01:20 pm
CaO attack on refractory materials of the system SiO2-Al2O3
Dr. Kathrin Weber | Refratechnik Cement GmbH | Germany
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Authors:
Dr. Kathrin Weber | Refratechnik Cement GmbH | Germany
Stefan Uhlendorf | Refratechnik Cement GmbH | Germany
Dr. Xenia Ritter | Refratechnik Cement GmbH | Germany
Dr. Hans-Jürgen Klischat | Refratechnik Cement GmbH | Germany
The thermochemical attack of CaO-rich kiln feed on refractory materials of the system SiO2-Al2O3 is a major cause of wear and limits their use in hot areas of e. g. cement clinker kilns. However, these materials could gain in importance compared to basic products due to their lower CO2 footprint and lower thermal conductivity, and even ample efforts towards lower firing temperatures for various mineral products, e.g. belite cement and the like.
To develop enhanced SiO2-Al2O3 products suitable for thermally/thermochemically higher stressed zones of cement clinker or lime kilns, the CaO attack on the system SiO2-Al2O3 was investigated. For this, a series of reaction tests was conducted along with drawing from decades of experience in examining wear mechanism in post-mortem analyses. For the reaction tests, crucibles were prepared from bauxite or tabular alumina, each mixed with bonding clay, filled with sulphate-resistant cement clinker, and fired at selected temperatures.
Subsequent analyses showed that while corundum remains comparatively stable, the component most reactive to CaO is a matrix phase consisting of a finely grained mixture of mullite and glass phase/cristobalite. The susceptibility of the matrix phase results from the high diffusion rates of Ca2+- and K+-ions and their high potential of forming eutectic melting phases as soon as CaO is added.
The findings support further development of enhanced high alumina and SiC enriched high alumina refractory products with increased CaO and alkali resistance.
01:40 pm
Impact of hydrogen on carbon monoxide disintegration of refractories
drs. Joeri Liefhebber | Tata Steel Nederland | Netherlands
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Authors:
drs. Joeri Liefhebber | Tata Steel Nederland | Netherlands
ir. Rinus Siebring | Tata Steel Nederland | Netherlands
In many critical installations for the steel industry, refractory materials are (locally) exposed to carbon monoxide (CO) gas. The reaction of CO gas to solid carbon and carbon dioxide (CO2), the Boudouard reaction, has been studied by many authors over the past decades. The reaction results in the deterioration of refractory materials, which can influence the lifetime and availability of production installations. To evaluate the resistance of materials to CO disintegration, the standard test method uses a 100% CO atmosphere. However, various studies have shown that the use of a few percent H2 together with the CO gas significantly increases the reaction rate and can reactivate catalyst particles that are otherwise inactive with a pure CO atmosphere. Current process gasses (like in the Blast Furnace) already contains 2 - 8 vol% H2 and this influences the reaction rates. In future processes, to reduce greenhouse gas emissions, this will further increase when more hydrogen containing gasses, like H2/CH4, are used. Also from a testing perspective it makes more sense to include hydrogen as faster reaction kinetics are observed to enhance the relative differences between materials’ resistance. Tata Steel has developed a testing method, for the evaluation of the resistance of refractory materials to CO disintegration. Clear differences are observed when comparing the same samples, which are tested with and without H2 in the gas mix. This approach and method is being discussed with refractory suppliers and used by both sides for the selection of refractory materials where appropriate for the application.
02:00 pm
The influence of the gas permeability of refractory materials on carbon deposition in CO containing atmospheres
Tobias Steffen | Forschungsgemeinschaft Feuerfest e. V. | Germany
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Authors:
Tobias Steffen | Forschungsgemeinschaft Feuerfest e. V. | Germany
Dr. Christian Dannert | Forschungsgemeinschaft Feuerfest e. V. | Germany
Prof. Dr. rer. nat. Olaf Krause | Hochschule Koblenz | Germany
Alexandra Koch | Hochschule Koblenz | Germany
The disintegration of refractory materials in CO containing atmospheres due to the deposition of carbon in the microstructure in the presence of reducible iron phases is a relevant issue for both refractory manufacturers and users.
With a new testing method, developed previously by Forschungsgemeinschaft Feuerfest e. V., reproducible and quantifiable in situ data on the reaction kinetics of this destruction process in unshaped refractory materials can be determined. During the process of collecting these data, a previously undescribed influence of the permeability of the materials on the kinetics of carbon deposition became apparent.
Based on this observation, we investigated the reaction kinetics of the carbon deposition in dependency of the pore structure of the refractory material using a new device to determine the gas permeability of dense unshaped refractory materials, thermogravimetrical analysis to collect data on reaction kinetics of carbon deposition in CO containing atmospheres as well as scanning electron microscopy to depict the spatially resolved degree of destruction by carbon deposition.
The results of these investigations are helpful to develop new CO resistant refractory materials and to optimize already existing material systems for the application in reducing conditions.
02:20 pm
Knowledge about the Carbon Deposition in the Microstructure of Refractory Materials
Alexandra Koch | Koblenz University of Applied Sciences | Germany
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Authors:
Alexandra Koch | Koblenz University of Applied Sciences | Germany
Prof. Dr. rer. nat. Olaf Krause | Koblenz University of Applied Sciences | Germany
Tobias Steffen | Forschungsgemeinschaft Feuerfest e. V. | Germany
Dr. Christian Dannert | Forschungsgemeinschaft Feuerfest e. V. | Germany
Carbon nanotubes (CNTs) are graphene sheets in cylindric structure and have exceptionally qualities. The single-walled CNTs, for example, depict excellent thermal and electrical properties. Whereby multi-walled CNTs present extraordinary mechanical qualities and a high aspect ratio. On the other hand, it is well known that carbon nanotubes build a remarkable tabular structure which expands enormously during formation in refractory castables. Hence, the CNTs are an unwanted and well documented byproduct in refractory materials by exposure in CO-atmosphere at intermediate temperatures. Solutions for CO resistance for refractory materials and, consequently, a better understanding of the decomposition and production process is desired and inevitable.
In this paper the knowledge of the development and destruction of the microstructure of refractory castables due to carbon deposition is given. For the first time an imaging method for the deposition of carbon in the microstructure of castables is presented and brought in correlation with the carbon deposition rate. By varying the duration of the dwell time under CO atmosphere the progression of the carbon deposition was analyzed and displays the influence of crack formation, built by the carbon nanotubes. The results were gathered by using a Field Emission Scanning Electron Microscope (FESEM).