ABSTRACT
Calcium phosphate scale formation on heated surfaces, including pipes and heat exchangers, has been reported in the dairy industry as a result of the processing of milk [1-4]. During milk processing, the pasteurization takes place at elevated temperatures (ca. 70°C), favoring the formation of salts with inverse solubility, such as calcium phosphate salts. In milk, about 25,000 casein molecules bind to calcium and phosphate and aggregate to form the heavily hydrated casein micelle of molecular weight 10,809 Da. Thus calcium phosphate is rendered soluble in milk (as the casein micelle itself can exist in milk as a stable suspension). The higher temperature provides the driving force for the precipitation of calcium phosphate in the uid phase [5,6]. Detailed thermodynamic analysis has shown that milk ultraltrate is thermodynamically unstable with respect to a number of calcium phosphate phases [7]. Simulation studies with milk ultraltrate have revealed the tendency for the formation of transient phases of calcium phosphate [8,9]. Belmar-Beiny and Fryer [10] and Schreier and Fryer [11] proposed that fouling was dependent on the bulk and
7.1 Introduction .................................................................................................. 117 7.2 Experimental ................................................................................................ 118
7.2.1 Preparation of 1,3,5-Triazinetriphosphonic Acid ............................. 118 7.2.2 Crystal Growth Experiments ............................................................ 119
7.3 Results and Discussion ................................................................................. 121 7.4 Summary ...................................................................................................... 129 Acknowledgments .................................................................................................. 130 References .............................................................................................................. 130
surface reactions and not on the mass transfer. Model experiments in which kinetics measurements were done at constant supersaturation in simulated milk ultraltrate have provided additional evidence that calcium phosphate crystal growth was controlled by a surface diffusion mechanism [9]. It should be noted that calcium phosphate deposits are more generally formed when the calcium in the feed water reacts with the phosphate residues coming from chemicals used for water treatment either directly (anticorrosion agent) or from the decomposition of antiscaling agents such as organophosphorus compounds. Precipitation from solutions supersaturated with respect to calcium phosphate may involve a number of mineral phases in the order of decreasing solubility: dicalcium phosphate dihydrate (CaHPO4·2H2O), octacalcium phosphate (Ca8H2(PO4)6·5H2O), tricalcium phosphate (Ca3(PO4)2, TCP), and hydroxyapatite (Ca5(PO4)3OH, HAP). At high supersaturations, it is possible that the formation of the thermodynamically most stable HAP is preceded by the formation of one or more transient crystalline phases [12].
