Assessment of Ammoniacal Leaching Agents for Metal Cation Extraction from Construction Wastes in Mineral Carbonation

The use of carbon mineralization to produce carbonates from alkaline industrial wastes is gaining traction as a method to decarbonize the built environment. One of the environmental concerns during this process is the use of acids, which are required to extract Ca2+ or Mg2+ from the alkaline waste to produce carbonates. Conventionally, acids such as hydrochloric, nitric, or sulfuric are used which allow for the highest material recovery but are corrosive and difficult to regenerate as they are utilized in a linear fashion and generate additional process waste. An alternative is to use regenerable protonatable salts of ammonia, such as ammonium chloride (AC) or ammonium sulfate, the former of which is used globally during the Solvay process as a reversible proton shuttle. In this study, we show that regenerable ammonium salts, such as AC (NH4Cl) and ammonium bisulfate (NH4HSO4), can be effectively used for material recovery and the production of calcium carbonate during the leaching of waste cement paste as an alternative to conventional acids such as HCl. Leaching kinetics, postreaction residue, and carbonate characterization were performed to assess the productivity of this system and potential uses of these materials downstream. The stabilization of vaterite was observed in the case of AC leaching, suggesting its importance in the kinetic stability of vaterite and suppression of calcite nucleation. Overall, this study motivates the use of alternative leaching agents, such as salts of ammonia, to facilitate material recovery and carbon capture from alkaline industrial wastes.