Accelerated mineral carbonation (AMC) method has been recognized as a promising measure for carbon dioxide fixation. The drawback of the AMC is the cost and power consumption for the acceleration of the carbonation reactions. Uses of chemicals such as acids, alkalis or some special reagents have been proposed for the acceleration reaction, mainly for the extraction of alkali earth metals from the sources. These chemicals should be recycled or reused; otherwise the emission amount of carbon dioxide associated with the production of these chemicals should outnumber the amount fixed in the process. The effective recycling or reusing method should be essential for the AMC as a practical option for carbon dioxide fixation.
The authors have proposed a new type of AMC process by using acids as an extraction reagent of calcium or magnesium from the sources and alkali metal hydroxide solutions as a capturing reagent of carbon dioxide in the flue gas. The solution of extracted magnesium or calcium will be mixed with the solution of alkali metal carbonate after capturing carbon dioxide to precipitate magnesium or calcium carbonates. The remained solution will then be treated with the electrodialysis process to recover the acid and the alkali.
We have examined the feasibility of the process by laboratory experimental studies. Since the capturing of carbon dioxide with alkali metal hydroxide solution and the precipitation of carbonates are well known processes, we concentrated to the extraction step and acid-alkali recovery steps. For the extraction step, we have examined the combination of solid sources such as wollastonite, serpentine, and waste cement and acid extractants such as nitric acid, hydrochloric acid, and other weak acids. The extraction reactions were found to progress rather rapidly, even for the cases using the weak acids, and the extraction of impurities in the sources such as iron, silicon could be minimized under a condition of proper solid-liquid ratios.
The recovery step with electrodialysis can be achieved by using the bipolar membrane electrodialysis (BMED). The alkali-acid pair was recovered for all the combinations studied. However, both the recovery rate and the energy-efficiency were significantly higher for the cases using the combination of weak acid and calcium or magnesium.
In conclusion, the use of weak acid as a extractant for the alkali earth metals is preferable for the process efficiency of the carbon dioxide fixation.