Scalable recycling of oyster shells into high purity calcite powders by the mechanochemical and hydrothermal treatments

Abstract

The massive oyster shells have been usually dumped into natural waters and landfills, causing water pollution and health/sanitation issues. It is highly desirable to convert the oyster shells into high value-added products. This study presents a two-step processing route to prepare high purity calcite powders from oyster shells, i.e. the mechanochemical and hydrothermal treatments. The mechanochemical milling produced nanosized powders with serious agglomeration, while the subsequent hydrothermal reaction achieved agglomerate-free and submicron calcite powders with high purity. The as-prepared calcite powders were characterized by various techniques. The processing parameters have been studied with respect to their influence on the microstructure, chemical composition, thermal stability and whiteness of powders. The thermal stability of calcite powders was significantly improved due to the removal of organic components. The whiteness of calcite powders decreased in the mechanochemical treatment, whereas it increased after the hydrothermal treatment, with a maximum value of 88.6%. The formation mechanism of calcite powders involves the hydrolysis, oxidation and recrystallization processes. The facile and scalable approach is suitable for the profitable reutilization of oyster shells, thus promoting the shellfish mariculture sustainability.