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.     

Effect of microporous aggregates and spinel powder on fracture behavior of magnesia-based refractories

The influences of microporous aggregates and spinel powder on the properties and fracture behavior of magnesia-based refractories were investigated by the three-point bending test and wedge splitting test with the digital image correlation method. With microporous aggregates instead of dense ones, lower thermal conductivity, higher cold modulus of rupture and compressive strength were observed for lightweight magnesia-based refractories. Besides, the results indicate that the strengthened interlocking interface between microporous aggregates and matrix in lightweight magnesia refractories decreased the proportion of crack propagation along the aggregate/matrix interface (P_AM). This reduced the tortuosity of crack propagation as well as increased the brittleness. With the addition of spinel powder in the matrix, the pregenerated microcracks by thermal mismatch increased the P_AM, which increased the tortuosity of crack propagation, improved fracture energy and reduced the brittleness. Lightweight magnesia spinel refractories merely showed a slightly higher brittleness than dense ones.     

Characterisation and properties of low-conductivity microporous magnesia based aggregates with in-situ intergranular spinel phases

Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al₂O₃. Intergranular MgAl₂O₄ phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al₂O₃ presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m^?1 k^?1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.     

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 Al₂O₃ and TiO₂. Moreover, the microporous magnesia aggregates mainly absorb SiO₂ by penetration, but excessive SiO₂ will lead to the serious corrosion of microporous magnesia aggregates.     

微孔富镁尖晶石对低碳MgO-C材料性能的影响

分别以微孔富镁尖晶石(5~3和3~1 mm)和电熔镁砂(5~3和3~1 mm)为粗骨料,以<1 mm的电熔镁砂为细骨料,以镁砂粉(≤0.088 mm)、鳞片石墨粉(≤0.088 mm)、金属铝粉(≤0.074 mm)为细粉,以酚醛树脂为结合剂,制备了w(C)=6%的两种低碳MgO-C材料,经220和1 500℃(埋焦炭)热处理后,测定其显气孔率、常温耐压强度、常温抗折强度、加热永久线变化率、抗热震性和抗渣性。结果表明:1)用微孔富镁尖晶石骨料取代普通低碳MgO-C材料中的部分镁砂骨料后,经220和1 500℃热处理后试样的显气孔率均比普通低碳MgO-C试样的大,体积密度均比普通低碳MgO-C试样的小;220℃固化后试样的强度比普通低碳MgO-C试样的小,但1 500℃热处理后试样的强度比普通低碳MgO-C试样的大;1 500℃热处理后试样的加热永久线变化率比普通低碳MgO-C试样的小。2)使用微孔富镁尖晶石骨料代替电熔镁砂骨料能有效提高低碳MgO-C材料的抗热震性,但对低碳MgO-C材料的抗侵蚀性不利。     

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 Al₂O₃ addition on the corrosion resistance of microporous magnesia aggregates against tundish slag is explored. The results show that the addition of nano-sized Al₂O₃ 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 Al₂O₃. Furthermore, MgO·Al₂O₃ spinel is formed in situ at the grain boundaries of Al₂O₃-bearing aggregates and the dissolution of MgO·Al₂O₃ spinel into molten slag damages the structure of aggregates. For the Al₂O₃-free microporous magnesia aggregates, as expected, the penetration of high basicity slag (CaO/SiO₂ = 9, mass ratio) into refractory is slighter than that of low basicity slag (CaO/SiO₂ = 4, mass ratio). But, for the Al₂O₃-bearing microporous magnesia aggregates, the corrosion of refractory by high basicity slag is severer. This is mainly because that MgO·Al₂O₃ spinel is more unstable in high basicity slag. Therefore, it is not suitable to add nano-sized Al₂O₃ for the preparation of microporous magnesia as tundish lining.     

Corrosion and adherence properties of cement clinker on porous periclase-spinel refractory aggregates with varying spinel content

The corrosion and adherence properties of cement clinker on porous periclase-spinel refractory aggregates with varying spinel content were examined using a static crucible test and a sandwich test, respectively. The reaction characteristics of porous periclase-spinel aggregates and cement clinker and the effects of spinel content on the adherence property were investigated through scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and FactSage^® thermo-chemical software. It was observed that the spinel content and pore characteristics strongly affected the corrosion results and thus affected the adherence ability of cement clinker on porous periclase-spinel aggregates. With an increase in the spinel content, the amount of glass phase formed from the reaction of the refractory and cement clinker increases because the rate of the spinel dissolution into the cement clinker is higher than that of periclase. The glass phase acts as a bridge between the cement clinker and the aggregate to enhance the adherence property, which depends on the amount, area distribution and viscosity of the glass phase and its penetration in aggregates. When the spinel content is 15–40 wt%, the refractory aggregate not only has a high cement clinker resistance but also a high adherence property. Once the spinel content exceeds 50 wt%, the skeletal structure of the aggregate will be destroyed, which will lead to a substantial decline in the cement clinker resistance.     

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.     

Effect of hollow spheres on the properties of lightweight periclase-magnesium aluminate spinel refractories

This study presents new lightweight periclase-magnesium alumina spinel refractories for the working lining of cement rotary kilns in which magnesium alumina spinel hollow spheres are used to replace conventional dense fused magnesia-aluminate spinel aggregates. The effects of adding spinel hollow spheres on the physical properties, mechanical strength, thermal conductivity, and slag resistance of the samples were explored. The results showed that compared with the sample prepared with dense aggregates, the sample prepared with hollow spheres had a 10.3% higher cold compressive strength, 44.1% higher modulus of rupture (MOR), and lower bulk density. Additionally, with increasing hollow spheres content, the thermal conductivity decreased from 3.79 W/(m·K) to 2.53 W/(m·K), and the high-temperature MOR increased from 2.82 to 4.09 MPa. The highest residual strength ratio was 90.73% (15 wt.% hollow spheres), which is 1.17 times that of the sample prepared without hollow spheres. Moreover, microstructure and energy dispersive spectroscopy of crucible specimens after corrosion by cement clinker showed that specimens with 15 wt.% hollow sphere additions had a better slag resistance. Introducing hollow spheres reduced the thermal conductivity of the refractories, providing a new strategy for improving the heat insulation performance of kiln linings.     

尖晶石不烧制品的组成、结构和力学性能

研究了全天然原料合成的矾土基尖晶石分别与电熔镁砂和亚白刚玉混合制备的不烧试样的组成、结构和力学性能;分析了试样经高温处理后基质和颗粒中的尖晶石相组成及化学成分的变化规律。     

Energy efficient lightweight periclase-magnesium alumina spinel castables containing porous aggregates for the working lining of steel ladles

This study presents a new lightweight periclase-magnesium alumina spinel castable (LPSC) for the working lining of steel ladles using porous periclase-spinel aggregates to replace conventional dense magnesia aggregates. The porous periclase-spinel aggregates were produced by an in-situ decomposition technique resulting in an apparent porosity of 23.3% and a median pore size of 5.66?μm. Scanning electron microscopy revealed a better porous aggregate/matrix interface bonding in the LPSC, which significantly improved its strength and thermal shock resistance. Additionally, the higher amount of micropores of the porous aggregates in the LPSC absorbed more penetrated slag from the matrix, which enhanced the slag resistance. Thus, compared with conventional castables, the LPSC had a lower bulk density of 9.2–10.8% and a lower thermal conductivity of 18.8% (1000?°C) while at the same time a higher strength, thermal shock resistance and slag resistance was achieved.     

Effect of Yb2O3 and TiO2 on reaction sintering and properties of magnesium aluminate spinel

Magnesium aluminate spinel with an initial MgO: Al₂O₃ molar ratio of 2:1 was prepared from its constituent oxides through a solid-state sintering process at temperatures ranging from 1550 to 1700 °C in a normal air atmosphere. The effect of varying amount (0.25–1.0 wt%) of TiO₂ and Yb₂O₃ on densification, phase assemblage, mechanical, thermo-mechanical properties and microstructure of magnesia-rich spinel were investigated under static heating condition. The addition of TiO₂ and Yb₂O₃ favours the densification of magnesia-rich spinel, which is discernible up to 1650 °C. This beneficial effect may be attributed to the development of the secondary phase and formation of solid solution due to the dissolution of the additive ions in the spinel structure. A marginal increase in the average grain size of the samples along with a narrower grain size distribution occurred with the incorporation of both the additives. Both the additives improved the mechanical properties of the magnesia-rich spinel; however, better room temperature flexural strength was achieved with Yb₂O₃ as compared to TiO₂ addition. For the samples sintered at 1550 °C, 1.0 wt% Yb₂O₃ addition resulted in 30% increase in flexural strength; however, same amount of TiO₂ addition increased the strength by 20%. In case of thermal shock resistance, 1.0 wt% TiO₂ and 0.25 wt% Yb₂O₃ addition demonstrated promising result among all the samples.     

SiC fines effects on the microstructure and properties of bauxite-based low-cement refractory castables

In the present paper, the effect of silicon carbide (SiC) fines was studied on the phase and microstructural properties of bauxite-based low-cement refractory castables (LCC) at different firing temperatures. XRD and SEM techniques were employed to evaluate phase and microstructure analysis, and physical/mechanical properties were measured according to standard methods. The results showed that the oxidation of SiC particles related to the oxygen partial pressure and active/passive oxidation behavior of SiC fines could markedly affect the microstructure and properties of bauxite-based LCC fired at high temperatures. Despite the negative influence of addition of SiC fines on cold mechanical strength, its usage up to 6 wt% could greatly improve the thermo-mechanical properties of LCC refractories due to increasing of matrix refractoriness.     

Microstructures and properties of novel lightweight refractory aggregates with microporous MgO@MgAl2O4 core-shell structures

Microporous aggregates are the key to the lightweight design and preparation of refractories for the working linings of the high temperature furnaces. In this work, the lightweight MgO-MgAl₂O₄ refractory aggregates with core-shell structures were prepared by in-situ decomposition synthesis technology using Mg(OH)₂ and nano-size Al₂O₃ as raw materials. The influence of the nano-size Al₂O₃ content on the microstructures and properties was thoroughly studied. The results demonstrated that the microporous MgO core structures were formed after the decomposition of Mg(OH)₂, and the MgAl₂O₄ bonds between the microporous MgO core structures were formed through the reaction between the nano-size Al₂O₃ and MgO. When the nano-size Al₂O₃ contents were less than 9 wt%, the MgAl₂O₄ bonds were discontinuous. With the increase of the nano-size Al₂O₃ contents to 9–15 wt%, more continuous MgAl₂O₄ bonds (i.e. MgAl₂O₄ shell structures) were formed at the surface of the microporous MgO core structures. Overall, the optimized specimens were lightweight MgO-MgAl₂O₄ refractory aggregates with the addition of 9 wt% nano-size Al₂O₃, which exhibited the microporous MgO@MgAl₂O₄ core-shell structures, a median pore size of 268 nm, a high compressive strength of 105 MPa, and a low thermal conductivity of 4.1 W/(m·K) at 800 ℃.     

富铝尖晶石对镁质耐火材料抗侵蚀性的影响

研究了富铝尖晶石对镁质耐火材料抗钢渣与抗钙处理钢侵蚀性的影响。结果表明 :随着富铝尖晶石加入量的增加 ,镁质耐火材料的抗钙处理钢和钢渣熔蚀性逐渐减弱 ,而抗钢渣渗透性逐渐增强 ;纯镁质和镁尖晶石质耐火材料在抗钢渣与抗钙处理钢侵蚀方面远远优于铝锆碳质材料     

Enhanced formation of magnesium silica hydrates (M-S-H) using sodium metasilicate and caustic magnesia in magnesia castables

Magnesium-silica-hydrates (M-S-H) is a promising binder for magnesia castables due to its bonding strength and progressive dehydration behavior over a wide temperature range during the heating-up stage. Sodium metasilicate and caustic magnesia were used to form M-S-H in magnesia castables. The results showed that M-S-H was remarkably produced in the caustic magnesia-microsilica slurries containing sodium metasilicate with increasing pH value, which activated the hydrolysis of microsilica into silicic ions and enhanced the M-S-H formation. When 0.3 wt% caustic magnesia and 0.05 wt% sodium metasilicate as additives were incorporated into magnesia castables, the cold crushing strength and cold modulus of rupture of castables after drying at 110 °C reached the maximum value of 68.3 MPa, which corresponded to ~ 40% improvement in comparison with those of caustic magnesia and sodium metasilicate-free magnesia castables. Besides, the enhanced formation of M-S-H bonding system contributed to a better explosion resistance of magnesia castables.     

尖晶石质浇注料显微结构的研究

对尖晶石质浇注料及高纯铝镁浇注料的基质进行了显微结构分析,分析了不同结合系统和硅灰纯度对显微结构的影响,结果表明：结合系统中主要由尖晶石、橄榄石和硅酸盐玻璃相构成,有水泥结合的系统中还有长石析出。     

Effect of magnesia-alumina spinel precursor sol on the sintering property of fused magnesia refractory

The sintering property of fused magnesia refractory was investigated by the fused magnesia powder as matrix and the synthetic magnesia-alumina spinel precursor sol as binder. The spinel precursor sol was prepared by co-precipitation method and was characterized by its particle size, thermal analysis, microstructure, phase development with temperatures, and so on. The effect of spinel precursor sol on the sintering property of fused magnesia refractory were studied after heat treatment at 1450?°C and 1550?°C. The results showed that the bulk density, flexural strength and linear shrinkage of the sintered samples firstly increase and then decrease with increasing spinel precursor sol. The bulk density and flexural strength (cold modulus of rupture) of the sample sintered at 1550?°C, introduced with 1?wt% spinel precursor sol, reached 3.10?g/cm³ and 47.25?MPa, respectively. From the experimental results, magnesia-alumina spinel precursor sol can replace the traditional binders and improve the sintering property of fused magnesia products.     

Microstructures and properties of microporous mullite-corundum aggregates for lightweight refractories

Five microporous mullite-corundum refractory aggregates were prepared from Al(OH)₃ and kaolinite gangue through in situ decomposition synthesis technique. The effects of the sintering temperature (1400–1600°C) and the particle sizes of raw materials (20.6–94.5 μm) on the microstructures and strengths of the aggregates were investigated through X-ray diffractometer, scanning electron microscopy, and energy-dispersive spectrometer etc., to find out the technological conditions to be controlled in industrial production. The higher sintering temperature promoted the reaction between Al(OH)₃ and kaolinite gangue, leading to the development of primary-mullite as well as the generation of secondary-mullite, which promoted the formation of the neck and improved the strength. Meanwhile, the dense mullite layers were formed continuously on the surface of Al(OH)₃ pseudomorphs, making the micropores inside the pseudomorphs become closed pores, which increased the closed porosity of the aggregates. The reduction of the particle sizes of raw materials changed the particle packing behavior, accelerated the rearrangement of the Al(OH)₃ pseudomorph particles during the process of reactive sintering, and then reduced the closed porosity. To realize the industrial production of microporous mullite-corundum refractory aggregate with high strength (103 MPa) and high closed porosity (16.1%), the sintering temperature should be at about 1600°C, and the median diameter of raw materials should be at 94.5 μm.