Analysis of Crystalline Structure of Sodium Tripolyphosphate: Effect of pH of Solution and Calcination Conditions

Phosphoric acid and caustic soda were reacted in a controlled environment to obtain a solution of orthophosphates. Three solutions were prepared at three different pHs. The solutions were dried and calcinated to produce STP (sodium tripolyphosphate). The calcination process was performed at two temperatures. The evolution of STP formation (phase characterization) with time during calcinations was studied. The type of crystalline phase as well as impurity formation during the calcination process were investigated using WAXD (Wide-angle X-ray diffraction) and FTIR (Fourier transform spectroscopy) techniques. At lower pH of 6.48 STP could be formed purely in either phase I or II by setting the calcination temperature at 560 degrees C for the former and 340 degrees C for the later. Impurities such as Na4P2O7 and NaPO3 in product were developed as the pH of the solution was increased. At high temperature such as 560 degrees C, STP formation was fast, and therefore the phase development was less time dependent. At a lower temperature of 340 degrees C and specially lower pH, the phase development for STP was noticeably time dependent. At pH of 6.57 and 6.77 both phases land II were observed for low calcinations temperature of 340 degrees C; however, at higher calcination temperature of 560 degrees C only phase I was observed. Although WAXD is usually used as a conventional method to study the phase characterization, it was found that FTIR can be considered a reliable technique to study such phase development.