A review on steel slag valorisation via mineral carbonation

Research pertaining to carbon dioxide sequestration via mineral carbonation has gained significant attention, primarily due to the stability of sequestered CO2 over geological time scales. The use of industry-derived alkaline wastes as feedstocks for mineral carbonation also provides an opportunity for waste valorisation. In this review, we describe the carbonation of steel slag, a calcium-rich solid waste from iron and steel industries, whose current utilisation is below par in major crude steel-producing countries. We review ex situ mineral carbonation routes, both direct and indirect, which produce weathered aggregates suitable for the construction industry and value-added chemicals such as precipitated calcium carbonate, respectively. We present an in-depth analysis of slag characteristics and its implications for dissolution and carbonation mechanisms. Current research efforts are focused on overcoming slow carbonation kinetics under atmospheric conditions. However, a good understanding of the rate limiting step and the role of product layers in reaction kinetics is incomplete and evolving. In this review, we summarize the experimental research efforts in understanding the rate limiting step and enhancement strategies for various process routes proposed in the literature. Steel slags are chemically heterogeneous residues and pose significant challenges in modelling the dissolution kinetics. To determine the controlling mechanism using models, it is necessary to model the competitive behaviour of ions during dissolution and precipitation, temporal evolution of the reactive surface area, and coupling of the surface reaction and mass transport at the fluid–solid interface. We present mathematical models that describe the surface reaction, role of polydispersity and heterogeneous morphology in determining reactive surface area evolution, and briefly inform the readers about the non-equilibrium thermodynamics approach for modelling coupled processes. Finally, the current state-of-art in reactor technologies and a step forward for commercialisation are commented on.