Permeability of High-Alumina Refractory Castables Based on Various Hydraulic Binders

This work investigated the changes in the permeability of high-alumina self-flowing refractory castables based on calcium aluminate cement (CAC) and hydratable alumina binder (HAB) pretreated between 110° and 1650°C. Permeability constants k ₁ and k ₂ were fitted from Forchheimer's equation based on airflow tests conducted at room temperature. The results indicated that dehydration was the main contributing factor for increased permeability in bodies pretreated up to 600°C and that sintering effects prevailed between 900° and 1650°C. Castables based on HAB were less permeable than those based on CAC, a behavior ascribed to the type of hydrates formed and to the particle-packing features of the matrix and the matrix–aggregate interfaces.     

无钙氧化铝结合的耐火浇注料干燥过程中的动态透气现象

了解与耐火浇注料脱水现象相关的值是必需的，尤其是透气值，它是控制过程最重要的参数。因透气值经常在室温下获得，因此，不能反映水排除过程中发生的明显的微观结构的变化。对于脱水过程仍不清楚且易发生剥落的以无钙结合剂结合的浇注料配方，这个问题变得更加迫切。这项研究已经从无钙高铝质耐火浇注料试样在700℃以上的干燥过程的动态透气性开始进行。试样在110—1650℃的温度范围内经预处理，以提供浇注料的可逆和不可逆的微观结构变化的信息。结果显示了在接近200℃时透气量的波动和明显下降，它有助于解释此类浇注料出现剥落现象的原因。     

Permeability of Refractory Castables at High Temperatures

Temperature is an important factor affectng the permeation of fluids in refractory castables. Because of experimental difficulties, however, permeability parameters are usually obtained at room temperature and then extrapolated to the temperature of interest, with no concern regarding porous structure modification (thermal expansion, etc.). In this work, an apparatus has been developed to evaluate the air-flow permeability of refractory castables at temperatures up to 800°C. The objective is to investigate modifications in the permeability constants obtained by Forchheimer's equation to provide a more realistic relationship with castable processing and molten-metal corrosion parameters.     

Assessment of Mass Loss and Permeability Changes during the Dewatering Process of Refractory Castables Containing Polypropylene Fibers

This work describes the use of a new permeametry technique to evaluate microstructural changes that take place in castables with high alumina-fiber content during the dewatering process. It is shown that the mass loss due to cement dehydration and the permeability increase caused by fiber burnout can be independently assessed, based on the fluid dynamic conditions established for the experiment. Control of the pressure drop through the sample was the key procedure used to analyze the phenomenon of interest.     

设计粒度分布的耐火浇注料的渗透性和干燥行为

控制粒度分布可以改变浇注料制品的流变学特性和机械性能。基于PSD设计软件的不同，对浇注料试样进行了渗透性试验和干燥试验，指出控制粒度分布对耐火材料用户而言是调整产品流动性能和机械性能的有效途径。     

Vaporization Processes and Pressure Buildup during Dewatering of Dense Refractory Castables

This work describes an experimental investigation on the dewatering kinetics of high-alumina refractory bodies under several heating conditions. The drying stages involving the removal of unbound water have been correlated with the two consecutive thermal processes by which liquid water transforms into vapor during heating: evaporation and ebullition. Thermogravimetric data have been used as the basis for a discussion of the parameters that affect the performance of both vaporization processes and guide the design of suitable heating schedules aimed at minimizing the drying time and the risk of harmful pore pressurization.     

In Situ Coagulation of High-Alumina Zero-Cement Refractory Castables

Coagulation methods originally developed for colloidal processing were investigated in this paper as alternative approaches to consolidate high-alumina refractory castables free of hydraulic binders (zero-cement). Three in situ reactions based on the direct coagulation casting (DCC) technique were evaluated to promote castable coagulation: (1) the autocatalytic hydrolysis of gluconic acid lactone, (2) the gradual dissolution of hydroxyaluminum diacetate particles in water, and (3) the enzyme-catalyzed hydrolysis of urea. The coagulating behavior of castables and matrix-representative suspensions was investigated with the help of zeta potential analysis, pH measurements, castable free-flow evaluation, and oscillatory rheological tests. The enzyme-catalyzed hydrolysis of urea seemed to be the most appropriate mechanism to promote the coagulation of initially self-flow zero-cement refractory compositions.     

Drying Stages during the Heating of High-Alumina, Ultra-Low-Cement Refractory Castables

The purpose of this work was to investigate the drying kinetics of high-alumina, ultra-low-cement refractory castables under continuous heating conditions. Three main drying stages were identified during the castable heat-up and were related to the phase change of free water and to the decomposition of hydrated products present in the body. A clear correlation was found between the actual heating profile inside the castable and the dewatering stages under various heating schedules. Thermal analysis was used to assess the drying temperature that represents the highest risk of steam pressure buildup and, thus, of explosive spalling.     

耐火浇注料的干燥性能

对浇注料干燥性能的了解是帮助耐火材料生产商优化加热过程的一个重要因素，由此可将在生产过程中替代损毁砖所花费的时间和能源费用降至最低。     

Influence of Air Compressibility on the Permeability Evaluation of Refractory Castables

Air compressibility is important in attaining an accurate quantification of the permeability parameters for refractory castables. Nevertheless, this effect is often neglected in the permeability equations that are found in ceramics literature. The consequences of omitting the compressibility effect in two of the main permeability equations are highlighted and discussed. The permeability to air flow of high-Al₂O₃ ultra-low-cement refractory castables has been quantified by Darcy's law according to U.S. ASTM C577–96 and European PRE-16th permeability methods, which propose distinct approaches for air compressibility. Forchheimer's equation, which establishes a more realistic dependence between fluid pressure and fluid velocity, also has been used to obtain the permeability constants k ₁ and k ₂. Results show that equations proposed by the ASTM and the PRE methods may yield very distinct values of Darcian permeability k ₁ for the same sample. Forchheimer's equation for compressible fluids provides the best fitting with experimental data.     

Permeable Porosity of Refractory Castables Evaluated by the Water-Expulsion Porosimetry Technique

In this work, the water-expulsion porosimetry technique was used to quantify the permeable pores of self-flow ultralow-cement refractory castables that were treated at 600°–1650°C. Results have shown that the method as proposed in previous works was not valid, because water was not removed continuously from pores, relative to increased pressure. Nevertheless, the maximum pore diameter obtained according to ASTM Method E128-89 could be successfully correlated with other castable physical properties (such as the permeability constants from the Forchheimer equation) and the apparent porosity obtained via the Archimedes technique. Although the apparent porosity decreased as the thermal treatment temperature increased, the pore morphology changed continuously, with the generation of less-tortuous and more-permeable paths for fluid flow.     

试样尺寸对高铝质浇注料干燥性状的影响

分析了试样尺寸对超低水泥高铝质浇注料的干燥性状的影响，通过从室温到800℃进行的热重量分析试验所获得的不同尺寸试样的重量损失数据，绘制出了试样的升温曲线和干燥曲线，而且查明了试样的尺寸对产生炸裂也有重大的影响。还论述了包括这些因素在内在工业上的应用。     

高铝质超低水泥浇注料在不同加热速率下的干燥行为

对高铝质超低水泥耐火浇注料在脱水过程中的热重变化进行了研究。对在8℃养护的含有2％铝酸钙水泥的试样以0.5～25℃·min~(-1)的速率加热进行测试。结果表明,均匀加热的浇注料中水分的减少发生于随加热速率的增大强度逐渐发生变化的阶段。干燥速率在10℃·min~(-1)以上时,干燥时间没有明显的减少,而当加热速率大于20℃·min~(-1)时,坯体出现爆裂。     

红柱石基材料中氧化铝原料的应用研究

实验研究了几种氧化铝原料的加入对高纯红柱石基耐火材料性能的影响,采用XRD,SEM等手段分析了氧化铝原料种类及加入量对试样相组成和显微结构的影响.实验结果表明,纯红柱石制品的烧结性能比添加氧化铝细粉的红柱石制品的好,而不同氧化铝细粉对红柱石制品烧结性能的影响与其纯度及活性有关;红柱石颗粒分解产生的SiO2玻璃相难以全部与氧化铝反应形成二次莫来石;在本试验条件下,当氧化铝细粉加入量达到15%左右时,在试样中引起的二次莫来石化效应和烧成膨胀最大.     

Thermal Resistance of Grain Boundaries in Alumina Ceramics and Refractories

The influence of grain boundaries on heat transfer through polycrystalline alumina has been investigated between 20° and 500°C. The thermal conductivities of small-grained porous ceramics and large-grained dense ceramics have been measured using the laser-flash technique. Two methods have been developed to assess the average thermal resistance of a grain boundary. The first method is based on the comparison of room-temperature thermal conductivities for dense ceramics that have various average grain sizes. This method yields a value of 0.9 × 10⁻⁸ m²·K·W⁻¹. The second method, particularly suitable for porous ceramics, is based on the extrapolation of the inverse of the thermal conductivity versus temperature to give an intercept with the axis at T = 0 K. This value is attributed to the thermal resistance of grain boundaries. By taking into account the influence of the pore content using an effective medium theory, the average thermal resistance of a grain boundary has been evaluated to be 1.3 × 10⁻⁸ m²·K·W⁻¹ in dense alumina and 2.2 × 10⁻⁸ m²·K·W⁻¹ in alumina containing a pore volume fraction of 0.3.     

氧化铝原料对红柱石基耐火材料莫来石化行为的影响

研究了几种氧化铝原料对红柱石基耐火材料莫来石化行为的影响。对试样XRD数据的分析结果表明,所引入的烧结氧化铝、活性氧化铝和棕刚玉都对红柱石的分解转化有不同程度的抑制作用,其中棕刚玉对红柱石分解转化的抑制作用尤为强烈,原因在于引入的TiO2。但TiO2对二次莫来石的生成没有影响。红柱石的分解转化受制于高温液相中的氧化铝浓度。活性氧化铝加入量小于20%时,其加入量增加,对红柱石分解转化的抑制作用随之增大。氧化铝原料的反应活性取决于其在高温液相中的溶解速度。     

高铝超低水泥耐火浇注料加热期间的干燥步骤

研究了连续加热的情况下，高铝超低水泥耐火浇注料的干燥动力学。鉴别出浇注料在加热升温过程中的三个主要干燥阶段与坯体中自由水的相变及水化物的分解有关。在不同的加热时间下，浇注料实际升温过程与脱水阶段之间有一个明显的关系。热分析用来估算干燥温度，此温度代表蒸汽压力即爆炸成碎片的最大危险值。     

结合剂对高铝矾土可塑料性能的影响

本文通过XRD物相分析和SEM微观形貌分析研究了不同结合剂对矾土基可塑料物理性能的影响,探明了其结合机理.结果表明:矾土骨料最佳颗粒级配为:3～1 mm/≤1 mm/≤0.074 mm=47/18/25,其中软质粘土占骨料总质量的10％;其中以六偏磷酸钠和偏硅酸钠为结合剂的试样常温性能优于其他试样,试样在添加剂中引入了Na+和Ca2,使得在试样在烧结中抑制了莫来石的形成,同时产生低熔物,提高烧结致密性,降低了显气孔率,导致其常温物理性能较好,但高温性能较差.     

Modified Pressure-Decay Technique for Evaluating the Permeability of Highly Dense Refractories

The experimental evaluation of permeability parameters encounters different difficulties, depending on the features of the materials. Traditional steady-state techniques for refractory castables and other highly dense ceramics demand arduous work and require reliable measurements at high pressure and for extremely low flow rates. In this paper, a recent and promising technique known as vacuum-decay permeametry is modified to consider the effect of gas compressibility and fluid inertia. Darcy's law is replaced by Forchheimer's equation, which has been proved to better represent the interactions between fluid and a castable porous medium. The modified method is modeled and applied experimentally to refractory samples with different permeability levels. Results that have been compared to a well-established steady-state permeametry method confirm the validity of the model.