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。     

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.     

Preparation of closed-pore MgO-MgFe2O4 aggregates with low thermal conductivity and high mechanical properties

MgO-MgFe₂O₄ refractory aggregates with high closed porosity were fabricated using MgO agglomerates and Mg(OH)₂ with introducing Fe₂O₃ additive. The evolutions of pores and microstructure and their relationship with the properties of the specimens were studied. The addition of Fe₂O₃ obviously promoted the MgO grain growth and conversion of large open pores into small closed pores, attributing to the formation of cationic vacancies and intergranular MgFe₂O₄ bonding phase. Owing to the presence of closed pores and networks of intergranular MgFe₂O₄, both thermal insulation and strength were enhanced significantly. Besides, the formed closed pores and MgFe₂O₄ phase could accommodate thermal stress and induce transgranular fracture and crack deflection, therefore effectively improving the thermal shock resistance. The specimen with 15 wt% Fe₂O₃ showed a apparent/closed porosity of 0.7%/10.1%, median pore diameter of 4.37 µm, thermal conductivity of 9.3 W/(m·K) (500 °C), flexural strength of 143.5 MPa, and residual flexural strength of 24.1 MPa after thermal shock.     

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.     

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.     

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.     

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.     

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     

Cr2O3对MgO-Al2O3系浇注料性能的影响

研究了氧化铬的引入方式及引入量对钢包渣线用MgO-Al2O3系浇注料性能的影响.结果表明,以工业氧化铬方式引入Cr2O3比以镁铬尖晶石方式引入Cr2O3对材料的热震稳定性更有利,Cr2O3的引入量控制在2%～4%时,材料的抗渣性可以得到显著改善.     

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.     

Optimized microstructure with nano-alumina and its effects on properties of corundum spinel castables

The mechanical properties, thermal shock resistance, and microstructure evolution of corundum spinel castables with different nano-alumina contents were investigated. The results show that the addition of nano-alumina has advantages on the microstructure evolution and properties of castables. An optimum nano-alumina amount is 2 wt%. When nano-alumina addition changes from 0 to 2 wt%, the cold modulus of rupture (CMOR) and cold compressive strength (CCS) improved by 69.7% and 78.1% after firing at 1100°C, respectively. The CMOR and CCS increased by 42.5% and 35.2% after firing at 1500°C, respectively. Meanwhile, the hot modulus of rupture (HMOR) was enhanced by 21.5% to 13.4 MPa. The retained Young's modulus values of castables were improved from 49.6% to 59.6% after eight thermal shock cycles. Furthermore, the HMOR and the retained Young's modulus values of castables slightly reduced when nano-alumina content up to 3 wt%. XRD, DSC, SEM, and EDS analyses revealed that the addition of nano-alumina leads to the formation of calcium dialuminate (CaO·2Al₂O₃) at 1100°C and it is beneficial to the formation of more platelet hibonite (CaO·6Al₂O₃) at 1500°C. As a result of using nano-alumina with large surface area, the solid phase sintering of the nanoscale particles can occur at lower temperatures. Moreover, the mechanical properties and thermal shock resistance of the castables were improved remarkably.     

矾土基电熔尖晶石对矾土基浇注料性能的影响

为了开发矾土基电熔尖晶石的应用,在矾土骨料、纯铝酸钙水泥、Al2O3微粉、SiO2微粉等原料加入量不变(分别为65%、4%、4%和1%)的条件下,变化矾土基电熔尖晶石的加入量(分别为0、6%、8%、10%和12%)和电熔白刚玉细粉的加入量,研究了矾土基电熔尖晶石对矾土基浇注料常温物理性能、热态抗折强度、抗热震性以及抗渣性的影响。结果表明,与不加矾土基电熔尖晶石的浇注料相比,尖晶石加入量为6%~12%时,浇注料的常温性能相差不大,但热态抗折强度及抗热震性均有所提高,抗渣性有很大改善。     

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.     

Structure and properties of lightweight magnesia refractory castables with porous matrix

To improve the energy-saving capacity of magnesia refractory castables for working lining of high-temperature kilns, this study presents the researches on microstructure and properties of lightweight magnesia refractory castables with porous matrix fabricated by direct foaming method. The results show that formation of closed-pores in the matrix significantly enhanced high-temperature thermal insulation performance of castables with minor changes of slag corrosion resistance. The thermal conductivity of the lightweight magnesia castables at 1000 °C was below 1.2 W/m·K, which is 47.8% lower than that of the referenced magnesia castable. The increasing content of SDS (foaming agent, over 0.02 wt%) led to increments of size and number of large-sized pores, resulting in the significantly decreased density and mechanical performances. The slag resistance mechanism reveals that, in addition to intergranular penetration, the accumulation of slag and penetration between adjacent pores were the major ways of slag mass transfer in lightweight magnesia castables. In conclusion, controlling the size (below 53.2 μm), number and distribution of closed-pores in the matrix is effective to realize the coupling of high thermal insulation, mechanical properties and slag resistance for lightweight magnesia castables used in the metallurgical field.     

铬矿对镁质浇注料性能的影响

在以8～5mm(18%,质量分数,下同)、5～3mm(18%)、3～1mm(22%)的97高纯镁砂和≤1mm(12%)的97电熔镁砂为骨料,≤0.088mm(20%)的97电熔镁砂,SiO2微粉(2%),α-Al2O3微粉(5%),亚白刚玉粉(3%)为基质细粉,六偏磷酸钠等为复合分散剂的镁质浇注料基础配方中,分别用5%、8%、15%的铬矿砂(0.833～0.178mm和≤0.088mm的混合料)等量替代≤0.088mm的电熔镁砂制成不同铬矿含量的镁质浇注料试样。分别测试样经110℃24h、1100℃3h和1550℃3h处理后的线变化率、显气孔率和抗折强度,并利用SEM和EDX分析1550℃3h处理后试样的显微结构。结果表明:(1)镁质浇注料中加入适量的铬矿,有利于提高方镁石晶粒间的直接结合强度,使材料抗折强度提高,显气孔率下降;适量微裂纹的产生有利于浇注料抗热震性的提高。铬矿的适宜加入量为5%～8%。(2)SEM和EDX分析表明,试样经1550℃3h处理后,铬矿和镁砂呈相互扩散,并脱溶出二次尖晶石,增强了晶粒间的直接结合程度,从而有效地提高了镁质浇注料的力学性能。     

含镁铝尖晶石的铝酸盐水泥的制备与应用

按天然白云石(≤0.1mm)与工业氧化铝粉(≤0.088mm)的质量比为45∶55配料,经混合、成型和烘干后,于1600℃3h煅烧,细磨烧结体得到含镁铝尖晶石的新型铝酸盐水泥。测量了新型铝酸盐水泥的凝结时间、耐火度以及用其所结合高铝矾土制成的耐火浇注料的早期强度;利用XRD、SEM和EDS分析了新型铝酸盐水泥的物相组成及其形貌和分布,同时采用静态坩埚法对比了新型铝酸盐水泥和纯铝酸钙水泥结合刚玉浇注料的抗渣性差异。结果表明:新型水泥的物相组成为镁铝尖晶石(MA)、一铝酸钙(CA)和二铝酸钙(CA2),且这3个物相的分布较为均匀;新型水泥的凝结时间正常,耐火度高于纯铝酸钙水泥;用新型水泥制得的刚玉浇注料抗渣性好;其原因是新型水泥组成中引入了镁铝尖晶石相,而镁铝尖晶石具有较高的熔点和抗熔渣侵蚀能力。     

沥青涂覆镁砂对MgO-C质底吹供气元件抗热震性的影响

为了提高顶底复吹转炉用MgO -C质底吹供气元件的使用寿命 ,研究了沥青涂覆量和沥青涂覆镁砂颗粒添加量对MgO -C砖性能的影响。结果表明 ,添加沥青涂覆镁砂颗粒能明显改善MgO -C砖的抗热震性能 ,同时在保证合理的体积密度、显气孔率和强度的基础上 ,得出了能够提高供气元件抗热震性能的合理配比方案为 :沥青涂覆量为 3% ,沥青涂覆镁砂和电熔镁砂的质量比为 1 1。MgO -C砖抗热震性能提高的原因是由于高温下沥青涂覆镁砂中的沥青炭化而在镁砂颗粒周围形成缝隙 ,这些缝隙能吸收和消除较多的变形能量     

Influence of bonding phases on properties of in-situ bonded porous SiC membrane supports

Porous SiC ceramic membrane supports are widely employed in a wide variety of high-temperature applications, such as hot flue gas filtration, porous burners and molten metal filters. Herein, SiC supports, with a porosity of ~37%, were prepared by using low-temperature bonding techniques and the influence of different bonding phases, such as mullite, cordierite and glass, on ambient-temperature flexural strength, hot modulus of rupture (HMOR), thermal shock resistance and oxidation resistance were systematically investigated. The results reveal that the glass-bonded SiC (GBSC) support exhibited the highest ambient-temperature flexural strength of 33.6 MPa, whereas the flexural strength of mullite-bonded SiC (MBSC) and cordierite-bonded SiC (CBSC) supports ranged from 22 to 25 MPa. However, the presence of glass phase deteriorated the high-temperature properties of the support. MBSC support rendered superior mechanical strength at high temperature and self-strengthening in a certain temperature range, such as HMOR improved 47.5% at 900 °C, but HMOR of glass-bonded support was only 57.4% of the ambient-temperature strength. Moreover, MBSC and CBSC supports exhibited better thermal shock resistance than GBSC supports and the critical temperature difference of water quenching for MBSC supports was ~200 °C higher than GBSC supports. In addition, MBSC support rendered superior oxidation resistance and exhibited a weight gain rate of ~0.1% at 1150 °C for 24 h, which is 54.4% and 42.2% lower than CBSC and GBSC supports, respectively.     

氧化铝微粉加入量对低碳镁碳砖性能的影响

以电熔镁砂、石墨、金属Al和氧化铝微粉为主要原料,热塑性酚醛树脂为结合剂,制备了w(C)5%的低碳镁碳砖试样,研究了氧化铝微粉加入量(其质量分数分别为0、3%、5%、7%、9%)对低碳镁碳砖的常温物理性能、抗热震性和抗氧化性的影响。结果表明:(1)随着氧化铝微粉加入量的增加,低碳镁碳砖试样的体积密度、常温强度和高温强度均先升高后降低,显气孔率则先降低后升高;抗热震性和抗氧化性随微粉加入量的增加呈先改善后变差的趋势;其中,微粉加入量为5%的低碳镁碳试样综合性能最好。(2)低碳镁碳砖试样性能的改善主要是由于加入的氧化铝微粉和MgO在高温下原位反应生成连续分布的尖晶石,增强了基质的陶瓷结合;但氧化铝微粉加入量过大时,由于大量原位反应引起材料产生过大的体积膨胀,会导致基质结构疏松,从而恶化镁碳砖的性能。