Research on the technology and mechanism of direct acidification of phosphogypsum and deeply solidify impurities with lime

Abstract

Phosphogypsum (PG) is a solid waste generated during the wet production of phosphoric acid, noted for causing significant environmental pollution due to low pH and contamination with phosphorus-fluorine impurities. This work proposes an innovative method of acid treatment to acidify PG by directly adding of sulfuric acid at suitable temperatures. Initially, this acid treatment process is employed to convert and remove phosphorus-fluorine impurities, followed by PG after acid treatment incorporation with lime for safe solidification. The conversion mechanisms of phosphorus-fluorine impurities during acidification and solidification are elucidated. Results demonstrate that the direct acidification method converts most fluoride into H2SiF6 and HF, which are subsequently released from the PG system after a temperature of 175 °C treatment. Phosphorus is wholly transformed into water-soluble forms (H3PO4/H2PO4-/HPO42-), especially as eutectic phosphorus is released and converted when CaSO4·2H2O crystals dissolve to II-CaSO4. Following acidification to achieve a high water-soluble phosphorus/fluorine ratio, lime is added to convert phosphorus-fluorine impurities into Ca5(PO4)3F, ensuring harmlessness. The addition of 3.7% lime to acidified PG, an acid amount of 2-times the theoretical requirement, yields a treatment solution with pH=8.65, ρ(P)=0.05mg/L and ρ(F-)=1.11mg/L, meeting Class V water discharge standards (GB 3838-2002). The solidified PG filter residue contains ω(P2O5) ≤ 0.01% and ω(F-) ≤ 0.0061%, suitable for safe storage. This work indicates that this process can separate phosphorus impurities with lower energy consumption, providing a harmless PG treatment. Furthermore, the treated PG can hydrate to gain strength, potentially enhancing its utility as a resource.