In a rotary kiln process for phosphoric acid production

the reaction between gaseous phosphorus pentoxide (P

2

O

5

) and phosphate ore and silica contained in feed balls (the so-called P

2

O

5

"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 P

2

O

5

with solid calcium phosphate (Ca

3

(PO

4

)

2

)

silica (SiO

2

) 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 (TGDSC)

and scanning electron microscopy and energy dispersive spectrometer (SEMEDS). Attentions were focused on apparent morphology

phase transformation and thermal stability of the products of the P

2

O

5

"absorption" at different temperatures. The results show that the temperature significantly affected the "absorption". The reaction between pure Ca

3

(PO

4

)

2

and P

2

O

5

occurred at 500℃. Calcium metaphosphate (Ca(PO

3

)

2

) was the primary product at the temperatures ≤ 900℃ with its melting point ≤ 900℃ while calcium pyrophosphate (Ca

2

P

2

O

7

) was obtained over 1000℃

which has a melting point ≤ 1200℃. The "absorption" by pure SiO

2

started at 800℃ and the most significant reaction occurred at 1000℃ with formation of silicon pyrophosphate (SiP

2

O

7

) product of melting point ≤ 1000℃. Using mixed Ca

3

(PO

4

)

2

and SiO

2

as raw material

the "absorption" by Ca

3

(PO

4

)

2

was enhanced due to existence of silica. At 600-700℃

silica was inert to P

2

O

5

and thus formed a porous structure in the raw material

which accelerated diffusion of gaseous P

2

O

5

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(PO

3

)

2

and SiP

2

O

7

were unstable at high temperatures and would decompose to Ca

2

P

2

O

7

and SiO

2

respectively

at over 1000℃ and 1100℃ with the release of gaseous P

2

O

5

at the same time.