Evaluating the dissolution behavior of secondary additives in various slag systems

Recycling of refractory materials has gained significant importance in recent years due to its potential to conserve raw materials, reduce energy consumption during calcination processes, minimize CO2 emissions, and reduce landfill volumes and environmental impact. Spent refractory materials are typically sorted into two main particle size categories: fractions larger than 80 mm and fractions smaller than 80 mm. The coarse fraction (>80 mm) is generally cleaned of slag deposits and metallic residues and reused directly for the production of new refractory products, contributing to circular economy goals. In contrast, the fine fraction (<80 mm) which can make up to 40% of the recycled material stream often contains impurities that impair its suitability for high-quality reuse. As a result, this fine fraction is frequently discarded or used in low-value applications. Developing methods to utilize this underused fraction presents a key opportunity for improving the sustainability and efficiency of refractory recycling. To address this challenge, these fine materials are repurposed as secondary metallurgical additives. Compared to traditional additives such as dolomitic lime or pre-melted slags, these recycled materials not only contribute valuable chemical properties but also have a significantly lower carbon footprint in steel production. Despite these advantages, the fundamental role of refractory materials as non-dissolving agents in slag presents a scientific challenge. This master¿s thesis aims to evaluate the dissolution behavior of Al2O3- and MgO-containing refractory recyclates in two different basic oxygen furnace (BOF) slags (sampled before and after deoxidation of the molten steel using metallic Al), as well as in a smelter slag. The study compares their dissolution rates and performance with benchmark additives to assess their suitability as secondary metallurgical additives, contributing to both scientific research and industrial applications. Given the heterogeneous nature and impurity content of secondary additives, as well as the need for industrially scalable results, a static dissolution experiment using dispersed loose particles was chosen for this study. The results showed that secondary additives exhibited comparable or higher dissolution rates than primary additives. Influencing parameters such as slag viscosity, density, and basicity played a crucial role in governing dissolution behavior. Lower slag basicity and higher viscosity were found to reduce dissolution efficiency, consistent with diffusion-controlled mechanisms. These findings provide valuable insights into the feasibility of utilizing refractory recyclates as secondary metallurgical additives, offering both environmental and operational benefits while highlighting key process parameters that influence their dissolution behavior in industrial steelmaking applications.