Sustainable Valorization of Steelmaking Slag: From Metal Extraction to Carbon Sequestration

This dissertation is focused on the development of novel, effective, and environmentally sustainable processes for the recovery of critical metals such as niobium, titanium, and magnesium from steelmaking slag and carbon dioxide (CO2) sequestration using steelmaking slag. In this work, a fundamental understanding of the metal extraction process and carbonation process was developed using principles of reaction kinetics, mass transfer, and thermodynamics to elucidate the underlying physicochemical mechanisms. This approach helps tackle the sustainability challenges associated with the supply of such materials, reduce the carbon footprint attributed to the steelmaking industry, and unveil the hidden value of industrial wastes.

In this dissertation, innovative pyro-hydrometallurgical processes were developed for the recovery of valuable components, i.e., niobium, titanium, and magnesium, from steelmaking slag. In this work, the physicochemical mechanisms of this process and their consequences on the process efficiency and operation of the process were investigated. The process mechanism was validated by trials using synthetic analogues to steelmaking slag, phase identification, morphology characterization, elemental mapping measurements, and microstructure measurements.

The focus of this dissertation was expanded to the development of a novel supercritical carbonation process for CO2 sequestration using EAF slag. A systematic design of experiments and response surface methodology were used to develop a model for calculating the carbonation efficiency as a function of various operating parameters, such as slag particle size, CO2 pressure, reaction temperature, water to slag ratio, and processing time. It was demonstrated that the empirical model built in this work is suitable for process optimization with the objective of maximizing the CO2 uptake of the slag. On the basis of the fundamental investigations, it was identified that the diffusion barrier to the carbonation process is a product calcium carbonate layer formed on the outer shell of the slag particle and the thickness of this layer is constant regardless of the slag particle size.

It is believed that the knowledge gained here will help enable the development of robust, cost-effective, and efficient pyro-hydrometallurgical processes for the valorization of industrial process residues, alongside the development of novel and environmentally-friendly supercritical carbonation processes for CO2 sequestration using industrial wastes.