Eggshells were collected and washed with detergent, then calcined in air at 900 °C for 10 h. During the first 30 min most of the organic materials were burnt out, then the eggshells were converted to calcium oxide. Calcined shells were crushed and milled in a ball mill or an attritor mill. The ball mill (Fritsch GmBH, Fig. 2.2a) was equipped with alumina balls and bowls, the attritor mill (Union Process, Fig. 2.2b) was fitted with zirconia tanks and zirconia balls (02 mm). The crushed eggshells were reacted with phosphoric acid (H3PO4) in an exothermic reaction. The mixtures were milled for 5 h at 4000 rpm (attritor milling) or for 10 h at 350 rpm (ball milling), to achieve homogenous mixtures and to prevent agglomeration. In all cases, the used shell: H3PO4 ratio was 50 : 50 wt.%16,17.

After milling, a small amount (approximately 0.5 g) of each type of HAp pow­der was sintered at 900 °C for 2 h in air. HAp powders usually degrade at high temperatures, the most common problem being CaO formation.