DEVELOPMENT OF A NEW TREATMENT PROCESS OF WASTE CONCRETE FOR CO_2 REDUCTION IN CEMENT INDUSTRY

摘要

A new type of treatment process of waste cement with pressurized CO_2 was developed as a CO_2 reduction measure for mitigating global warming. Waste cement, a by-product of recycling process of waste concrete to recover aggregates, is used as a calcium source to produce calcium carbonate, CaCO_3. The waste cement will be treated by a two-step reaction scheme; (1) extraction of calcium from the waste cement into the aqueous phase with pressurized CO_2, and (2) precipitation of calcium carbonate from the extracted solution under a reduced pressure of CO_2. The CO_2 necessary for the extraction reaction will be supplied from a flue gas of point sources such as thermal power plants after capturing and separation. The final product, calcium carbonate (CaCO_3), could be used as a raw material in the cement production industry. The net reduction of CO_2 emission could be realized at the amount of CO_2 from the recycled CaCO_3 in the calcination process (otherwise the same amount of virgin limestone should be mined and release that amount of CO_2) minus the CO_2 emission owing to the power consumption in the recycling process. The process feasibility was examined based on laboratory-scale experiments. The extraction reaction of calcium from the waste cement can proceed at relatively high reaction rates: the concentration of calcium in the aqueous phase increased rapidly over an initial period of 10-20 min and far exceeded the equilibrium solubility. The precipitation reaction of calcium carbonate from the solution proceeds by reducing CO_2 pressure. The purity of the obtained calcium carbonate crystals was as high as 97%. A process design was completed for a system treating CO_2 emitted from an oil-fired 100-MW thermal power plant, and the power consumption and cost was estimated based on the experimental results. The power consumption for operating the proposed process was 52.8 MW for the removal of all the CO_2 emitted per 100-MW power generation. The operation cost for the emission reduction of CO_2, excluding costs for capital, maintenances, is about USD 80 A- CO_2.