The objective of this thesis was to study precipitation of calcium hydrogen phosphate dihydrate (dicalcium phosphate dihydrate, DCPD) in semi-batch and continuous stirred tank crystallizers and continuous plug-flow reactor. The literature part deals with phosphate reserves and fertilizer production in different countries, phosphorus removal methods used in wastewater treatment, and methods for phosphorus recycle from waste streams as slow-release fertilizers. Finally, different calcium phosphate phases and their precipitation are compared to DCPD precipitation which is used as model substance. The calcium phosphate precipitation was expected to proceed in steps, from precipitation of hydroxyapatite via slow transformation to DCPD due to pH change. The changes in calcium phosphate phase were seen from suspension pH evolution. The precipitated samples were examined using X-ray diffraction (XRD) analysis and Scanning Electron Microscope (SEM). Also, volumetric size distributions (Malvern Morphologi) were determined. The results from the continuous plug-flow crystallizer showed that initial pH of reagents affected significantly precipitation, not only to amounts precipitated, but also pH influenced on produced calcium phosphate properties as well. Furthermore, it could be deduced from pH, conductivity developments and size distributions when suspension sample reached steady-state after sampling, or when desupersaturation still occurs in solution. This is an important information for example in terms of filtration so that filtration equipment would not clog during the filtration because of precipitation. In continuous stirred tank experiments, high phosphate precipitation was achieved regardless of residence time or mixing rate. Precipitated calcium phosphate was almost completely DCPD regardless of residence time. High phosphate precipitation rates with short residence times also changed crystal morphology during the experiments. It was also noted, that anhydrous form was often co-precipitating when unstable hydroxyapatite was transforming into more stable form.
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