Continuous production of hydroxyapatite powder by drip pyrolysis in a fluidized bed

Hydroxyapatite (HAp) powder was produced by drip pyrolysis in a fluidized bed (DPFB) at 913–1113 K with dry air. In this study, HAp powders derived using different concentrations of a solution-type precursor material were compared with those derived using slurry-type precursor materials. From a solution-type precursor containing calcium nitrate and ammonium phosphate dibasic, fine HAp powders with mean particle sizes of approximately 8–40 μm were produced, depending on the solution concentration and reaction temperature. In these cases, bimodal particle size distribution was observed. Spherical alumina of 250 μm was found to be superior to silica sand of 270 μm as the coarse medium particles in reducing contamination of the product powder. The thermal stability of HAp derived from a slurry-type precursor after 1-day aging was improved by DPFB. HAp powders derived from the solution-type precursor material were Ca-deficient, and their Ca/P molar ratio increased with bed temperature. In contrast, the Ca/P molar ratio of HAp powders from the slurry-type precursor material was nearly stoichiometric. Morphology strongly depended on the starting precursor material. The solid collection ratio was closely related to the population of product particle sizes larger than 25 μm.