地球温暖化対策としての二酸化炭素の炭酸塩鉱物化プロセス

Abstract

The mineral carbonation method is an effective countermeasure for global warming. In the mineral carbonation method, anthropogenically emitted carbon dioxide is captured and fixed as carbonates by using calcium or magnesium sources. For a practical mineral carbonation process, the carbonation reaction should be accelerated, and a huge amount of calcium or magnesium sources should be secured. Waste concrete is a calcium source with the highest CO_2 fixation potential in Japan at about 6 million tons annually. We have developed a new type of CO_2 fixation process by using waste concrete as a calcium source. The process is composed of four steps, namely, (1) absorption of CO_2 from flue gas stream by using an alkaline metal hydroxide solution such as sodium hydroxide, NaOH, (2) extraction of calcium ions from waste concrete by using a strong acid such as hydrochloric acid, HCl, (3) carbonation by mixing the two solutions obtained in the absorption step and the extraction step to form calcium carbonate as a precipitant, CaCO3. The obtained calcium carbonate, where CO_2 in the flue gas is fixed in a solid form, can be used as a raw material for widerange industries such as fillers in paper or plastics, additives, or cement. The remained solution of sodium chloride, NaCl, after carbonation and filtration of calcium carbonate, can be treated by the bipolar membrane electrodialysis (BPED), where sodium hydroxide and hydrochloric acid can be regenerated by electric power. By the regeneration step (4), the total process is closed as a cyclic process without consuming alkaline and acid. We have examined the characteristics of each step based on laboratory-scale experimental studies, and it was confirmed that the method can realize a negative CO_2 emission. Further reduction of the power consumption for step (4) is the key to improve the process efficiency.