Real-time Raman Monitoring of Calcium Phosphate Precipitation in a Semi-batch Stirred Crystallizer

Recently, recovery of phosphorus has become a subject of great research interest. This is not only because phosphorus is an important, geographically concentrated, nonrenewable resource to support food production, but also because the natural reserves like phosphate rocks are reducing [1]. Moreover, high amount of phosphorus causes eutrophication in rivers, lakes and seas all over the world [2]. Various techniques including biological nutrient removal methods and physicochemical treatment methods have been employed for phosphate removal [3, 4].

Precipitation of calcium phosphate is an attractive option due to its wide applications. Different sparingly soluble crystalline phases such as decalcium phosphate dihydride (DCPD or brushite, CaHPO4∙2H2O), octacalcium phosphate (OCP, Ca8H2(PO4)6∙5H2O) and calcium hydroxyapatite (HAP, Ca5OH(PO4)3) can be formed during the reaction, which makes the precipitation process particularly complex. The formation of these phosphates depends on operational conditions such as the concentration of reactants, pH, supersaturation level and temperature. Crystals with desired particle size and morphology can be obtained at optimum operating conditions, which requires a particular tool to control the whole process. Raman spectroscopy has the potential to monitor the changes taking place in solid-liquid suspensions in crystallization process [5, 6].

To our best knowledge, application of Raman spectroscopy on real-time monitoring calcium phosphate precipitation has not been studied widely. The present work focuses on the precipitation of calcium phosphate from the reaction between CaCl2 and KH2PO4 at ambient temperature and pressure. Semi-batch crystallization experiments were carried out in a 2 L glass reactor. CaCl2 solution was added into the solution of KH2PO4 with different feeding rates. The initial pH of KH2PO4 solution was adjusted with NaOH solution. A multi-parameter analyzer (consisting of a pH meter, a conductivity meter and a temperature meter) connected to a PC was used to inline monitor the changes of suspension. An inline Raman probe was immersed into the crystallizer to monitor the whole process. After addition of CaCl2 solution, the suspension was stirred for 1 h to be steady. Then the crystals were filtered and dried in the oven. Mixing rate was also considered as a variable. Figure 1 shows the schematic experimental set-up. Particle size distribution were determined by a Malvern Master sizer. Composition and morphology of the crystals were examined by X-ray diffraction and scanning electron microscope, respectively.

From the semi-batch crystallization experiments, the obtained results clearly indicated that the formation of calcium phosphate was considerable sensitive to pH. The temperature was kept constant for the entire crystallization. DCPD was precipitated as the main product with the studied experimental conditions. The ability of inline Raman spectroscopy monitoring precipitation process was evaluated. Furthermore, the effects of stirring speed and feeding rate were investigated. These real-time monitoring results could provide systematic information of the complex reactive crystallization and give better understanding of fundamental mechanism and kinetics of calcium phosphate precipitation process. Thus, the whole process could be optimized and controlled well for developing future research work with various purposes.

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