Infrared study on the by-product gypsum from the phosphoric acid process and on the solid solutions of the system CaSO
4⋅2H
2O-CaHPO
4⋅2H
2O was made to confirm the previous assumption that water-insoluble P
2O
5 in the by-product gypsum is due to the formation of the solid solution of gypsum and secondary calcium phosphate dihydrate.
Infrared absorption spectrum of the by-product gypsum showed, besides the absorption spectrum of gypsum, weak absorptions at 1015cm
-1 and 837cm
-1 in the related characteristic absorption bands of HPO
42-.
The corresponding absorptions of secondary calcium phosphate dihydrate were at 984cm
-1 and 873cm
-1. From the study on the absorption spectra of the system CaSO
4⋅2H
2O-CaHPO
4⋅2H
2O, these displacements can be ascribed to the formation of the solid solution of gypsum with secondary calcium phosphate dihydrate equivalent to the amount of water-insoluble P
2O
5 in the by-product gypsum.
Some characteristics of the by-product gypsum can well be explained by the formation of the solid solution.
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