(122c) Development of a New Recycling Process of Shell Waste Using High-Pressure Carbon Dioxide Solution

A considerable amount of seashell waste has been generated in Japan, of which the annual rate has become as large as 20,000 ton in recent years. Most of the waste seashell has been incinerated or land filled at the present stage, but because of environmental concerns, development of an effective utilization process is demanded. The shell is composed of calcium carbonate with a trace protein, which can be reused as a raw material of concrete or fertilizer. However, seashells grown near the outlet of cooling water stream from power plants may not be edible, and both the flesh as well as the shell should be treated as a waste. The present study aims to develop an effective utilizing process of such kind of waste seashell with flesh. In the proposed process, the shell and the flesh will be separated by treating with high-pressure CO2 (~ 3 bar) aqueous solution; the shell will be dissolved into the aqueous phase, while the flesh is unaffected with high pressure CO2 and then the aqueous phase and the remained flesh will be separated. The calcium in the aqueous solution can be recovered in the form of pure calcium carbonate by reducing the CO2 pressure. On the other hand, the shell flesh will be used to generate methane gas via fermentation process, and the methane can be used for power generation. Thus, the proposed process can achieve the recovery of calcium and the energy production simultaneously from waste seashells. On the other hand, if the aqueous solution dissolved with calcium ions with high-pressure CO2 is disposed on in the deep ocean, it can realize a CO2 reduction process in the form of bicarbonate ions. Process feasibility for the proposed process was examined by laboratory-scale experiments both for the dissolution of several types of seashells with high-pressure CO2, and the fermentation of the shell flesh. Based on the experimental results, process design of the seashell treatment was conducted, and the process energy balance was evaluated and the cost was estimated.