Structural and chemical changes of thermally treated bone apatite

The thermal behaviour of the animal by-product meat and bone meal (MBM) has been investigated in order to assess how it is affected structurally and chemically by incineration. Initially composed of intergrown collagen and hydroxyapatite (HAP), combustion of the organic component is complete by 650 degrees C, with most mass loss (50-55%) occurring by 500 degrees C. No original proteins were detected in samples heated at 400 degrees C or above. Combustion of collagen is accompanied by an increase in HAP mean crystallite size at temperatures greater than 400 degrees C, from 10 nm to a constant value of 120 nm at 800 degrees C or more. Newly formed crystalline phases appear beyond 400 degrees C, and include beta-tricalcium phosphate, NaCaPO4, halite (NaCl) and sylvite (KCl). Crystallite thickness as judged by small angle X-ray scattering (SAXS) increases from 2 nm (25-400 degrees C) to 8-9 nm very rapidly at 550 degrees C, and then gradually increases to approximately 10 nm. The original texture of HAP within a collagen matrix is progressively lost, producing a porous HAP dominated solid at 700 degrees C, and a very low porosity sintered HAP product at 900 degrees C.