spinels and Al4SiC4 flakes, owing to the synergistic effect of Al and Si. Further, some physical properties are also characterized.

Advanced lightweight periclase-magnesium aluminate spinel refractories with high mechanical properties and high corrosion resistance

A new strategy to improve the mechanical strength and the cement clinker corrosion resistance of refractories for cement kiln was proposed in this work. For this purpose, advanced lightweight periclase-magnesium aluminate spinel refractories (LPSR) with high mechanical properties and excellent cement clinker resistance were fabricated using microporous magnesia aggregates. Their microstructures and properties were then compared to conventional dense periclase-magnesium aluminate spinel refractories (DPSR) containing sintered magnesia aggregates. The apparent porosities of the microporous and dense magnesia aggregates were 43.1% and 5%, respectively. Scanning electron microscopy observation results showed that the interface between the microporous aggregates and the matrix was better, which significantly improved the strength and thermal shock resistance of the LPSR. Additionally, the microporous magnesia aggregates absorbed some of the penetrating slag from the matrix, which prevented further infiltration. Thus, after substituting the microporous magnesia aggregates for dense magnesia aggregates to fabricate LPSR, the practically same cement clinker resistance was obtained for the LPSR compared to DPSR. Meanwhile, the bulk density was decreased from 2.87 g/cm3 to 2.61 g/cm3, while the compressive strength and flexural strength increased from 48.7 MPa to 73.1 MPa and 6.0 MPa to 10.1 MPa, respectively. Meanwhile, practically the same cement clinker resistance was obtained for the LPSR compared to DPSR.

Effect of microporous magnesia aggregates on microstructure and properties of periclase-magnesium aluminate spinel castables

The present study investigated three lightweight periclase-magnesium aluminate spinel castables containing microporous magnesia aggregates with a varying apparent porosity (12.8%, 30.8% and 39.3%). The effect of the apparent porosity of the aggregates on the phase composition, microstructure, fracture behavior and strength of the lightweight castables was investigated by XRD, SEM and three-point bending tests. Large cracks between the aggregates with an apparent porosity of 12.8% and the matrix reduced the strength of the castable. For the aggregates with an apparent porosity of 30.8%, an excellent interlocking interface with the matrix increased the strength considerably, but also reduced the fracture toughness. At the highest level of the apparent porosity of the aggregates of 39.3%, the formation of a small number of microcracks between the aggregates and matrix reduced the strength, while the fracture toughness was only slightly affected. The lightweight castables with the best combination of properties were achieved at an apparent porosity of the aggregates of 30.8% since they had a low bulk density of 2.63 g/cm3 as well as a high compressive and flexural strength of 70.2 MPa and 20.9 MPa, respectively.

Comportamiento termomecánico y termoquímico de ladrillos refractarios MgO-C

El desarrollo tecnológico de los ladrillos de MgO-C y su posición en el mercado, genera un gran interés en continuar estudiando ciertos temas que aún son de discusión. El desarrollo del tema de esta tesis es de utilidad, tanto para el sector académico como el industrial, en la generación de conocimiento integral de los refractarios MgO-C. El objetivo es estudiar el efecto que tiene la calidad de las materias primas en el comportamiento químico, mecánico y térmico de estos materiales. Para esto se formularon ladrillos con dos calidades de agregados de magnesia (electrofundidos y sinterizados) y con diferentes grados de pureza (entre 97,6 - 99,1% MgO). Los materiales para este estudio se prepararon a partir de 90% en peso de magnesia (MgO) y 8,2% en peso de grafito (C), que se completó con la adición de antioxidantes y resina ligante. Se determinaron: (i) composición química y mineralógica, (ii) análisis microestructural, (iii) densidad y porosidad (aparente y total), (iv) resistencia al ataque químico por escorias a 1600°C, (v) comportamiento mecánico en compresión a temperatura ambiente y a 1400 °C, y (vi) evolución térmica hasta 1400°C.