The lifetime of aluminium reduction cells is of considerable importance in the operating economics of an aluminium smelter. Not only are the cell materials costly, but cell life is also key in minimizing production downtime and the waste material generated in the cutting-out and relining of a reduction cell. This lifetime is increasingly driven by two components of the cell lining: the carbon cathode and the sidewall refractories which are the primary containment of the metal and electrolyte. Silicon nitride bonded silicon carbide (SNBSC) refractories are the current state-of-the-art sidewall materials. These materials are formed by encapsulation of SiC particles in a silicon nitride matrix, by nitridation of silicon at high temperatures under a nitrogen atmosphere. In this study the mechanisms of the corrosion of silicon nitride bonded silicon carbide refractories in an aluminium reduction cell environment were examined. Microstructural analysis of SNBSC materials by: X-ray diffraction, solid state NMR, Raman spectroscopy, XPS, and SEM, identified variations in α/β Si3N4 ratio and porosity in the binder phase, with higher porosity levels and β Si3N4 content found in the interior part of the brick. Unreacted metallic silicon was observed only as a crystalline phase inside SiC grains and is not associated with the binder phase. SNBSC samples from eight different commercial refractory manufacturers were tested in an industrial aluminium cell environment, both by immersion in industrial cells and in the laboratory. The samples showed extensive attack and considerable differentiation between the samples. The effects on the corrosion rate of porosity levels, amount of binder, α/β Si3N4 ratio, free Si content in the binder, and different factors in the environment were examined in the laboratory scale trials. A laboratory scale testing rig with some novel features, such as a rotating anode, was developed to examine corrosion resistance of samples under electrolysis conditions in molten cryolite at up to 1000oC. Corrosion test results, supported by thermodynamic calculations, identified the Si3N4 binder as the reactive phase in SNBSC materials, especially in the gas phase above the bath level. High porosity levels and high β Si3N4 fraction in the binder showed a statistically significant contribution to the corrosion rate in lab-scale corrosion trials. Commercial materials are also highly inhomogeneous in the distribution of the β Si3N4 phase as mentioned above, which complicates sample selection and the interpretation of corrosion test results. The crystal morphology of β Si3N4 is suggested as the reason for the high reactivity of these materials. This morphology characterised by elongated rod-like shape crystals with hexagonal cross section, presents a higher surface area compared to α Si3N4 crystals containing mainly flat matte crystals. In the light of this observation, the binder phase in selected samples was deliberately modified to convert a significant fraction of the α Si3N4 to β Si3N4. Corrosion rates measured from these modified samples in general support the argument that the crystallographic form of silicon nitride contributes to corrosion rate. The proposed corrosion mechanism for SNBSC materials in the aluminium reduction cell atmosphere is based on combination of oxidation of the binder followed by attack of corrosive gases to produce volatile SiF4. Thus the binder phase is initially passivated below the electrolyte level on the sidewall, where exposure to corrosive gases is limited, but occurs more rapidly in the area above the electrolyte/air interface. The intrusion of electrolyte into the refractory and capillary transport up the sidewall is a key in accelerating this reaction.

The lifetime of aluminium reduction cells is of considerable importance in the operating economics of an aluminium smelter. Not only are the cell materials costly, but cell life is also key in minimizing production downtime and the waste material generated in the cutting-out and relining of a reduction cell. This lifetime is increasingly driven by two components of the cell lining: the carbon cathode and the sidewall refractories which are the primary containment of the metal and electrolyte. Silicon nitride bonded silicon carbide (SNBSC) refractories are the current state-of-the-art sidewall materials. These materials are formed by encapsulation of SiC particles in a silicon nitride matrix, by nitridation of silicon at high temperatures under a nitrogen atmosphere. In this study the mechanisms of the corrosion of silicon nitride bonded silicon carbide refractories in an aluminium reduction cell environment were examined. Microstructural analysis of SNBSC materials by: X-ray diffraction, solid state NMR, Raman spectroscopy, XPS, and SEM, identified variations in α/β Si3N4 ratio and porosity in the binder phase, with higher porosity levels and β Si3N4 content found in the interior part of the brick. Unreacted metallic silicon was observed only as a crystalline phase inside SiC grains and is not associated with the binder phase. SNBSC samples from eight different commercial refractory manufacturers were tested in an industrial aluminium cell environment, both by immersion in industrial cells and in the laboratory. The samples showed extensive attack and considerable differentiation between the samples. The effects on the corrosion rate of porosity levels, amount of binder, α/β Si3N4 ratio, free Si content in the binder, and different factors in the environment were examined in the laboratory scale trials. A laboratory scale testing rig with some novel features, such as a rotating anode, was developed to examine corrosion resistance of samples under electrolysis conditions in molten cryolite at up to 1000oC. Corrosion test results, supported by thermodynamic calculations, identified the Si3N4 binder as the reactive phase in SNBSC materials, especially in the gas phase above the bath level. High porosity levels and high β Si3N4 fraction in the binder showed a statistically significant contribution to the corrosion rate in lab-scale corrosion trials. Commercial materials are also highly inhomogeneous in the distribution of the β Si3N4 phase as mentioned above, which complicates sample selection and the interpretation of corrosion test results. The crystal morphology of β Si3N4 is suggested as the reason for the high reactivity of these materials. This morphology characterised by elongated rod-like shape crystals with hexagonal cross section, presents a higher surface area compared to α Si3N4 crystals containing mainly flat matte crystals. In the light of this observation, the binder phase in selected samples was deliberately modified to convert a significant fraction of the α Si3N4 to β Si3N4. Corrosion rates measured from these modified samples in general support the argument that the crystallographic form of silicon nitride contributes to corrosion rate. The proposed corrosion mechanism for SNBSC materials in the aluminium reduction cell atmosphere is based on combination of oxidation of the binder followed by attack of corrosive gases to produce volatile SiF4. Thus the binder phase is initially passivated below the electrolyte level on the sidewall, where exposure to corrosive gases is limited, but occurs more rapidly in the area above the electrolyte/air interface. The intrusion of electrolyte into the refractory and capillary transport up the sidewall is a key in accelerating this reaction.

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A indústria siderúrgica busca continuamente melhorias no desempenho dos refratários, como forma de melhorar sua competitividade, em um mercado caracterizado pela competição global. Um dos principais fatores de desgaste dos refratários é a corrosão, que no ambiente siderúrgico, está diretamente relacionada com a qualidade das escórias, na maioria dos equipamentos. Nesse contexto, o presente trabalho apresenta um estudo dos principais fatores que desgastam quimicamente o revestimento dolomítico, com o objetivo de entender os fenômenos que estavam impactando no desempenho dos refratários utilizados na panela de aço da Siderúrgica Riograndense. Para o estudo em questão, o comportamento químico dessas escórias foi monitorado de setembro de 2008 a outubro de 2009. Para análise didática os dados foram estratificados em três períodos, onde o período 01 corresponde aos meses de setembro, outubro e novembro de 2008, período 02 englobam os meses de fevereiro a junho de 2009 e o período 03 revela os resultados de julho a outubro de 2009. Os resultados de monitoração da qualidade da escória foram apresentados por equipamento forno elétrico a arco (FEA) e forno panela (FP). Os principais parâmetros de processo avaliados, foram a basicidade binária, teor de MgO e oxidação da escória. Os resultados foram relacionados com o desempenho do revestimento refratário aplicado na panela de aço. Para a análise dos dados foi utilizado o Minitab um software que oferece um conjunto de ferramentas estatísticas. Pode-se inferir, a partir da análise dos resultados apresentados nesse trabalho, que as ações de redução da oxidação da escória do forno elétrico através de ajustes no sistema de injeção de oxigênio das injetoras, propiciaram uma redução do óxido de ferro nas escórias do forno panela, permitindo correções da basicidade binária nesse mesmo equipamento. Estas ações em conjunto com a elevação na concentração do óxido de magnésio, através da introdução de cal dolomítica no processo, promoveram melhor formação do coating no revestimento refratário. As alterações no processo de produção do aço promoveram um significativo aumento na vida média das panelas de aço desta aciaria. / The steel industry is continually seeking improvements in the performance of refractories, as a way to improve their competitiveness in a global market. A major factor in refractories wear is chemical corrosion. In the steel industry this is directly related to the quality of the slag used in most of the equipment. In this context, this work presents a study of the main factors that causes chemical wear in dolomite refractory. The main objective was the understanding of the phenomena that were impacting on the performance of refractories used in steel ladle of Siderúrgica Riograndense. For this study, the chemical behavior of slags was monitored from September 2008 to October 2009. For better understanding the analysis, the data were stratified into three periods: period 01, from September to November 2008; period 02, from February to June 2009; and the period 03, from July to October 2009 . The results of slag quality monitoring were presented separated from the electric arc furnace (EAF) and the ladle furnace (LF). The main process parameters evaluated were the binary basicity, content of MgO and oxidation of the slag. The results were correlated with the performance of refractories used in steel ladle. For the analysis of data, it was used Minitab software which offers a set of statistical tools. It can be inferred, through out the analysis of the results presented here, that the actions to decrease oxidation of the EAF slag, by adjusting the feeding system of oxygen injectors, propitiated a reduction of iron oxide in the slag of ladle furnece, which allowed corrections in binary basicity in the same equipment. These actions together with the increase in the concentration of magnesium oxide in the FP slag, through the introduction of dolomitic lime in the process, promoted the creation of a better coating on the refractory lining. The changes in the process of steel production promoted a significant increase in average life of ladle refractory in this steel making plant.

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A indústria siderúrgica busca continuamente melhorias no desempenho dos refratários, como forma de melhorar sua competitividade, em um mercado caracterizado pela competição global. Um dos principais fatores de desgaste dos refratários é a corrosão, que no ambiente siderúrgico, está diretamente relacionada com a qualidade das escórias, na maioria dos equipamentos. Nesse contexto, o presente trabalho apresenta um estudo dos principais fatores que desgastam quimicamente o revestimento dolomítico, com o objetivo de entender os fenômenos que estavam impactando no desempenho dos refratários utilizados na panela de aço da Siderúrgica Riograndense. Para o estudo em questão, o comportamento químico dessas escórias foi monitorado de setembro de 2008 a outubro de 2009. Para análise didática os dados foram estratificados em três períodos, onde o período 01 corresponde aos meses de setembro, outubro e novembro de 2008, período 02 englobam os meses de fevereiro a junho de 2009 e o período 03 revela os resultados de julho a outubro de 2009. Os resultados de monitoração da qualidade da escória foram apresentados por equipamento forno elétrico a arco (FEA) e forno panela (FP). Os principais parâmetros de processo avaliados, foram a basicidade binária, teor de MgO e oxidação da escória. Os resultados foram relacionados com o desempenho do revestimento refratário aplicado na panela de aço. Para a análise dos dados foi utilizado o Minitab um software que oferece um conjunto de ferramentas estatísticas. Pode-se inferir, a partir da análise dos resultados apresentados nesse trabalho, que as ações de redução da oxidação da escória do forno elétrico através de ajustes no sistema de injeção de oxigênio das injetoras, propiciaram uma redução do óxido de ferro nas escórias do forno panela, permitindo correções da basicidade binária nesse mesmo equipamento. Estas ações em conjunto com a elevação na concentração do óxido de magnésio, através da introdução de cal dolomítica no processo, promoveram melhor formação do coating no revestimento refratário. As alterações no processo de produção do aço promoveram um significativo aumento na vida média das panelas de aço desta aciaria. / The steel industry is continually seeking improvements in the performance of refractories, as a way to improve their competitiveness in a global market. A major factor in refractories wear is chemical corrosion. In the steel industry this is directly related to the quality of the slag used in most of the equipment. In this context, this work presents a study of the main factors that causes chemical wear in dolomite refractory. The main objective was the understanding of the phenomena that were impacting on the performance of refractories used in steel ladle of Siderúrgica Riograndense. For this study, the chemical behavior of slags was monitored from September 2008 to October 2009. For better understanding the analysis, the data were stratified into three periods: period 01, from September to November 2008; period 02, from February to June 2009; and the period 03, from July to October 2009 . The results of slag quality monitoring were presented separated from the electric arc furnace (EAF) and the ladle furnace (LF). The main process parameters evaluated were the binary basicity, content of MgO and oxidation of the slag. The results were correlated with the performance of refractories used in steel ladle. For the analysis of data, it was used Minitab software which offers a set of statistical tools. It can be inferred, through out the analysis of the results presented here, that the actions to decrease oxidation of the EAF slag, by adjusting the feeding system of oxygen injectors, propitiated a reduction of iron oxide in the slag of ladle furnece, which allowed corrections in binary basicity in the same equipment. These actions together with the increase in the concentration of magnesium oxide in the FP slag, through the introduction of dolomitic lime in the process, promoted the creation of a better coating on the refractory lining. The changes in the process of steel production promoted a significant increase in average life of ladle refractory in this steel making plant.

Made available in DSpace on 2014-10-09T12:44:27Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:23Z (GMT). No. of bitstreams: 1 06908.pdf: 2703320 bytes, checksum: 38c6007057a454b93e257e7f851f366a (MD5) / Dissertacao [Mestrado] / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

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For a given coal gasification atmosphere, the reactions between fired alumina-chromia solid solution refractories and alkali (sodium and potassium) with and without sulfur at varying alkali concentrations were thermodynamically calculated using the SOLGASMIX-PV computer program and the results were experimentally confirmed. In addition, the kinetics of alkali diffusion into the refractory were experimentally determined as a function of time and temperature. The results, both experimental and theoretical, show formation of alkali-aluminate (Na₂O⋅Al₂O₃, K₂O⋅Al₂O₃) and β-alumina (Na₂O⋅11Al₂O₃, K₂O⋅11A₂O₃) compounds with formation of several metastable alkali compounds in a coal gasification environment. Sulfur did not appear to affect the reaction products. Alkali distribution into the alumina-chrome refractory is rapid and the formation of the Na₂O⋅Al₂O/K₂O⋅Al₂O₃ compounds cause large volume expansion from the reaction surface which causes poor thermal shock resistance and eventual refractory failure. The hot face of an alumina-chrome refractory in service in an alkali environment will be prone to failure by alkali attack. / Master of Science

Made available in DSpace on 2016-12-08T17:19:20Z (GMT). No. of bitstreams: 1 resumo.pdf: 33365 bytes, checksum: c0d79b06b712dd7d33a99ff2c405578a (MD5) Previous issue date: 2013-02-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The silica refractory is nowadays one of the most used linings in the world due to its excellent cost/benefit ratio. The silica sand is available in abundance in the market for use as a raw material for production of ordinary applications refractories, and this does not encourage the search for new usages of its wastes. However, in some industrial sectors where the natural raw materials are used, it has been noticed an increased difficulty to find materials with adequate purity grades to its applications. Furthermore, most of the productive sector is looking for alternatives to reduce the generation of industrial wastes that must be disposed in specific landfills. In this study one aimed to evaluate the potential use of a waste from the synthesis of SiO2 refractory for the application with Alkaline Sodium Silicate and Silica Refractory Paste. This paste is utilized as lining for Ingots and Pouring Ladles in the Foundry Process. This proposal is supported by the demand of Tupy S/A to recover that waste. This company is totally committed to the environment care of your productive processes, always aiming the reduction of wastes generation and its discard. In this proposal it was considered an initial processing stage to partially remove the excess iron present in the wastes that is deleterious to the silica lining. The refractories were produced making waste additions up to 100% in weight. The obtained refractories characterizations, as well as each process step efficiency evaluation, were done through techniques like thermal analysis, X-ray diffraction analysis, grain size distribution curves and ambient temperature compression strength measurements. The results showed good performance with the addition of silica waste, especially a significant increase in ambient temperature compression strength; provided by incorporation of the waste with higher amount of grain small, medium and large. The economic viability of the economy is considerable and preservation of raw materials, conventional silica sand, and the reuse of waste silica mix, avoiding disposal at a landfill. / O refratário de sílica é hoje um dos revestimentos mais utilizados no mundo pelo seu excelente custo/benefício. A areia de sílica como matéria prima para a obtenção de refratários de uso convencional está disponível no mercado em abundância, o que não incentiva a busca por alternativas para a reutilização de seus resíduos. Entretanto, em diversos setores industriais onde matérias primas naturais são utilizadas tem notado o aumento na dificuldade de obtenção de materiais com grau de pureza adequado a suas aplicações. Além disso, todo o setor produtivo tem buscado por alternativas para redução da geração de resíduos industriais a serem descartados em aterros específicos. O presente trabalho busca avaliar o potencial do uso de um resíduo refratário na síntese de refratário de SiO2 para aplicação com Massa Refratária de Silica e Silicato de Sódio Alcalino (MRSNA) para revestimentos de lingoteiras e panelas de vazamento utilizados no processo de Fundição. Esta proposta está baseada na necessidade da reutilização desse resíduo pela empresa Tupy S. A. Esta empresa tem como premissa a constante preocupação ambiental nos seus processos produtivos, buscando sempre a redução da geração de resíduos ou descarte destes. Para o desenvolvimento desta proposta foi considerada inicialmente uma etapa de beneficiamento para a remoção parcial do excesso de ferro presente no resíduo que é um contaminante para o revestimento de sílica. Os refratários foram produzidos considerados adições de resíduos em teores em peso de resíduo de até 100%. Para a caracterização dos refratários obtidos, assim como para avaliação da eficiência das diferentes etapas de processamento foram utilizadas ferramentas como análise térmica, difratometria de raios-X, construção de curvas de distruibuição granulométrica e medidas de resistência à compressão a tempertura ambiente. Os resultados mostraram um bom desempenho com a adição do resíduo de sílica, especialmente um aumento significativo na resistência à compressão à temperatura ambiente; proporcionado por uma curva de distribuição granulométrica heterogênea (grãos pequenos, médios e grandes). A viabilidade econômica é considerável pela economia e preservação da matéria prima, areia de sílica convencional, e pela reutilização do resíduo de sílica mix, evitando o seu descarte em aterro industrial.