Characteristics of metastable zone in the crystallization process: a case study of sparingly soluble hydroxyapatite

摘要

Hydroxyapatite (HAP) crystallization for phosphorus recovery from wastewater is an economical and efficient approach. The metastable zone of solution is the precondition of crystallization process and is also the key to obtain high quality crystal products. HAP is a sparingly soluble salt, and the research on the metastable zone of HAP seems more complex than that of soluble salts. In this study, the characteristics of metastable zone of HAP crystallization at different saturation temperatures were studied by using the electrical conductivity and turbidity method. Effects of various experimental factors (crystal seeds, initial pH, stirring intensity, and impurity ions) on metastable zone width (MZW) were investigated. The results indicated that MZW was increased with the rise of cooling rate, and the presence of crystal seeds delayed the occurrence time of critical conductivity thus increasing the MZW. Calcite as the crystal seed exhibited the best performance on improving the MZW due to its specific surface area and composition among three tested seeds. MZW became narrow with the increase of the stirring intensity, and the acidic solution (pH 6.0) had a larger MZW than that in alkaline solution (pH 8.0). Several typical impurity ions could broaden the MZW of HAP, and the order of the impact on.MZW was: CO32-> Fe3+> Cu2+> SO42-.