Dicalcium Phosphate Dihydrate Precipitation: Characterization and Crystal Growth

The precipitation of dicalcium phosphate dihydrate, brushite, by mixing a calcium hydroxide suspension and an orthophosphoric acid solution in equimolar quantities, has been investigated in a batch system at 25°C. The concentration of calcium hydroxide and orthophosphoric acid, before mixing, ranged from 50 to 400 mmol dm−3. The precipitation process occurs in five stages, being the hydroxyapatite, Ca10(OH)2(PO4)6, the first phase to be precipitated, as shown in a previous work. This present work deals with the phenomena occurring during the last stage, according to the model presented: brushite grows due to direct consumption of calcium in solution, and due to the transformation of HAP into brushite. An equilibrium and mass balance equations system is formulated describing the chemical species present in solution and the change of their concentrations along each run. This system was validated through conductivity measurements. From this system, the type and composition of precipitate is determined, and the evolution of particle size is estimated and compared with experimental results. The kinetics of brushite growth due to calcium consumption (determined from the decrease of calcium concentration) was studied. The results obtained in these studies allow validating the proposed model concerning the transformation of HAP and brushite growth.