Development of a process for producing high‐purity calcium carbonate (CaCO3) from waste cement using pressurized CO2

A new type of process for producing high-purity calcium carbonate from waste cement powder was developed. The process consists of two main reactions; extraction of calcium in the form of calcium ions from waste cement powder in a water slurry by pressurized CO2 (typically at several MPa), and precipitation of calcium carbonate (CaCO3) from the extracted solution by reduction of the CO2 pressure. The process can be recognized as the emission reduction process of CO2 as well. Laboratory-scale experimental studies were conducted for both the extraction reaction and the precipitation reaction, to examine the feasibility of the process. The extraction process was found to proceed at a relatively high rate, especially during the initial 10 min. The concentration of calcium ions exceeded the thermodynamic solubility of CaCO3 (supersaturation) when abundant waste cement was available. After filtration of the residues, CaCO3 particles with >98% purity were obtained by depressurizing the CO2. The dependency of the reaction rates on the operating conditions, such as the ratio of waste cement to water, CO2 pressure, and particle size of waste cement particles, was investigated. A process design was carried out based on the experimental results, assuming that the present process is applied to two types of CaCO3 production process: flue gas desulfurization and production of ultrahigh-purity CaCO3. The estimated costs per 1 metric ton of CaCO3 were USD 136 for desulfurization and USD 323 for high-purity CaCO3. © 2005 American Institute of Chemical Engineers Environ Prog, 2005