화학공학소재연구정보센터
Chinese Journal of Chemical Engineering, Vol.26, No.4, 795-805, 2018
Study on reactions of gaseous P2O5 with Ca-3(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 (Ca-3(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 Ca-3(PO4)(2) and P2O5 occurred at 500 degrees C. Calcium metaphosphate (Ca(PO3)(2)) was the primary product at the temperatures <= 900 degrees C. with its melting point <= 900 degrees C while calcium pyrophosphate (Ca2P2O7) was obtained over 1000 degrees C, which has a melting point <= 1200 degrees C. The "absorption" by pure SiO2 started at 800 degrees C and the most significant reaction occurred at 1000 degrees C with formation of silicon pyrophosphate (SiP2O7) product of melting point <= 1000 degrees C. Using mixed Ca-3(PO4)(2) and SiO2 as raw material, the "absorption" by Ca-3(PO4)(2) was enhanced due to existence of silica. At 600-700 degrees 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 degrees C and 1100 degrees C with the release of gaseous P2O5 at the same time. (C) 2017 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.