Batchwise Leaching of Calcium from Demolished Concrete Fines in an Aqueous Ammonium Nitrate Solution: Part 1 Experimental Study

In this work, the dissolution step of the indirect mineral carbonation process was investigated; first, a process-relevant thermodynamic framework is introduced, and then, an extensive experimental campaign with aqueous ammonium nitrate (AN) as the solvent is presented. Steady-state experiments after 24 h of leaching under different solvent concentrations and cement dosing were studied, confirming the applicability of the developed thermodynamic framework and its mathematical description. Afterward, batch experiments were performed to study the kinetic leaching behavior from different particle sizes, at different temperatures, and under a variety of industrially relevant conditions. Smaller particles release more calcium faster than larger particles as their calcium content is higher, and they have more specific surface available and less intraparticle diffusion limitations. Between temperatures of 5 and 35 degrees C, a 20% increase in dissolution performance was observed, which is due to an increase in reaction kinetics and/or diffusivity. Experiments under industrially relevant conditions reveal that increasing both the AN concentration and the solid to liquid ratio increases the amount of calcium in solution but has contradicting effects on extraction efficiency. For the typical conditions of a solid to liquid ratio of 0.2 and an AN concentration of 0.5 M, about 32% of the available calcium was leached after 20 min, resulting in a calcium concentration of 0.065 M; thereafter, this leaching process slows down sharply. The knowledge developed in this work forms a solid foundation for future modeling and scaling up of the indirect mineral carbonation process.