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

doi:10.1016/j.cjche.2017.11.016

Abstract

Abstract: 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℃. Calcium metaphosphate (Ca(PO₃)₂) was the primary product at the temperatures ≤ 900℃ with its melting point ≤ 900℃ while calcium pyrophosphate (Ca₂P₂O₇) was obtained over 1000℃, which has a melting point ≤ 1200℃. The "absorption" by pure SiO₂ started at 800℃ and the most significant reaction occurred at 1000℃ with formation of silicon pyrophosphate (SiP₂O₇) product of melting point ≤ 1000℃. 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℃, 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℃ and 1100℃ with the release of gaseous P₂O₅ at the same time.

Key words: Kiln phosphoric acid, Kiln ring, Absorption, Calcium metaphosphate, Calcium pyrophosphate, Back reaction

摘要： 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℃. Calcium metaphosphate (Ca(PO₃)₂) was the primary product at the temperatures ≤ 900℃ with its melting point ≤ 900℃ while calcium pyrophosphate (Ca₂P₂O₇) was obtained over 1000℃, which has a melting point ≤ 1200℃. The "absorption" by pure SiO₂ started at 800℃ and the most significant reaction occurred at 1000℃ with formation of silicon pyrophosphate (SiP₂O₇) product of melting point ≤ 1000℃. 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℃, 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℃ and 1100℃ with the release of gaseous P₂O₅ at the same time.

关键词: Kiln phosphoric acid, Kiln ring, Absorption, Calcium metaphosphate, Calcium pyrophosphate, Back reaction

Qiang Liu, Weizao Liu, Li Lü, Ruhu Li, Bin Liang, Hairong Yue, Shengwei Tang, Chun Li. Study on reactions of gaseous P₂O₅ with Ca₃(PO₄)₂ and SiO₂ during a rotary kiln process for phosphoric acid production[J]. Chin.J.Chem.Eng., 2018, 26(4): 795-805.

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

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

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