Study on reactions of gaseous P2O5 with Ca3(PO4)2 and SiO2 during a rotary kiln process for phosphoric acid production

In a rotary kiln process for phosphoric acid production, the reaction between gaseous phosphorus pentoxide (P2O5) and phosphate ore and silica contained in feed balls (the so-called P2O5 “absorption”) not only reduces phosphorous recovery but also generates a large amount of low melting-point side products. The products may give rise to formation of kiln ring, which interferes with kiln operation performance. In this study, the reactions of gaseous P2O5 with solid calcium phosphate (Ca3(PO4)2), silica (SiO2) and their mixture, respectively, were investigated via combined chemical analysis and various characterizations comprised of X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis and differential scanning calorimeter (TG&DSC), and scanning electron microscopy and energy dispersive spectrometer (SEM&EDS). Attentions were focused on apparent morphology, phase transformation and thermal stability of the products of the P2O5 “absorption” at different temperatures. The results show that the temperature significantly affected the “absorption”. The reaction between pure Ca3(PO4)2 and P2O5 occurred at 500 °C. Calcium metaphosphate (Ca(PO3)2) was the primary product at the temperatures ≤ 900 °C with its melting point ≤ 900 °C while calcium pyrophosphate (Ca2P2O7) was obtained over 1000 °C, which has a melting point ≤ 1200 °C. The “absorption” by pure SiO2 started at 800 °C and the most significant reaction occurred at 1000 °C with formation of silicon pyrophosphate (SiP2O7) product of melting point ≤ 1000 °C. Using mixed Ca3(PO4)2 and SiO2 as raw material, the “absorption” by Ca3(PO4)2 was enhanced due to existence of silica. At 600-700 °C, silica was inert to P2O5 and thus formed a porous structure in the raw material, which accelerated diffusion of gaseous P2O5 inside the mixture. At higher temperatures, the combined “absorption” by calcium phosphate and reaction between silicon dioxide and the “absorption” product calcium pyrophosphate, reinforced the “absorption” by the mixture. Besides, it was found that both Ca(PO3)2 and SiP2O7 were unstable at high temperatures and would decompose to Ca2P2O7 and SiO2, respectively, at over 1000 °C and 1100 °C with the release of gaseous P2O5 at the same time.