A CO2 fixation process with waste cement powder via regeneration of alkali and acid by electrodialysis

Abstract

A mineral carbonation process for CO₂ sequestration has been developed, and its process feasibility was examined based on laboratory-scale experimental studies. The process is composed of four steps: (i) extraction of calcium ions from waste cement powder with nitric acid. (ii) Absorption of CO₂ in sodium hydroxide solution from CO₂ emission sources. (iii) Precipitation of calcium carbonate particles by mixing the calcium leached solution and the CO₂ absorbed solution. (iv) Regeneration of nitric acid and sodium hydroxide from the remaining solution of sodium nitrate from step (iii) with bipolar membrane electrodialysis. The regenerated acid and alkali can be reused in steps (i) and (ii) with newly fed waste cement powder. The overall mass balance of the process is the input of CO₂ and calcium and the output of calcium carbonate and residues of the leaching. The effects of the operation conditions in the calcium leaching step (i) and regeneration step (iv) were experimentally investigated with a laboratory-scale experimental apparatus. It was found that calcium leaching can be easily and quickly performed with nitric acid, and high-purity calcium carbonate can be obtained when the operation conditions are set to obtain a final pH of about 7. Regeneration of nitric acid and sodium hydroxide were favourable when the current density and the electric potential were high and the feed concentration was low in terms of the power consumption. Thus, the total process can be operated under appropriate conditions, and its CO₂ fixation performance was evaluated.