Procedures for the Chemical Analysis of Impurities of Phosphogypsum

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Phosphogypsum byproduced from the wet process of phosphoric acid manufacture is one of the most valuable sources of gypsum as setting modifier of portland cement and raw material of gypsum board and plaster in Japan. In general, phosphogypsum contains some amounts of impurities, such as phosphoric acid, calcium phosphates, sodium silicofluoride and organic materials. These impurities have some unfavorable effects on setting of portland cement and on hardening of gypsum board and plaster. For practical utilization of phosphogypsum, it is necessary to know the exact amounts and the detailed character of these impurities. However, no procedures for quntitative analysis of the impurity of phosphogypsum have been established with the exception of the slightly unsure procedures for analysis of the water soluble P2O5 and the water soluble F of phosphogypsum. Moreover the impurities of phosphogypsum, for example, the water soluble phosphates such as phosphoric acid, monocalcium phosphate and dicalcium phosphate and sodium silicofluoride etc. are observed in the state of adhesion to the surface of gypsum crystals, holdings between the polycrystalline aggregates of phosphogypsum. Practically, some of SO42- ion are replaced by HPO42- ion in the crystal lattice of phosphogypsum. These impurities are expressed as follows (a) water soluble P2O5 on the surface of gypsum crystal (b) water insoluble F on the surface of gypsum crystal (c) P2O5 in the crystal lattice of gypsum (d) total P2O5 (e) total F The detailed operations of analytical method on the above impurities are given by the following descriptions. (a) All the sample phosphogypsum must be crushed under 325 mesh, because it is impossible to leach the impurities held between the polycrystalline aggregates of phosphogypsum by the simple extraction as leaching by the boiling distilled water. (b) Soluble P2O5 adhered to the surface of phosphogypsum are leached by the boiling distilled water saturated with pure gypsum. If the distilled water without gypsum is used, it will be extracted not only soluble P2O5 on the surface of phosphogypsum, but also substituted P2O5 In the crystal lattice of phosphogypsum, because the gypsum has a relatively large solubility in pure water. (c) In general, impurities adhered on the surface of phosphogypsum are easily removed by the boiling of the dilute mixed acid solution (HCl : HNO 3 : 1). Therefore, the substituted P2O5 contents in the crystal lattice of gypsum are obtained by the determination of total P2O5 in phosphogypsum with chemically cleaned surface as above mentioned. (d) In order to analysis the insoluble F contents in pulverized phosphogypsum previously, the water soluble F in phosphogypsum must be removed by washing with a relatively large amounts of distilled water. Consequently, total F contents of the cleaned phoshogypsum, which corresponds to the insoluble F contents in the orignal sample phosphogypsum, are obtained by the steam distillation in the presence of sulfuric acid and silica powder and by the colorimetry with Th-alizarin. (e) Total P2O5 and total F contents of phosphogypsum are obtained by the conventional procedures. The results show that the present procedures are suitable for the determination of impurities of phosphogypsum, and that remarkable differences are found in data given by two representative extraction processes.
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Procedures for the Chemical Analysis of Impurities of Phosphogypsum

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