Properties of silica sol bonded corundum‐spinel castables for steel ladles

In order to overcome the shortcoming of the calcium aluminate cement (CAC) bonded castables, we prepared corundum‐spinel castables using silica sol as binder and tabular corundum, sintered magnesia‐alumina spinel, and reactive alumina as raw materials in this study. The effect of spinel grain size and solid content of silica sol on the flow value, sintering, mechanical strength and microstructure of the specimens treated at varying temperature of 400, 1000, 1500, and 1650°C for 5 hours in an air atmosphere were studied by SEM and EDS analyses. The results indicate that silica sol is suitable as a binder for corundum‐spinel systems. And silica sol with solid content of 25% bonded samples containing ≤90 μm spinel perform quite better than the others. At the same time, silica sol bonded samples had high strength in medium temperature. This is because that the closer proximity of silica sol and alumina powder and the high activity of nanometer SiO₂ in silica sol are beneficial for the reaction of SiO₂ and Al₂O₃ to generate mullite needed for reinforcement of castables matrix.     

陶瓷梗对致密浇注料性能的影响

对比研究了添加莫来石质陶瓷梗和不锈钢纤维分别对刚玉质超低水泥浇注料和高铝质超低水泥浇注料性能的影响 ,并从显微结构分析了其影响机理。研究表明 :莫来石陶瓷梗能显著改善烧成后致密浇注料的抗热震性和体积稳定性 ;而含不锈钢纤维的浇注料烧成后 ,纤维氧化成氧化铁和氧化铬 ,促进了烧结以及纤维与基体之间的结合 ,使浇注料的刚性增加 ,高温抗折强度提高 ,但抗热震性下降 ,同时 ,高温下钢纤维氧化生成更大体积的氧化铁 ,导致了浇注料本体的膨胀 ,使其烧成线变化增加。     

High alumina castables reinforced with SiC

Abstract

To improve the sintering, mechanical and refractory properties of high alumina castables, different contents of SiC (up to 8 wt.-%) were added at the expense of high alumina cement. Cold crushing strength of the green samples was measured and hydration behaviour was studied using differential thermal analysis (DTA). After firing at 1550°C for 3 h, the compositions of the fired samples were investigated using X-ray diffraction. Sintering parameters (bulk density and apparent porosity) and mechanical (cold crushing strength) as well as refractory (thermal shock resistance, permanent linear change, load bearing capacity) properties were tested according to standard specifications. It was concluded that increasing content of added SiC results in an improvement in sintering, mechanical and refractory properties but adversely affects green strength. However, castables containing 6 wt.-% SiC show a reasonable compromise between acceptable green strength and improved sintering, mechanical and refractory properties. The improved properties of the fired samples are related to the formation of SiC-mullite system in the matrix.     

高强铁沟浇注料的研制与应用

高炉用Al2O3-SiC-C质铁沟浇注料高温性能低主要与水泥、硅微粉的加入量有关,以棕刚玉、碳化硅等为主要原料,研究了硅微粉、水泥加入量对浇注料强度的影响,研究结果表明:通过采用高铝80水泥代替原用70水泥并适当降低两者的加入量,可以保证材料的常温强度不下降的前提下,显著提高高温抗折强度,其原因在于形成了发育良好的莫来石晶体,将该技术在1080 m3高炉的铁沟使用,通铁量提高3.8万吨,取得了良好的使用效果.     

Low Temperature Synthesis of High Alumina Cements by Novel Co‐Melt Precursors and Their Implementation as Castables with Some Micro Fine Additives

In the present investigations nano size high alumina cements (HAC) were prepared by very effective co‐melt precursor sintering technique from their metal nitrate precursors. The prime cementing phases observed were CA, CA₂, and C₁₂A₇. The addition of nano structured cements in refractory castables has improved the thermo‐chemical‐mechanical properties to a significant extent. Each batch of low cement castables (LCC) was prepared from calcined Chinese bauxite, HAC, and superfine additives. The effect of HAC in bauxite castable with the additives similar to Silicon Carbide, reactive alumina, and micro‐fine silica on the sinterability and properties of these castables was investigated. Physical properties such as apparent porosity and bulk density, mechanical properties such as hot modulus of rupture (HMOR), cold and hot modulus of rupture (CMOR), and cold crushing strength (CCS) of hydrated and sintered castables were studied. The sintered castables were also characterized for their solid phase compositions and microstructure using X‐ray diffraction (XRD) and FE‐SEM, respectively. In the castables new phases such as mullite, α‐alumina were formed at the expense of bauxite and silica. Solid solution of mullite formed at high temperature acts as a bonding phase and is accounted for high HMOR, CMOR, and CCS values. These excellent properties of such castables may enable their uses in various applications such as refractory lining for fabrication of steel, aluminium, copper, glass, cement, chemicals, and ceramics.     

Effect of Bauxite Aggregate on Properties of Al_2O_3-SiC-C Iron Trough Castables

Al2O3- SiC- C iron trough castables were prepared using high alumina bauxite clinker GL-90( 1 mm) or homogenized bauxite GL-88 to replace the traditional brown corundum aggregates( 1 mm) to reduce the cost of iron trough castables. Effects of the two bauxite aggregates on properties of iron trough castables were investigated. The results show that the two kinds of bauxite affect flowability,bulk density,strength and slag corrosion resistance of trough castables differently. Compared with homogenized bauxite,high alumina bauxite clinker is more suitable to substitute brown corundum. The castables prepared from the bauxite clinker have similar performances with those prepared from brown corundum,especially used in medium or small blast furnaces with weak thermal impact and lower temperature hot metal.However,in large blast furnaces with severe thermal impact and high temperature hot metal,the performances of the two castables are significantly different. So it is suggested to use the brown corundum based castables in the front gyral zone of the slag skimmer.     

硅溶胶结合刚玉-莫来石快干浇注料的性能与应用

对比测试了常温浇注成型及在模拟热态修补条件下浇注成型的硅溶胶结合刚玉-莫来石快干浇注料分别在815、1 100和1 400℃保温3 h热处理后的常温抗折强度、常温耐压强度、体积密度和加热永久线变化以及常温浇注成型的硅溶胶结合刚玉-莫来石浇注料和矾土水泥结合刚玉质浇注料的抗热震性,并在钢厂对硅溶胶结合刚玉-莫来石浇注料进行了现场应用试验。结果表明:硅溶胶结合刚玉-莫来石浇注料的烘干强度与普通低水泥刚玉质浇注料的相当,815和1 100℃热处理后强度比烘干强度高;硅溶胶结合刚玉-莫来石浇注料热震循坏(1 100℃,水冷)100次后基本上没有出现裂纹,其耐压强度损失率仅为18.7%;在模拟热态条件下成型的硅溶胶结合刚玉-莫来石浇注料的性能与常温浇注成型的相当。在现场应用试验中,硅溶胶结合刚玉-莫来石浇注料的脱模时间和烘炉时间大大缩短,并且可以热态浇注,具有良好的使用性能。     

Effect of grinding on the hydration of hydratable alumina and properties of hydratable alumina-bonded castables

Hydratable alumina (HA) is a superior Ca-free refractory binder, but the quick hydration rate restricts the working time of castables bonded with HA. In this work, HA was grounded for 1 h and 6 h by a rotational ball mill to study the effect of grinding on the hydration of HA and properties of HA-bonded castables. HA samples with and without grinding were cured at 30 °C and then terminated by freeze-vacuum drying. The phase composition and microstructure of the dried HA samples were then examined. Moreover, flow ability and mechanical strength of castables containing ungrounded and grounded HA were also investigated. The results indicate that the specific area of HA particles were decreased by grinding as the micro-pores and micro-cracks on the surface of HA particles were blocked by smaller HA particles, thereby decreasing the hydration rate of HA and increasing the flow ability of castables.     

水泥含量对高纯铝尖晶石自流浇注料性能的影响

以致密刚玉和尖晶石为主要原料,铝镁尖晶石的含量为19%,在颗粒组成(5～0 mm)不变的情况下,研究了水泥含量分别为3%、8%和13%对高纯铝尖晶石自流浇注料性能的影响.结果显示1)随着水泥含量的增加,浇注料的加水量、干燥后致密度和强度均增加,但烧后致密度和强度都下降;2)水泥含量的变化对自流浇注料热态强度的影响不明显;3)随着水泥含量的增加,自流料烧后线变化增加,但水泥含量达到8%以后变化不明显;4)对于LF精炼炉渣,不管水泥的含量多少,自流料都不被渗透,但随水泥含量增加,自流料的抗熔蚀性变差;5)对于转炉钢包渣,随着水泥含量的增加,自流料的抗渗透性增强,抗熔蚀性变差,而渗透与熔蚀量之和减少;6)对于转炉末期渣,随着水泥含量增加,浇注料的抗渗透性增强,抗熔蚀性变差;7)应用相图分析了铝尖晶石浇注料的侵蚀机理,认为渣中Fe2O3或低熔点相CF的增加是渣渗透的主要原因.     

Controlling hydration of hydratable alumina via co-grinding with Mg–Al hydrotalcite

The on-site industrial application of hydratable alumina (HA)-bonded castables is inhibited by the high hydration rate of HA. In this study, the hydration behavior of HA co-ground with sheetlike Mg–Al hydrotalcite (MAT) is investigated. The properties of castables bonded with MAT-bearing HA are systematically assessed. The hydration rate of HA co-ground MAT decreases as this allows MAT sheets to be effectively inserted into the microcracks of HA particles during grinding, thus decreasing the direct contact area between HA and water. The strength of MAT-bearing castables (0.5 and 1 wt%) fired at 800°C improved slightly owning to the generation of magnesia–alumina spinel. The mechanical strength of castables fired at 1100 and 1550°C decreased as the MAT content increased owing to an increase in porosity. Based on an analysis of the hydration behavior of HA and the properties of HA-bonded castables, the optimal MAT/HA weight ratio is approximately 1/10.     

Effects of cement content on the microstructural evolution and mechanical properties of cement-bonded corundum castables

The cement-bonded corundum castables are often subjected to great temperature gradient in the service process of purging plugs for refining ladle. The mechanical properties of such castables are of particular interest in the serviceability and the safety of purging plug, which are significantly influenced by the microstructures including the amount, size and morphology of materials. The reported cement contents in compounds of purging plugs are generally low or ultralow, which inevitably limits the adjustable range of the mechanical properties enhancement by regulating their microstructures. In the present research, a serial of comprehensive experiments have been carried out, including the high cement contents (10–15 wt%), so as to further understand the role of cement content on the microstructural evolution and mechanical properties of castables. It is found that the phase compositions and microstructure can be optimized through adjusting the cement content in corundum castables. When the cement content is below 10 wt%, the hexagonal flake CA₆ grains are platelet-shaped both in the matrix and at the border of the aggregates, their amount and size increase and distributions are more uniformly with cement content. When the cement content is above 10 wt%, small amount of granular crystals CA₂ are detected in matrix, and CA₆ crystals transform to equiaxial morphologies after cement content of 10 wt%. This research also provides quantitative relationship between the mechanical properties of the castables and the cement content. Castables with cement content of 10 wt% contain most hexagonal flake CA₆ crystals, so that they have the highest CMOR and HMOR after heating at 1600 °C. However, CMOR decreases after cement content of 10 wt% due to the porosity and volumetric expansion from the formation of in-situ CA₆ and CA₂.     

Effect of hydromagnesite on the hydration of hydratable alumina and properties of corundum-based castables

Hydratable alumina (HA) was premixed with hydromagnesite (BMC) to investigate the BMC impact on the hydration behavior of HA and the thermo-mechanical properties of HA bonded (HAB) castables. The phase composition, microstructure and mass changes of dried HA samples, were characterized by XRD, SEM, and TG, respectively. Flow ability, microstructure, and thermo-mechanical properties of HAB castables were studied. Results indicated that BMC effectively lowered HA hydration rate due to the decreased specific surface area. The hot modulus of rupture strength of castables was improved because the sintering of Al₂O₃ was enhanced by the MgO from BMC decomposition.     

Phase evolution and hydraulic properties of low cement castables matrixes

Abstract

To identify the hydration products of aggregate-free, low cement castables (LCC), cement matrixes were examined. Two series of cementing batches based on 33·3 and 50·0 wt-% high alumina cement (HAC) were processed by adding ultra-fine calcined alumina/fumed silica mixtures (FA/FS) with weight ratios of 1·0, 1·5, 2·3, 4·0, and 9·0 to each series. The hydrated batches were investigated for their hydraulic properties. Batches showing the highest cold crushing strength with minimum water of consistency (WOC) and reasonable setting time (ST) were selected and characterised with respect to phase composition, microstructure, and microchemistry, before and after firing up to 1400°C, by XRD, DTA, and SEM techniques. Cementing batches containing ≈33 wt-% HAC, and ≈67 wt-% FA/FS mixtures with ratios of 4·0–9·0 show optimum particle packing and hydration conditions with the least amount of WOC. This results in increases in cold crushing strength (CCS) of cementing batches up to 58 MPa after hydration for 3 d. The hydrated batches are composed mainly of unreacted α-Al₂O₃ particles bonded by CAH₁₀, AH₃, and C₂ASH₈ phases. On firing up to 1400°C, the hydrated phases are transformed into anhydrous CA₂ and CA₆ patches enclosing limited amounts of CAS₂ and/or C₂AS phases. Such batches are suitable for application as cementing matrixes for high alumina low cement castables. The low HAC content with high FA/FS ratio in the presence of more fine alumina in the matrixes of such castables leads to significant improvement in their hot mechanical properties.     

新型结合剂SioxX-Zero对喷煤管用无水泥浇注料显微结构和性能的影响

喷煤管是水泥回转窑的关键承力部位,长期承受高温熟料粉尘和气流的剧烈冲刷,需具备优异的高温力学性能。本工作分别以新型SioxX-Zero和传统铝酸钙水泥为结合剂,采用板状刚玉、α-Al₂O₃微粉和碳化硅等为主要原料,分别在1 100 ℃和1 400 ℃热处理3 h,制备了喷煤管用无水泥和低水泥结合Al₂O₃-SiC质浇注料,并研究了新型SioxX-Zero结合剂对浇注料显微结构、常规物理性能和高温力学性能的影响。结果表明:与传统铝酸钙水泥相比,SioxX-Zero能有效提高浇注料流动值和延缓流动值衰减,改善其施工性能。同时,含SioxX-Zero的Al₂O₃-SiC质浇注料中原位生成了数量较多、尺寸较大的针状莫来石,大幅改善了材料的常温抗折/耐压强度、高温抗折强度和高温耐磨性能。     

The role of hydraulic binders on magnesia containing refractory castables: Calcium aluminate cement and hydratable alumina

Previous work by the authors has shown that the effects of calcium aluminate cement (CAC) and hydratable alumina (HA) can modify the magnesia hydration behavior in aqueous suspensions. As a consequence of these studies, the present paper highlights how varying the content of these binders can affect magnesia hydration in refractory castables using pH, apparent volumetric expansion, mechanical strength and porosity measurements and hydration–dehydration tests. Furthermore, as mechanical strength, porosity and refractoriness also play an important role in these materials, binder-free, magnesia-free and magnesia-and-binder-free samples were also tested as references. It was found that the deleterious effects of magnesia hydration can be greatly minimized by the binder and its selection content.     

不同结合系统刚玉-方镁石-尖晶石浇注料的性能

对比研究了水泥结合和水合氧化铝结合的刚玉 -方镁石 -尖晶石浇注料的性能。结果表明 :二者相比 ,水合氧化铝结合浇注料的需水量较大 ,流动性较差 ,烧结致密化温度高 ,在 1 5 5 0℃下烧结后的强度和致密度低 ,但其抗热震性能及抗渣性能较好。     

Microstructure and phase evolution of alumina-spinel self-flowing refractory castables containing nano-alumina particles

The microstructure and phase composition of alumina-spinel self-flowing refractory castables added with nano-alumina particles at different temperatures are investigated. The physical and mechanical properties of these refractory castables are studied. The results show that the addition of nano-alumina has a great effect on the physical and mechanical properties of these refractory castables. With the increase of nano-alumina content in the castable composition, the mechanical strength is considerably increased at various temperatures. It is shown that nano-alumina particles can affect formed phases after firing. The platy crystals of CA₆ are detected inside the grain boundaries of tabular alumina and spinel grains in samples fired at 1500 °C. CA₆ phase can be formed at lower temperatures (1300 °C) with the addition of nano-alumina particles. As a result of using nanometer-sized alumina particles with high surface area, the solid phase sintering of the nano-sized particles and CA₆ formation can occur at lower temperatures.     

Enhancement of bonding network for silica sol bonded SiC castables by reactive micropowder

Silica sol bonded castables have obvious advantages over low cement or hydratable alumina bonded castables in drying performance and sintering properties for SiC castables. However, they are not widely used due to their weak strength at low temperature. The efficiency of bonding network for silica sol bonded SiC castable in the presence of different reactive micropowder such as SiO₂ micropowder and α-Al₂O₃ micropowder was evaluated through oscillatory tests, sintered properties and microstructural analysis. Results show that the polymerization reaction between SiO₂ micropowders enhanced the siloxane network and reinforced the bonding strength, furthermore, the addition of α-Al₂O₃ micropowder contributed to accelerating the formation of the siloxane network and hardening of the silica sol at lower temperatures and shorter time. Silica sol performed well as a binder agent for SiC castables with an addition content of 3 wt% SiO₂ micropowder and 2 wt% α-Al₂O₃ micropowder, which showed high strength and good workability at room temperature. And Silica sol bonded SiC castable with the above micropowder contents possessed the best mechanical behavior after heat treatment due to combined binding of SiC whiskers and mullite.     

耐火材料用有机-无机复合结合剂的研制

以水泥为结合剂的浇注料中含有CaO,在烧成过程中能够生成低共熔相而影响了其高温性能。为了提高浇注料的高温使用性能,需要在保证浇注料具有一定的常温强度的基础上,研制低水泥或者无水泥的新型结合剂来替代水泥结合剂。以丙烯酰胺体系为基础,探讨了丙烯酰胺系有机-无机结合剂在矾土基浇注料中的实际使用效果。结果表明,浇注料在110℃烘干后的体积密度、显气孔率、耐压和抗折强度等常温性能良好,尤其是抗折强度得到很大的提高。在不同高温烧成后,有机结合剂的浇注料性能得到提高。     

Room temperature mechanical properties of high alumina refractory castables with spinel, periclase and dolomite additions

The mechanical properties of refractory castables at room temperature are critical parameters for selecting suitable operating conditions for the structural design of refractory components. In this work, high alumina refractory castables based on the alumina-rich zone of the Al₂O₃–MgO–CaO ternary phase equilibrium diagram were prepared by adding synthetic spinel, periclase and dolomite via three processing routes. Bending strength studies at room temperature under several thermal treatments and the analysis of the elastic modulus of the refractories and their matrices point to two different mechanical behaviours. From room temperature to 1000 °C the refractory castables present a pronounced non-linear stress–strain behaviour both in the uniaxial tensile and compressive modes, as a result of damage to the microcrack network. Above 1000 °C the refractory castables begin to sinter owing to a transitory liquid phase, the crystallization of calcium aluminate cement phases (such as CA₂ and CA₆, for example) and the self-forming spinel phase (refractory castables with periclase or dolomite additions). At higher firing temperatures the sintering process leads a strengthening of the mechanical properties.