Influence of Milling Speed on the Structural and Hardness Properties of Eggshell-Derived Hydroxyapatite

Abstract

Hydroxyapatite (HA) has incredible compatibility for use in the field of biomedical applications. Nevertheless, conventional HA synthesis frequently uses energy-intensive processing and high-purity chemical precursors, which raise manufacturing costs and have an adverse effect on the environment. This study offers a sustainable method for producing value-added biomaterials from biowaste by assessing the impact of ball milling speed on the structural and hardness characteristics of HA synthesised from eggshell-derived calcium oxide (CaO). Eggshells were calcined to produce CaO, which was then combined with diammonium hydrogen phosphate and ball-milled for four hours at 200, 300, and 400 rpm before being heated to 800 °C. Field-emission scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (FESEM–EDX), X-ray diffraction (XRD) was used to characterize the produced powders. The findings demonstrated that increasing milling speed promoted a more homogenous microstructure while improving phase formation, crystallinity, and particle refinement. EDX analysis revealed that after heat treatment, Ca/P ratios converged toward the stoichiometric value of HA (1.67) across all milling speeds, with the 400 rpm condition achieving 1.67 post-calcination. Additionally, hardness increased dramatically from 551.43 MPa at 200 rpm to 730.83 MPa at 400 rpm, suggesting better densification and less porosity. Overall, the results show that a crucial processing factor affecting the mechanical and structural characteristics of HA produced from eggshells is milling speed. The superior crystallinity and hardness of the optimized HA produced at 400 rpm demonstrated the potential of eggshell waste as a sustainable and eco-friendly precursor for biomedical-HA.