Precipitation Process of Calcium Phosphate from Calcium Carbonate Suspension

The Ca-HA is synthesized by reacting calcium carbonate (CaCO3) and ammonium dihydrogen orthophosphate (NH4H2PO4) in stoichiometric proportions. In this precipitation process, the ratio of Ca/P by moles is 1.67. The experiments were performed in a batch reactor at 25 degrees C. From control parameters, the pH and the temperature were measured in line for all experiments. The tests were performed with two liquid-solid mass ratios (H2O/CaCO3) of 3 and 5. For each mass ratio, three stirring rates of 260, 400, and 600 rpm were tested. Samples of the synthesis were collected at different intervals and analysed by laser granulometry and by environmental scanning electron microscopy. For the synthesis conducted at 260 rpm, synthesis monitoring was made using a contact probe in solution coupled to a Raman spectrometer in order to follow the formation of solid phase. This technique is valuable to follow the synthesis of Ca-HA in a concentrated solids suspension (around 20-30 wt%). The results make it possible to propose a mechanism of the precipitation process of Ca-HA. It can be divided into four main stages: (i) dissolution of calcium carbonate (CaCO3), (ii) precipitation of brushite (CaHPO4.2H(2)O), (iii) transformation of brushite into Ca-HA (Ca-10(PO4)(6)(OH2)) and (iv) nucleation, growth and agglomeration of Ca-HA.