Indirect mineral carbonation of recycled concrete aggregate: Enhancing calcium extraction using a packed bed reactor

Mineral carbonation of industrial residues is gaining increasing interest, with the first pilot and industrial-scale applications being implemented. The process offers a low-risk, easily available solution for permanent carbon dioxide storage while concurrently valorising industrial wastes. This work investigates the indirect mineral carbonation of recycled concrete aggregate (RCA), in which calcium is first leached from the RCA into an aqueous ammonium nitrate (AN) solution, followed by carbonation to yield high-purity precipitated calcium carbonate. The study focuses on the first step, calcium dissolution, which traditionally takes place in a stirred batch reactor (SBR). However, SBRs often encounter solubility constraints, leading to low calcium extraction yields. In this investigation, we explore the application of a packed bed reactor (PBR) and demonstrate its superior performance, resulting in a calcium extraction enhancement of 27% to 55% relative to SBRs, across AN concentrations ranging from 0.25 to 2.0 mol/kg, at 25 degrees C and atmospheric pressure. The PBR's superior performance can be attributed to the more compact design, enabling longer dissolution times, and to the continuous supply of fresh solvent, effectively reducing solubility limitations. The impact of key operating parameters (AN concentration, liquid flow rate and extraction time) are systemically assessed experimentally, accurately modelled using a shrinking-core model, and illustrated in process performance maps. Furthermore, this research highlights the significance of ammonium nitrate recovery from the wet solids after filtration to ensure the process's environmental viability. The packed bed reactor (PBR) functions as an integrated filter, facilitating the straightforward recovery of 80% of the trapped ammonium nitrate solution through simple water flushing. In conclusion, the utilisation of PBRs in indirect mineral carbonation presents an innovative and efficient approach for enhancing calcium extraction while addressing environmental concerns through ammonium nitrate recovery. The applicability of PBRs extends beyond recycled concrete aggregates, and could also be successfully applied for other coarse industrial residues such as steel slags.