Kiln phosphoric acid (KPA) technology could produce P

2

O

5

with high purity and has been applied in thermal phosphoric acid industry; however the formation of fouling in the high-temperature rotary kiln restricts the stable and long-term operation. In this paper

the reaction of phosphate ores with gaseous P

2

O

5

was investigated in a high-temperature reactor

and the CaO-SiO

2

-P

2

O

5

ternary phase diagram was analyzed to understand the fouling formation mechanism. The results showed that the low-melting-point products

such as Ca(PO

3

)

2

and Ca

2

P

2

O

7

are responsible for the fouling in the KPA process. In addition

a small amount of impurities

e.g

.

aluminum and iron

could facilitate the generation of the low-melting-point products and cause serious fouling. Based on the high-temperature SiO

2

-P

2

O

5

and CaO-SiO

2

-P

2

O

5

phase diagram analysis

the control of Si/Ca molar ratio (

e.g

.

Si/Ca = 2.0) was found to avoid fouling formation in the

kiln. These results could provide the operation parameters of reaction temperature and feeds composition to suppress the fouling in the kiln reactor for the phosphoric acid production in industry.