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Study on the occurrence state of main components of phosphogypsum dihydrate and its impurity distribution.
Abstract
The dihydrate phosphoric acid process is the mainstream technique. However, the phosphogypsum (PG) produced contains high levels of impurities such as phosphorus and fluorine, severely constraining its valorization. In order to elucidate the occurrence patterns of phosphorus and fluorine impurities in PG, this study employed analytical methods including XRF, XRD, AMICS (Automated Mineralogy Integrated with Chemistry System), XPS, and chemical element balance analysis. We investigated the occurrence states of phosphorus, fluorine, silicon, iron, and aluminum elements in PG from wet-process phosphoric acid production, as well as the distribution characteristics of phosphorus and fluorine impurities. Additionally, we utilized Density Functional Theory (DFT) calculations to determine the binding energies of major minerals with water-soluble phosphate and fluoride groups, and analyzed the zeta potentials of gypsum and quartz mineral surfaces. The results indicate that the main mineral phases in PG are gypsum, quartz, potassium silicate minerals, aluminosilicate minerals, and hematite, predominantly occurring in monomineralic forms. Phosphorus impurities primarily exist in calcium silicate and hematite minerals, while fluorine is mainly associated with gypsum and potassium silicate minerals. DFT calculations demonstrate strong binding energies between calcium silicate and hematite minerals with PO43-, as well as between gypsum and quartz minerals with F–. The acidic conditions in the separation tank during wet-process phosphoric acid production may contribute to the distinctive distribution characteristics of phosphorus and fluorine impurities in PG.
Excerpt:
4. Conclusions
(1) The process mineralogical analysis reveals that gypsum has a significant intergrowth relationship with potassium silicate, calcium aluminate, biotite, and wollastonite, while quartz minerals exhibit a significant intergrowth relationship with rutile, biotite, pyrite, and chlorite. The distribution of phosphorus impurities in calcium aluminate and rutile is remarkably higher than the average. The distribution ratio of fluorine impurities in gypsum is obtained up to 73.94%. The distribution of iron impurities in the impurity mineral phase was relatively balanced, and the distribution of aluminum impurities in quartz and calcium aluminate was significantly higher than in other components. The results of XPS valence analysis indicate that phosphorus is mainly present in the form of PO43- in PG, while fluorine mainly exists in the form of metal fluoride salt in PG, followed by fluoride, Fe element mainly exists in insoluble form, and K element mainly occurs in potassium silicate mineral.
(2) The results of the balance analysis of chemical elements show that phosphoric acid is enriched in the mineral phase of calcium aluminate and rutile, as well as fluorine within gypsum and potassium silicate minerals, which demonstrates significant enrichment and distribution characteristics. The results of DFT calculations reveal that the binding energy between phosphate and calcium aluminate and rutile is significantly higher than that and other minerals. In addition, the binding energies among the fluoride, quartz, and potassium silicate minerals are significantly higher than in other cases. In the production process of WPPA, PO43- most likely had an electrophilic adsorption reaction with quartz under the operating conditions of pH = 2 in the filter tank, which may be the main reason for the distribution characteristics of phosphorus and fluorine impurities in PG.
(3) In the harmless treatment of PG via the curing method, the use of a curing agent to neutralize the acidity of PG is able to achieve efficient curing of water-soluble phosphorus and fluorine in PG. For the dosage of curing agent of 3.0% in the presence of the curing temperature of 30 °C for 150 min, the curing rate of water-soluble phosphorus in PG is obtained as 98.5%, whereas the curing rate of water-soluble fluorine impurities reaches more than 99.0%. After the curing process, PG exhibits good environmental friendliness. The main product of water-soluble fluoride in PG is CaF2 precipitation when using a curing agent for harmless disposal of PG. Therefore, the research in this paper can provide theoretical guidance for the efficient recovery of fluoride resources from PG.