Application of Wastes from Pulp and Paper Industries for steelmaking Processes
Abstract: Over the past several years, considerable efforts have been made to increase the efforts to move towards a circular economy. The ambition to reuse, remanufacture and recycle materials is of great significance for the establishment of a sustainable society. An extended longevity of materials helps to decrease the amount of wastes in landfills, as well as to reduce the mining of natural assets. Pulp and paper and steel industries are two large basic industries and export industrial sectors in Sweden. The pulp and paper industries generate large amounts of organic and inorganic wastes. A significant part of these wastes ends up in landfills. The deposit of wastes in landfills is becoming more challenging to employ and more costly, due to stronger regulations and demands with respect to the environment. During Electric Arc Furnace (EAF) and Argon Oxygen Decarburization (AOD) stainless steelmaking operations, burnt lime (primary lime) is charged together with other slag forming components in the furnace or converter to obtain a specific basicity of the slag and to attain purification from undesired elements by chemical reactions between the steel and slag. In blast furnace (BF) operations, carbon is charged in the form of coke and injected as powder to provide heat to melt the iron ore and to reduce the iron oxides to iron by separating iron from oxygen. The use of this primary lime and fossil carbon does not comply with the closing the loop idea that is being prioritized in modern society. However, some of the CaO-containing wastes from pulp and paper mills can be used as slag formers in steelmaking processes, and to neutralize and clean acidic wastewaters from the pickling process in steelmaking to replace the currently used primary lime. Moreover, some C-containing organic sludges can be upgraded to hydrochar and partly replace fossil carbon in blast furnaces. This thesis presents results from preliminary experiments for examining the replacement of primary lime with recovered lime from pulp and paper production waste as slag formers in EAF and AOD converters and neutralizing agents for acidic wastewaters, as well as hydrochar from organic sludges as a carbon source in BF. The gained results showed a possibility of using up to 30% CaO from recovered lime as a replacement for primary lime in the EAF. Furthermore, the amount of ferrosilicon alloy additions can be decreased by up to 3kg/ton of steel. In the AOD process, the use of recovered lime didn´t show any negative effects on the decarburization process nor on the reduction process. Moreover, the desulphurization function was similar when using recovered lime compared to when using the primary lime. However, the P content in the metal increased slightly. Thus, the replacement ratio of recovered lime will be limited by the acceptable P level in the final steel due to higher P content in recovered lime materials from pulp and paper mills compared to that in primary lime. Furthermore, the obtained results of laboratory-scale trials using recovered lime materials as neutralizers showed that they can be successfully used instead of primary lime to significantly raise the pH level of the wastewater as well as to decrease the concentrations of Cr, Fe, Ni, Mo and Zn. Also, the results from the initial industrial-scale experimental trials using hydrochar derived from mixed organic biosludge and from green waste showed that most of the technological parameters of the BF process, such as the production rate of hot metal, amount of dust, fuel rate and amount of injected coal, amount of slag, contents of FeO in slag and %C and %P in the hot metal in the experimental trials were very similar compared to those in the reference periods (two days before and two days after the trials) without using these new innovative charge materials. Thus, it was proven that hydrochar derived from various types of organic residues could be used for metallurgical applications. However, the replacement ratio was very low and longer trial campaigns are needed to obtain more certain results. Moreover, it was revealed during this study that slags from AOD converters can be used as binding agents for briquetting of these recovered lime materials. These results show that waste materials/byproducts from two of the most important Swedish base industries can be used to make metallurgical briquettes, uniting two major industrial sectors in a circular symbiosis towards obtaining a more sustainable future.
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