Simultaneous immobilizations of soluble phosphorus and fluorine in phosphogypsum by milling with calcareous and aluminiferous samples: Reaction products and immobilization performances

Phosphogypsum (PG) is a by-product of phosphorus chemical industry, which contains a large amount of soluble phosphorus and fluorine impurities to pose a great ecological risk to the environment and human health. In this study, we proposed one-step mechanical ball milling as a cleaner operation for simultaneous immobilization of phosphorus and fluorine in PG waste with different calcareous and aluminiferous immobilization reagents. The results show that, the remaining soluble PO43− and F− concentrations could reduce to 0.056 and 0.99 mg/L in PG-Ca(OH)2–Al(OH)3 system, respectively, when the total mass ratio at 5% of Ca(OH)2 and Al(OH)3 was added to PG. Furthermore, mineralogical and chemical evidence indicated that, the main immobilization mechanisms of soluble phosphorus and fluorine in PG are as follows: a) the transformation of co-crystal phosphorus (Ca((SO4)x,(HPO)4)1-x), CSHP) into insoluble phosphorus during milling process, b) the exchange of F− by OH− in Katoite formed by Ca(OH)2 and Al(OH)3, c) the formation of calcium aluminum fluorosilicate and hydroxyapatite due to the increased activity of silicate. Additionally, when using kaolinite and calcium carbonate slag as calcareous and aluminiferous alternative reagents, the concentrations of PO43− and F− were 0.056 and 7.42 mg/L in leaching solution, within the permitted level for the GB8978-1996 (0.5 mg/L for PO43−, 10.0 mg/L for F−). The application of calcium base, aluminum base solid wastes could provide a potential solution for large-capacity disposal of industrial waste and sustainable utilization of PG.