The influence of calcination parameters on free calcium oxide content in natural hydroxyapatite

Effect of calcining temperature and time on the content of free CaO in hydroxyapatite of natural origin is presented. Hydroxyapatite was obtained from pork bones in the course of a three-stage process: hydrolysis with the application of lactic acid, pre-calcination at 600 °C and main calcination stage within the temperature range of 750–950 °C. Calcination was conducted in an electrically heated stationary chamber oven in air atmosphere and in a laboratory scale rotary kiln equipped with a gas burner. The FT-IR spectra confirmed that all organic substances were removed during the calcination process. An increase in free calcium oxide content in hydroxyapatite from 0.003% to 0.023% was caused by the increase of calcining temperature from 750 °C to 950 °C respectively. Calcining time at 950 °C gave a distinct impact upon free CaO content ranging from 0.014% (2 h) to 0.023% (3 h). Hydroxyapatite calcined in the rotary kiln contained the lowest amount of free calcium oxide: 0.002% (750 °C) and 0.003% (950 °C). A method of neutralizing of free calcium oxide, present in calcined hydroxyapatite powders of natural origin, through CaO transformation into hydroxyapatite by application of diluted thermic phosphoric acid has been developed.