The environmental sustainability of calcined calcium phosphates production from the milling of eggshell wastes and phosphoric acid

Abstract

Industrial waste shells of egg cause environmental and health issues. Diversion of eggshells from landfill to a useful product or used in a better way towards the fabrication of marketable/valuable products is of high economic and ecologic interest. Different structures of calcium phosphates were prepared by milling eggshells with different amounts of phosphoric acid (H3PO4) in the presence of gelatin. The amounts of H3PO4 were varied over wide ranges (2.90 ml–8.73 ml), and the impact of such variations on calcined samples was investigated through the characterization of their morphologies by field emission scanning electron microscopy (FESEM). Long milling time is associated with size reduction of the eggshell powders. It was demonstrated that addition of 8.73 ml H3PO4 reduced the d50 eggshell particles from ∼ 1 μm to 0.96 μm after being milled for 8 h. X-ray diffraction (XRD) has been used to identify the crystalline phases present after milling and also after heat treatment for various times of up to 5 h at temperatures in the range 700–1200 °C. Monetite (CaHPO4) is the characteristic phase of ball-milled samples over the whole ranges of phosphoric acid. The thermal treatment applied to the ball-milled samples led to the interaction between the constituents, causing a modification in the proportion of calcium phosphate structures. At 2.90 and 3.88 ml of H3PO4, both tri-calcium phosphate Ca3(PO4)2 and hydroxyapatite Ca5(PO4)3OH phases are the major crystalline structures developed at 1000 and 1200 °C, while Ca2P2O7 and Ca3(PO4)2 were mainly the observed phases at lower sintering temperatures. The present work is establishing a treatment strategy to produce tailor-made calcium phosphate structures for bio-applications.