Papers by Keyword: Refractory

Abstract: Concrete has remarkable fire resistance properties. In the case of fire, it is found that the concrete affected by fire depends to a great extent on the intensity and duration of fire. Previous experience has shown that concrete structures are likely to have a good fire rating than structures made of other materials. Nevertheless, concrete undergoes important chemical and physical changes, starting at 400°C - 500°C. As calcium hydroxide and other hydration products start to decompose, concrete tends to lose its strength, typically around 600°C - 700°C. In order to improve the high temperature properties of concrete, admixtures can be used in concrete. In the following, a study on the effects of different admixtures on the properties of concrete at high temperature is presented.

Abstract: Monolithic refractories are now well established as linings for a range of holding and melting applications for the processing of aluminium. The refractory lining in an aluminium furnace has to withstand a wide variety of physical and chemical environments. Each of the different furnace zones presents a different set of operating conditions, in terms of peak temperature, temperature fluctuation, metal contact, flux contact, impact from ingot loading, etc. Therefore, in order for a monolithic material to successfully perform in a particular area of the furnace, it needs to be able to cope with the specific environmental conditions in that region of the furnace. Aluminium producers continue to increase productivity through their Melt-Hold furnaces to maintain competitiveness. The use of more powerful burners to increase heat input to the furnace is therefore becoming increasingly common practice. But faster melting leads to increased metal losses from surface oxidation and to segregation from large heat gradients. These effects are countered by increased use of fluxes and increased stirring. Given the increasingly challenging environment within which the refractory lining has to work, traditional lining solutions can no longer be relied upon to provide the service lives that were previously achieved. Therefore, a new generation of furnace lining materials is required to cope with today’s aluminium furnace. This paper describes one such newly developed monolithic material, designed specifically to improve performance in the superstructure zone of Aluminium furnaces. The non-metal contact, superstructure regions of aluminium furnaces present their own unique set of challenges for the refractory lining. Refractories in these regions – roof, upper walls and flue – have to cope with excessively high levels of alkali vapour and thermal shock. This paper reviews the operating conditions found in the superstructure areas of a typical melting and holding furnace and the implications these have on monolithic lining material design and performance. The improved behaviour of the newly developed monolithic material against the critical performance criteria in these furnace regions is demonstrated in the laboratory, compared to existing industry leading materials, using industry standard test methods.

Abstract: As one of the important refractory raw materials, the impurities of magnesite raw material had been strictly required. Magnesite processing methods can be divided into three categories, Physical beneficiation, chemical beneficiation and bacteriological beneficiation. And the flotation method is currently widely used in magnesite ore processing, but it also exists some problems.

Abstract: The Al₂O₃-Cr₂O₃ slag as raw material was analyzed by scanning electron microscopy (SEM). The effects of different size distribution of MgO-Al₂O₃ spinel grain on physical properties of refractory bricks made from Al₂O₃-Cr₂O₃ slag was studied according to YB/T376.1-1995（water quenching）, GB/T 5072－1985, GB/T 2997-1982 criterion. The results show that the main crystal phase of the refractory bricks were chromium corundum, corundum. The refractory bricks containing 10 wt% of the maximum size of 3.0 mm of MgO-Al₂O₃ spinel grain can significantly improve the thermal shock resistance of this refractory bricks. At the meanwhile, This kind of refractory bricks can be absolutely satisfied with the requirements about cold crushing strength, bulk density and apparent porosity.

Abstract: The effect of Yb₂O₃ addition on sintering of MgO ceramics was studied in this work. Increasing of Yb₂O₃ addition is helpful to densification and grain growth. The effect of Yb₂O₃ amount on densification decreased when the sintering temperature was increased from 1400°C to 1550°C. Quick densification and rapid grain boundary mobility in Yb₂O₃ doped samples come from the point defect because the dopant mainly located at the grain boundary, and this is meaningful for grain growth and densification. The results also illustrate that sintering time and sintering temperature would noticeably affect grain growth and densification process, which is not related to the amount of Yb₂O₃ addition.

Abstract: Oxidation of carbon is the main problem in magnesia-carbon refractory bricks. Al₄SiC₄ powder is an excellent antioxidant which has obvious practical significance. It was prepared by vacuum sintering at 1700°C in vacuum sintering furnace. The effect of Al₄SiC₄ on the oxidation resistance of magnesia-carbon bricks were investigated at 1500°C. The thickness of decarburized layer of bricks was measured and the oxidized situation of the bricks was examined by XRD, SEM and EDAS. The decarburization layer became thin with adding Al₄SiC₄ powder. Magnesium alumina spinel (MgAl₂O₄) was determined to be present by characterization studies on Al₄SiC₄ added specimens and phase evolution of MgAl₂O4 spinel formation were investigated by XRD. The optical microscope and EDAS indicated that the spinel diminished the open pores owing to its volume expansion and formed a protective layer on the surface. The results suggested that the spinel had an excellent effect on the oxidation resistance of bricks. In addition, compared with adding other aluminum-containing antioxidants, the specimens added Al₄SiC₄ had better stability from high temperature to room temperature.

Abstract: In order to study the influence of the geometry and material properties on the service life of converters, different geometry and material properties of the converters were analyzed by using finite element method in this paper. A mechanical and thermal coupling diagram was introduced to optimize the geometric figure and material properties of the converter by software ABAQUS. The damage of converters was investigated, especially in the lower brick layers of the wall (the bottom wall transition zone). The result indicates both the ladder of the converter with small steps and a proper inclination at the joint between the bottom and wall of converters can reduce the stresses, which make the refractory material damage decrease. The working lining with high conductivity and low thermal expansion will make the thermal stress of the converter lower, which reduces the crack of the converter. The relations between the obtained results and the corresponding lining design and the material properties give a good insight into the reasons of refractory material failure and help to find counter-measures.

Abstract: Aluminium titanate, Al₂TiO₅ (AT) with the pseudobrookite structure is the only compound in the alumina-titania system. It is an excellent refractory and thermal shock resistant material due to its relatively low thermal expansion coefficient (1 ´10^-6 °C ^–1) and high melting point (1860°C). However, its low mechanical strength, hardness and fracture resistance together with susceptibility to decomposition in the temperature range 900–1200°C has limited its wider application. In this paper, the innovative tailored design of functionally- graded Al₂TiO₅ – based ceramics system was presented. This involves the use of a vacuum heat-treatment or die-pressing to form hard graded layers of alumina on Al₂TiO₅. These hard outer layers will provide hardness and wear resistance to protect the softer but damage resistant underlayers. The results will also explore unresolved issues concerning the effect of graded interfaces on their physical and mechanical performance properties.

Abstract: This work addresses damage evaluation of porous mullite refractory subjected to thermal shock. Incommunicating circular pores were distributed randomly at a volume percentage up to 40% in a cylinder of 20 cm diameter. The analysis was performed by means of the software ANSYS® combined with a pre-program that generates randomly distributed pores of given size. The analysis procedure was divided into two stages. In the first, transient thermal analysis considering temperature-dependent material property was dealt with different thermal shock temperatures under natural cooling condition. The following structure analysis ran based on the obtained temperature distribution. The material damage was defined by that the local tensile stress reached to or was over the strength of the refractory. The extent of damage was determined as the ratio of the area of the damaged regions to the section area of the cylinder. The results show that the porosity, thermal shock temperature and cooling time have a high effect on the material damage. The lower the porosity is, the larger the extent of damage. The thermal damage increases with the raise of thermal shock temperature and the cooling time. The damage develops rapidly within 10 minutes but slows down after one hour cooling. The damage difference at high shock temperature stage (≥ 1000°C) is less than at low shock temperature stage. The pore size effect gets into practice only at high shock temperature stage: the damage increases with the raise of the pore size. The present research confirms that high porosity and small pore size could decrease greatly thermal shock damage and should be considered in the micro structural design of refractory.

Abstract: A pulse-echo technique, based on ultrasonic "long-bar" mode (LBM) velocity measurements, working up to 1700°C is described. Magnetostrictive transducers and ultrasonic lines used in a 40-350 kHz frequency range are detailed. The conditions of choice of fundamental parameters (frequency, line geometry, sample size) are discussed in relation with the nature and the microstructure of the materials under test. This technique can be used to study the variations of elastic moduli of materials at high temperature.