Recyclability evaluation through empirical and environmental criteria based on multi-criteria decision-analysis applied to tundish working lining refractories

Tundish refractories for the working lining, acting as a sacrificial layer, are commonly composed of highly pure MgO-based materials. Their lifetime spans from few to tens of hours and it is estimated they made up to 20% of the total refractory waste produced during steel production process. Despite this significant amount, recycling of tundish working lining refractories has received scarce attention. Some of the reasons can lie in the abundance of low-cost virgin raw materials and low disposal costs of the largely inert wastes. However, recent environmental sensitivity, circular economy policies, increasing landfill and carbon-tax costs foster attention on spent refractories. Recycling tundish working lining refractories in soil conditioners, roadbed foundations, or slag additives does not exploit the full intrinsic value of the materials. Hence, an additional drawback concerns the opening of the process loop. Even if high value recycling is still a minor share, this alternative solution can mitigate rising prices and supply chain issues generating a closed-loop process taking advantage of the high-purity constituents composing these refractories. The study presented here investigates the influence of empirical and environmental criteria on the recyclability convenience of working lining refractories for the tundish used in steel continuous casting. Three distinct formulations of slurry gunning refractory mixes, exhibiting different particle size distributions and similar chemical composition, were studied. Each of the three mixes has been considered in the new form containing only virgin raw materials, and with various fractions of secondary raw material. MgO and impurities (SiO2 and Fe2O3) content, open porosity, water requirement, permeability, corrosion resistance, and slag infiltration depth were experimentally determined and considered as empirical criteria. Literature data and LCA open-source databases have been used to estimate carbon footprint, energy consumption and production costs of the refractory mixes and taken into account for the environmental criteria. The influence of criteria weights variation has been assessed and exploited to generate objective functions of best convenience.