Mechanisms of sintering of macroporous hydroxyapatite adsorbents

The behaviour during calcination of two hydroxyapatites (HAP), one calcium deficient and the second with calcium excess, was investigated. A structural change in terms of the crystallization, combined with significant decreases of the specific surface area by sintering process, was observed. This process begins near 600degreesC and reaches a maximum near 900degreesC. The decay in specific surface area observed is described accurately by an empirical equation of the form dS/dt = -B'(T)S-h where S is the specific surface area, t is the time, b and B'(T) are respectively the reaction order and the constant rate which depends on the calcination temperature. These constants vary swiftly with changing temperature, suggesting that the mechanism of the sintering may change with the temperature. The results for the two macroporous HAPs are compared and discussed in terms of various possible mechanisms of materials transport involved in the sintering process.