Stepwise treatment of ashes and slags by dissolution, precipitation of iron phases and carbonate precipitation for production of raw materials for industrial applications.

The purpose of this study was to test the feasibility of a specific mineral carbonation reaction route applied to different types of alkaline industrial residues, i.e. biomass, paper sludge and municipal solid waste incineration bottom ashes and stainless steel slags and dust. This new approach includes the dissolution of industrial residues in hydrochloric acid (HCl), followed by precipitation of iron compounds from the resulting aqueous solutions and the precipitation of calcium carbonates to employ in industrial applications (Carbon Capture, Utilisation and Storage, CCUS). The aim of this work is to apply this stepwise treatment to different types of poorly valorised industrial residues to assess which may be the most promising ones to employ for the process, in terms of total content of specific elements in the obtained products. Our results clearly indicate that the investigated ashes and slags consist of 20-30 wt% CaO which is bound in a broad variety of mineral phases. Reaction of slags and ashes with HCl leads to the formation of Si-rich solid residues and Ca-rich aqueous solutions. Dissolution residues from ash treatment might be used as lightweight concrete aggregate in case of appropriate mechanical properties, whereas dissolution residues from slag treatment might serve as metallurgical Cr concentrates. Resulting aqueous solutions show high concentrations of Ca (>10 g/L), up to 27 g/L of Fe and significant amounts of heavy metals like Pb, Ba, Zn, Cu, Ni. The concentration of dissolved Fe decreases to 2 mg/L by adding NH3 which leads to the precipitation of amorphous iron phases. Finally, calcium carbonates with a purity of 79-97% are precipitated by injecting CO2 at pH 9. These carbonates present lower heavy metal contents than the input materials (e.g. 0.3 wt% ZnO compared to 0.9 wt% for EAF-FD).