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, CaCO ₃ 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.