Study of the castable selection for blast furnace blowpipe

The contradictory properties required of castable refractories makes selecting castable refractories for industrial applications challenging. This paper seeks to describe the material selection for a blast furnace blowpipe application that is subjected to sudden temperature changes and must prevent heat loss. Three commercial high alumina castables containing andalusite or mullite from different manufacturers were characterized. Thermal shock damage resistance was evaluated using thermal shock damage resistance theory and experiments. The castables’ coefficient of thermal expansion was estimated using quantitative X-ray diffraction. Crack propagation resistance was measured using the work-of-fracture technique. Thermal shock damage was experimentally evaluated by measuring the modulus of elasticity and rupture prior to and after thermal cycles. Ultimately, the microstructure of the castables was related to the thermal shock damage behavior by estimating the aggregate size and the fracture toughening mechanisms using light optical and scanning electron microscopes. Heat loss was evaluated by calculating the blowpipe shell temperature using a one-dimensional steady-state heat conduction model. The best commercial castable refractory for blowpipe showed high thermal shock damage resistance and low thermal conductivity. The results in this study agreed with thermal shock damage resistance parameters and showed a correlation between coarse microstructure with large aggregate and higher thermal shock damage resistance.     

On the problem of heat resistance of chamotte refractories

Results of a study of the heat resistance of chamotte refractories as a function of the composition of their binder are presented. The introduction of a refractory particolored clay or clay DN2 rich with alkaline and alkaline-earth oxides into chamotte-kaolin mixtures hampers the formation of cristobalite. The cristobalite effect and the temperature coefficient of linear expansion of the specimens decreases up to a temperature of 600°C, which enhances their heat resistance     

High-temperature mechanical characterisation of an alumina refractory concrete for Blast Furnace main trough: PART II. Material behaviour

This paper deals with the experimental mechanical characterisation of a high alumina carbon containing refractory concrete at temperatures ranging from room to high temperature (1500 °C). Uniaxial compression, indirect tensile, creep and cyclic loading–unloading tests have been performed. Scale factor effect, influence of interfaces lubrication, material initial heat treatment as well as oxidising or reducing atmosphere test conditions have been investigated. Some of the material mechanical properties have been found to depend not only on the level of applied temperature but also on its duration revealing then the sensible influence of chemical transformation kinetics within the material. An analysis of the global macroscopic mechanical behaviour of the material through the range of temperature is lastly exposed. The data collected are aimed at identifying damage behaviour constitutive equations for numerical simulation of structures built within this material.     

Procedural elements in estimation of the thermal shock resistance of different types of refractory concrete based on chamotte filler

A comparative analysis of the thermal shock resistance of refractory concrete based on chamotte filler, such as traditional concrete, traditional concrete modified with the addition of microsilica, and medium-cement concrete, is performed. It is shown that the method employed to determine thermal shock resistance with the use of cooling of the concrete samples with water cannot be applied to determine this indicator in the case of traditional and modified types of concrete due to the reaction of the water with minerals in the cement. The thermal shock resistance of different types of concrete determined by means of ultrasonic equipment and the calculated thermal shock resistance criterion R _st supplies the most accurate estimate of the thermal shock resistance of the types of concrete studied here.     

Drying and firstheat up of a kiln unit with cyclone heat exchangers with a lining of refractory concretes

Conclusions An accelerated drying and first heat up cycle was developed for a kiln unit for dry production of clinker with a capacity of 3000 tons/day with cyclone heat exchangers with a lining of refractory concretes of high-alumina cement with a chamotte aggregate. The drying of the lining and the heating of the unit were done in 4 days. The results of the work indicate the desirability of use of refractory concretes for lining the cyclone heat exchangers of kiln units for dry production of clinker.Translated from Ogneupory, No. 3, pp. 46–49, March, 1985.     

Role of fatigue in damage development of refractories under thermal shock loads of different intensity

Refractories insulation of industrial furnaces often fail under repetitive thermal shock. Degradation of silica refractories under thermal shock loads of different intensity was studied. The load variation was achieved by utilisation of geometrically similar samples of different dimensions. Finite element method modelling predicted loads developing during the test. Resulting damage was determined by the ultrasound velocity and crack patterns. Tests involving up to 150 cycles demonstrated the role of fatigue in enabling sub-critical crack formation and countering the crack arrest. Repetitive cycles reduce crack wake friction and intensify loading due to crack debris re-location. Damage saturation, sigmoidal and near-exponential damage growth was typical for low, intermediate and high loads, respectively. Similar trends of damage accumulation were observed in mechanical displacement controlled cyclic fatigue tests performed in wedge splitting set-up. Strain and strain energy based criteria of thermal shock intensity seem to have complimentary value in predicting the crack formation and growth. Thermal shock damage after the first cycle seems to be an effective parameter to predict overall resistance to the degradation in the sample. Load reduction due to previous crack formation related to the fatigue potential for subsequent crack development can explain the crack size variation typically observed in refractories after multiple thermal shocks. For thermal shock tests, the variation of sample size, instead of the temperature interval, is a suitable alternative for refractories with strongly temperature dependant material properties.     

Influence of the mechanical behaviour specificities of damaged refractory castables on the Young's modulus determination

This paper deals with the problematic of the determination of the Young's modulus of refractory castables by the way of mechanical tests. Two materials are considered: a cordierite based refractory castable that is reinforced with short steel fibres and an andalusite based refractory castable. Discrepancies in Young's modulus values are noticed depending on whether they are determined on direct tensile test curves, four points bending test curves or compression test curves. Damage due to a first thermal cycle is underlined as enhancing these discrepancies. Original mechanical tests have been performed in order to understand the influence of such a damage on the four points bending and compression behaviours. Results show that depending on the method that is used to measure displacements and strains, the calculated Young's modulus values can be highly influenced by local strain effects that occur at the contact between the sample and the loading system. Related to the damage that develops in these materials during the first heat treatment, these effects are more important when samples have been previously fired.     

The Monolithic Refractory Lining for Furnaces Fired by Wood Fuel

Service conditions of the refractory lining of the furnaces of drying drums fired by wood fuel (polishing dust for wood fiber boards) are analyzed. Combustion of this fuel produces 9% of solid-state ash containing low-melting compounds capable of infiltrating into the chamotte refractory. Because of the frequent thermal shocks associated with heat-cool operating regimes and the attack by molten ash slag, the chamotte lining undergoes degradation. To improve the operating conditions and extend the service life of the lining, the use of a castable monolithic material with superior resistance to high temperature and corrosion is proposed.     

Determination of mechanical properties and thermal treatment behavior of alumina-based refractories

Two different types of alumina-based refractories have been investigated. Chamotte and bauxite were the raw materials, being molded using a system of aluminate cement and water or a phenol–formaldehyde resin, as binder. Different raw material's grain size, water content, molding pressure, and firing temperature were selected for the preparation of the specimens. The mechanical strength of all specimens was determined using compressive tests, where parameters with physical meaning, such as maximum stress, maximum strain, elasticity parameter, and viscoelastic parameter were obtained after modeling the collected stress–strain data. In addition, water quenching test was performed for both samples, in order to present their thermal stability and mechanical wear after certain cycles.Process conditions as well as the raw materials’ characteristics were correlated with the compressive properties. More specifically, maximum stress seemed to increase when increasing all parameters except grain size both for bauxite and chamotte samples. The increment of bonding phase, grain size, and molding pressure caused an increment in maximum strain for both types of refractories. However, the higher the firing temperature was the higher the maximum strain of bauxite appeared, while the maximum strain of chamotte samples was decreased. Modulus of elasticity seemed to increase with bonding phase content and firing temperature and decreased with grain size for both types of refractories. For bauxite samples higher molding pressure caused a decrement in elasticity parameter while for chamotte in caused an increment. As far as the quenching tests are concerned chamotte samples seemed to be more tolerant to thermal shock. After data acquisition, the change presented in elasticity parameter (E) was also modeled with the number of cycles. Both bauxite and chamotte samples showed a decrement in maximum stress after thermal treatment, while elasticity parameter seemed to have a more severe decrement for bauxite samples.     

Thermal shock resistance of refractory composites with Zirconia and Silicon-Carbide inclusions and alumina binder

With the goal of designing a castable refractory for an aerospace application with optimum resistance to thermal shock, three different particle-reinforced ceramic composites are designed and compared. Different volume fractions of Silicon Carbide (SiC) particles, Zirconia (ZrO₂) bubbles, and Zirconia solid particles dispersed in an alumina (Al₂O₃) matrix are used in the fabrication of these castables. Destructive and nondestructive testing procedures are implemented to evaluate their thermomechanical properties, both before and after a custom designed thermal shock cycle. After demonstrating the applicability of thermal shock indices, the variation of these indices due to thermal shock is measured experimentally and utilized as a design tool. Multiple micro-scale damage mechanisms were observed, all of which are various forms of structural deformation.     

A new arrangement of SiC particles in reaction bonded Si3N4-SiC ceramic refractory and crack behaviors under thermal shock

High temperature protection brick lining is important for super-charged boilers. In practice, Si₃N₄ bonded SiC ceramics are usually chosen as the raw material of refractory bricks due to their excellent performance under high temperature. In the field of the ceramic refractory material, a main goal is to improve the resistance of ceramics under thermal shock because their inherent brittleness may cause failure under sudden change in temperature. In this paper, we fulfilled this goal by introducing a new particle arrangement called “double dispersion” for the SiC particle-reinforced ceramic refractory material. And we established the micro-structure models for both the original and the modified ceramic refractory material. To study the influence of the particle arrangement on the fracture toughness, we performed simulations of the crack initiation and propagation under the same thermal load for the original and the modified material. The results showed that the “double dispersion” method can improve the thermal shock resistance of the reaction-bonded Si₃N₄-SiC ceramic refractory.     

Refractory clays of Stepnyi deposit

Conclusions The investigated sample of Stepnyi deposit clay is suitable for production of type ShA normal-purpose chamotte parts. The use of this clay in the production of certain forms of chamotte parts such as bottom teeming, open hearth, and other parts is possible. Being quite plastic, such clays may serve as a binder refractory raw material and be used for production of more critical parts in combination with chamotte from more refractory clays (bauxite and others). Chamotte of Stepnyi deposit clay may be used in combination with enriched Eltai kaolin. This in full measure refers to Stepnyi deposit clays of the better grades corresponding to types VGO-1 and VGM. Taking into consideration the low content of the coloring oxides, Stepnyi deposit clays may also be used as a white-firing raw material in the production of various ceramics.Translated from Ogneupory, No. 4, pp. 24–26, April, 1993.     

Investigation of Thermal Shock in a High-Temperature Refractory Ceramic: A Fracture Mechanics Approach

The results of a study on the thermal shock behavior of a high-temperature refractory ceramic that is used as a furnace liner in the melting of steels are presented in this paper. The experimental studies show that thermal shock damage initiates by edge cracking after the first shock cycle. Subsequent subcritical crack growth occurs by the incremental extension of dominant cracks until catastrophic failure occurs. The observations of the crack profiles also reveal the formation of viscoelastic bridges that promote crack-tip shielding/toughening via crack bridging. Following a brief discussion of the respective mechanisms of fracture and thermal shock damage at different temperatures and temperature ranges, the implications of the results are discussed for refractory ceramics that are toughened by viscoelastic crack bridging.     

Effect of the quality of microsilica on the flow properties of cement slurry and characteristics of low-cement refractory concrete on a chamotte filler

The effect of impurity content in different grades of microsilica (MS) on the physicomechanical characteristics of a low-cement refractory concrete based on a chamotte filler is considered. The high content of impurities in MS degrades the flow properties of cement slurry and the respective concrete slurry. It is shown that as ambient temperature grows to 30°C, the impurities in MS increase the cement slurry thickness and impede the placement of refractory concrete in summer period. If low-quality MS is used in concrete, an increased crack formation is observed under the cyclic high-temperature effect. __________ Translated from Novye Ogneupory, No. 5, pp. 41–46, May, 2007.     

平板微热管阵列-泡沫铜复合结构相变蓄热装置蓄放热特性

相变蓄热技术是节能减排的一个重要手段,在太阳能利用、余热回收和电力削峰填谷等领域发挥重要的作用。设计了以平板微热管阵列-泡沫铜复合结构为基础,多孔扁管为载热流体通路,水为载热介质,石蜡为相变材料的热管式蓄热装置。通过实验研究了蓄放热过程中装置内部石蜡的温度分布情况,不同载热流体温度和流量下的蓄放热功率变化,以及装置蓄放热效率等特性。实验结果表明,平板微热管阵列-泡沫铜复合结构可以使箱体内石蜡温度分布更加均匀;增加载热流体和相变材料的温差以及增大流量都可以提高蓄放热功率。实验条件下,该装置的最大蓄热功率为1.24 kW,最大放热功率为1.43 kW。装置蓄热效率为92%,放热效率为94%,总效率为87.4%。     

蓄热型热泵热水器单级与复叠循环性能比较

引言 热泵机组的复叠循环,可以降低在寒冷气候下压缩机的压缩比,提高其制热效率[1].Agyenim等[2]采用石蜡为相变蓄热材料(PCM),分析了空气源热泵热水器内部传热特性,由于石蜡的热导率较小导致蓄热放热效率较低,PCM和水之间换热效率只有52%,为此开发了一种新型套管式换热器用于加强PCM和水之间的换热.Long等[3]数值模拟和实验分析了相变蓄热水箱的蓄热放热过程,在室外于湿球温度24/20℃,进水温度24℃条件下,热泵系统平均COP为3.08.陈光明等[4-5]设计一种新型的复叠式空气源热泵装置,提出单级向复叠运行转换足由两者运行时的吸、放热量、耗功量和运行性能系数决定,同时还与压缩机的形式有关,输气系数、绝热效率随压缩比变化越小,复叠运行优势越小.吴青吴等[6]以常规工质对复叠式热泵热水器在不同运行工况下的循环特性进行了理论计算,提出冬季运行复叠式循环,夏季单独运行高温级循环,有利于系统的节能.     

In Situ Mechanical Properties of Chamotte Particulate Reinforced,Potassium Geopolymer

Geopolymers are an inorganic polymeric material composed of alumina, silica, and alkali metal oxides. Monolithic geopolymer is brittle and susceptible to dehydration cracking at elevated temperatures. The addition of a reinforcing phase not only improves strength and toughness but also maintains the structural integrity of the material at elevated temperatures. For this study, potassium‐based geopolymer (KGP) is reinforced with varying weight percent of chamotte particles. Chamotte is kaolinite grade clay calcined at 1350°C to produce 38% crystalline mullite, as well as metastable cristobalite and quartz. The chemical composition of the chamotte is almost identical to that of the metakaolin used to create the geopolymer, however, its crystalline nature prevents reactivity with the caustic potassium silicate solution and it remains as a particulate reinforcement. Flexural strength is evaluated at room temperature and in situ at elevated temperatures to just below the leucite crystallization temperature. Reinforcement with 25 wt% chamotte has shown a two‐fold increase in room‐temperature flexural strength. Flexural strength is also evaluated at room temperature after heating above the leucite crystallization temperature to determine if the chamotte aids in maintaining structural integrity during the volumetric contraction and destructive transformation from cubic to tetragonal symmetry upon forming leucite.     

Behavior of silicon carbide/cordierite composite material after cyclic thermal shock

In the present work Mg-exchanged zeolite and silicon carbide were used as starting materials for obtaining cordierite/SiC composite ceramics with weight ratio 30:70. Samples were exposed to the water quench test from 950 °C, applying various number of thermal cycles (shocks). Level of surface deterioration before and during quenching was monitored by image analysis. Ultrasonic measurements were used as non-destructive quantification of thermal shock damage in refractory specimens. When refractory samples are subjected to the rapid temperature changes crack nucleation and propagation occurs resulting in loss of strength and materials degradation. The formation of cracks decreases the density and elastic properties of material. Therefore by measuring these properties one can directly monitor the development of thermal shock damage level. Dynamic Young's modulus of elasticity and strength degradation were calculated using measured values. Level of degradation of the samples was monitored before and during testing using Image Pro Plus program for image analysis. The capability of non-destructive test methods such as: ultrasonic velocity technique and image analysis for simple, and reliable non-destructive characterization are presented.     

Variation of Poisson's ratio of refractory materials with thermal shocks

Investigations into the changes of Poisson's ratio as well as other mechanical properties of selected types of refractories subjected to thermal shocks have been carried out. The investigations included several types of porous refractory materials e.g. magnesia, magnesia–spinel, magnesia–chrome, chrome–magnesia, mullite, silica and low cement chamotte castable. The Poisson's ratio value of the examined materials decreased after each thermal shock. Decreasing of Poisson ratio caused by a stronger drop in Young's modulus than that for shear modulus was explained.It was found that some tested materials were characterized by low values of Poisson's ratio, which after series of severe thermal shocks decreased even to negative values. In order to explain the observed phenomenon, an attempt to correlate the obtained results with the microstructure of materials before and after a series of thermal shocks was undertaken. A hypothetical model of the microstructure of a porous ceramic material with negative Poisson ratio was proposed.     

细小尺度下潜热型功能热流体压降与传热特性

实验研究了相变微胶囊颗粒(囊芯材料为正十六烷,壳材为尿素-甲醛树脂)和去离子水混合制成的潜热型功能热流体流过等热流细小圆管的流动与传热特性,同时以去离子水作为传热工质在相同条件下进行了对比实验。得到了压降随质量流量的变化规律,实验段出、入口温度以及量纲1出口温度随Reynolds数变化规律,量纲1壁面温度沿轴向的分布规律,平均Nusselt数随Reynolds数的变化关系。结果表明,相变微胶囊颗粒的加入会导致流动压降增大,但随着流量增加,流动压降逐渐与单相液体的接近;出口温度及壁面温度要比相同条件下单相液体的低;含有较小相变微胶囊颗粒浓度的潜热型功能热流体的平均Nusselt数是相同条件下单相液体的2.0～4.0倍。