Development of a new recycling process of shell wastes using high-pressure carbon dioxide solution

A new recycling process of shell wastes using high-pressure (∼30 bar) carbon dioxide (CO₂) solution has been developed. The waste shell composed of the shell part and the flesh part will be crushed and then treated with a high-pressure CO₂ solution. The shell part, mainly composed of calcium carbonate, will be dissolved into the CO₂ solution, and will be separated from the flesh part. The flesh part will be treated in a fermentation process to produce methane gas. The dissolved shell part will be either recovered as calcium carbonate (Process (i)) or will be directly disposed of in the ocean (Process (ii)). Process feasibility was examined with laboratory-scale experimental studies. The dissolution behavior of the blue mussel shell samples with a high-pressure CO₂ solution were investigated under various operation conditions such as the CO₂ pressure, temperature, stirring speed, sample size. The effects of these operation parameters on the shell dissolution rate were elucidated. It was confirmed that the shell sample could be dissolved into high-pressure CO ₂ solution with a reasonably high rate, suggesting that the separation of the shell part could be realized by the CO₂ treatment. The methane fermentation experiments demonstrated that the high-pressure CO ₂ exposure for 1 hr did not affect the methane fermentation performance of the flesh part of blue mussel. Based on the experimental results, the process design was carried out and the cost was estimated. As a result, the cost of the process (i) (recovery of calcium carbonate) was about 18000 JPY / t-shell, and that of the process (ii) (ocean disposal) was 80,000 JPY /t-shell waste, reflecting the high cost for CO₂ recovery and compression from the flue gas. The process (i) is competitive with the waste disposal cost for simple disposal and more environmentally benign.