A Low Cost of Burning Raw Material For PO43−-P And F− Solidication/Stabilization In Phosphogypsum

Phosphogypsum (PG) contains a lot of soluble phosphate (PO 4 3− -P) and uorine ion (F − ), which seriously has hindered the sustainable development of phosphorous chemical industry. In this study, a new burning raw material (BRM) was used for the stabilize of PO 43− -P and F − in PG. The characteristics of PG and BRM, stabilize mechanism of PO 43− -P and F − , leaching test and economic evaluation were investigated. The effect of PG and BRM weight ratio, solid to liquid ratio, reaction time and reaction temperature on the concentrations of PO 43− -P and F − were studied. The results showed that the concentration of F − in PG leaching solution was 8.65 mg/L and the removal eciency of PO 43− -P was 99.78 %, as well as the pH of PG leaching solution was 8.12, when the weight ratio of PG and BRM was 100:2, and the solid to liquid ratio was 4:1, reacting for 24 h at the temperature of 30 ℃ . PO 43− -P and F − were mostly solidied as Ca 5 (PO 4 ) 3 F, CaPO 3 (OH), Ca 5 (PO 4 ) 3 (OH), Ca 2 P 2 O 7 ·2H 2 O, CaSO 4 PO 3 (OH)·4H 2 O, CaF 2 , and CaFPO 3 ·2H 2 O. Leaching test results indicated that the concentrations of PO 4 3− -P, F − and heavy metals were less than the integrated wastewater discharge standard (GB8978-1996). Economic evaluation revealed that the cost of PG treatment was $ 0.88/ton. This study provides a new low cost and harmless treatment method for PG. FT-IR spectrometer (PerkinElmer Frontier) via the KBr pellet method. Phase composition, microstructure and composition of PG, BRM and stabilized samples were analyzed by X-ray diffractometer (XRD; JapanD/max-IIIA), Scanning Electron Microscopy (Slgma 300), X-ray spectroscopy system (EDS; ΣIGMA + X-Max20, Zeiss, Germany), and X-ray uorescence (XRF; PANalytical B.V., Axios; Netherlands), respectively. other study

environmental protection in the production of gypsum board or cement retarder were application. But the price of quicklime was higher than a new alkaline reagents of burning raw material (BRM), and the concentration of PO 4 3− -P still is higher than the integrated wastewater discharge standard (GB8978-1996) (Shu et al.,2020).
PG will produce a lot of PO 4 3− -P and F − leachate during the storage process. Water contaminated with PO 4 3− -P and F − is harmful to human health and environment. Too much PO 4 3− -P can lead to contact dermatitis, harm to marine life, soil pollution and deterioration of water quality (Shaddel et al.,2020). F − will disrupts the normal metabolism of calcium and phosphorus, and damages teeth, heart muscle and skeletal muscle (Shu et al.,2020). Therefore, it is urgent to nd a low cost alkaline material for PO 4 3− -P and F − solidi cation/stabilization in PG.
BRM is a intermediate product in the cement production process (Huang et al.,2020 Puri cation System (HMCWS10) provided the deionized water in this study. The PG radioactivity in this experiment were tested (Each sample was tested in three parallel groups). As shown in Table 1 of the supporting information, both the internal and external exposure indexes (IRa, Ir) of PG were less than the standard of building materials radionuclide limit (GB 6566 − 2010. IRa ≤ 1.0, Ir ≤ 1.3). Therefore, the activity concentration of the radionuclides in PG was not measured in leaching process. can be dissolved in water as alkaline agents to adjust the pH. In Fig. 1c, SEM images of BRM also showed that relatively uniform particles with a particle size range from 300 nm to 500 nm. A high magni cation SEM image of PG reveals that ake morphology with a particle size range from 50 µm to 80 µm, and the surface content of precipitate particles mainly contains of O, Ca, S, C, F, Mg, P, and Al according to EDX analysis (In Fig. 1d).   Fig. 2a, the pH of leaching solution increased with the weight ratios of PG and BRM increased at the given reaction time, due to the concentration of OH − increased with the dosage of BRM increased; And the pH not much has changed with the reaction time was higher than 5 h, at the given PG and BRM weight ratio. The pH was less than 9.00, which meet the integrated wastewater discharge standard (GB8978-1996), when the weight ratios of PG and BRM was 100:2 and reaction time was higher than 5 h. Figure 2b showed that the removal e ciency of PO 4 3− -P increased as the weight ratios of PG and BRM increased from 100:0.7 to 100:3 at a given reaction time. The concentration of PO 4 3− -P was higher than the integrated wastewater discharge standard (GB8978-1996) when the reaction time was less than 24 h at any PG and BRM weight ratios. In Fig. 2c, the concentration of F − decreased with the weight ratios of PG and BRM increased at a given reaction time, due to F − was began to react with Ca 2+ and Mg 2+ to form uoride precipitation with the pH of In Fig. 3c, the concentration of F − decreased with the increased of reaction temperature at a given reaction time, because F − began to react with Ca 2+ and Mg 2+ to form uoride precipitation with the temperature increased; At a given reaction temperature, the concentration of F − decreased as the increased of reaction times. As shown in Fig. 3d, the removal e ciency of PO 4 3− -P increased when the reaction temperature increased from 20 ℃ to 40 ℃, because PO 4 3− -P began to react with Ca 2+ and Mg 2+ to form phosphate precipitation (Qiu et al.,2020). According to the concentration of F − , and the removal e ciency of PO 4 3− -P, as well as economic costs in the S/S system. The solid to liquid ratio was 4:1 and reaction temperature at 30 ℃ were selected as the optimal reaction condition. The surface of the large sheet structure is covered with a small amount of non-uniform impurity particles were found when the weight ratio of PG and BRM was 100:0.7 (Fig. 5a). particles mainly was newly formed phosphate and uoride precipitate. The surface of the sheet structure is covered with more and more non-uniform particles with the weight ratio of PG and BRM increased from 100:1 to 100:1.5 (Fig. 5b-Fig. 5c). The results showed that more Ca 2+ -1996).

Stabilize mechanism of PO
In this study, PO 4 3− -P and F − S/S process was analyzed by economic viewpoint. The costs of the chemicals reagents were only considered. The price of BRM was $ 44.0/ton according to cement industry in China, and the dosage of BRM was 20 kg when treatment per ton PG. Therefore, the total cost of harmless treatment PG by BRM was $ 0.88/ton. In addition, the price of quicklime was $ 92.86 /ton in China, and the dosage of quicklime was 20 kg when treatment per ton PG, and the total cost of harmless treatment PG by quicklime was $ 1.86/ton. Moreover, the concentration of PO 4 3− -P in PG can only be reduced to 10 mg/L when quicklime as the stabilizing agent. The concentration of PO 4 3− -P in PG can be less than 0.5 mg/L when BRM as the stabilizing agent. The results indicated that the removal e ciency of PO 4 3− -P and the cost of PG treatment was better than that of quicklime.

Conclusion
In this study, BRM was used for PO 4