| Phosphorus is a kind of non-renewable, indispensable elements which accounts greatly in human life. With increasingly global consumption of phosphate rock, the grade of phosphate rock constantly reduces. Nowadays, 90% of the phosphate rock in China is mid and low-grade phosphate rock. Under the new situation of phosphate rock mineral resource, research on reasonable utilization of low grade phosphate rock has become a challenge for all phosphorus chemical practitioners. Thermal phosphoric acid process has better prospects on dealing with mid and low-grade phosphate rock, and therotical instructions can be made by research on the melting characteristics of phosphate rock. In this paper the effects of SiO2, MgO, and Al2O3 on the melting characteristics of phosphate rock were studied. In further, the reasons and trends of melting temperature change were analyzed after X-ray diffraction by using phase diagram theory. Main experimental contents and conclusions were as follows:（1） The effects of SiO2, MgO, and Al2O3 on the melting characteristics of experimental materials were studied firstly using tricalcium phosphate without concerning the effects of other impurities in phosphate rock. The results indicated that all three oxides alone couldn’t affect the melting temperature effectively, and effects of the three oxides were as follows: SiO2>Al2O3>MgO. The addition of the mixture of both SiO2-MgO and SiO2-Al2O3 could reduce the melting temperature effectively. One low melting temperature area existed in SiO2-MgO system, and SiO2/CaO was 0.751.75, with MgO/CaO to be 0.751.5. And two low melting temperature areas existed as “S”-shaped distribution in SiO2- Al2O3 system, the range of ingredients of one area were as follows: SiO2/CaO was 0.31.25 and Al2O3/CaO was 0.251.0; the range of ingredients of the other area were as follows: SiO2/CaO was 1.41.75 and Al2O3/CaO was 0.31.25.（2） Study on the effects of SiO2, MgO, and Al2O3 on the melting characteristics of phosphate rock indicated that: SiO2 alone could help reduce the melting temperature in certain degree, MgO and Al2O3 alone couldn’t reduce the melting temperature effectively, and effects of the three oxides were as follows: SiO2>MgO>Al2O3. The low melting temperature area of SiO2-MgO system was the “V”-shaped area which symmetrically distributed along the SiO2/MgO=1.0 line. Two low melting temperature areas existed in SiO2-Al2O3 system, the range of ingredients of the two areas was as follows: SiO2/CaO was 1.21.6 and Al2O3/CaO were 0.51.0 or 1.21.4 independently.（3） The effects of SiO2-MgO system and SiO2-Al2O3 system on the melting temperature were compared based on the results. It turned out that for both tricalcium phosphate and phosphate rock, the effect of SiO2-MgO system was much better than that of SiO2-Al2O3 system. The addition of MgO and Al2O3 did help reduce the melting temperature of the materials, however, they reduced the reduction rate of phosphate rock at the same time, so the range of ingredients for the industry process should be based on actual situations. The low melting temperature area of phosphate rock was bigger than that of Ca3（PO4）2 materials with the addition of SiO2-MgO. However low melting temperature area of phosphate rock was smaller than that of Ca3（PO4）2 materials with the addition of SiO2-Al2O3.（4） Based on X-ray diffraction and phase diagram theory, the mechanisms of the melting process with the existence of MgO and Al2O3 were concluded as follows: with the existence of MgO, forsterite, monticellite and other minerals were generated in the mineral system, and further eutectic minerals could be generated through the reactions between these minerals. The formation of these eutectic minerals helped to reduce the melting temperature of phosphate rock. With the existence of Al2O3, anorthite, gehlenite and other minerals were generated in the mineral system, and further eutectic minerals could be generated through the reactions between these minerals. The formation of these eutectic minerals could also help to reduce the melting temperature of phosphate rock. |
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