Fluoride Solidification Efficiency Of Modified Phosphogypsum Backfill And Its Impact On Groundwater Environment

Phosphogypsum（PG）is an industrial solid waste inevitably generated from the wet process of phosphoric acid production.The toxic components（e.g.,F and P）in PG limit its secondary recycling and lead to safety and environmental problems such as occupying land,damaging the environment,and polluting groundwater.PG-based cemented paste backfill（PCPB）has been reported as a prevalent and robust technique for safely and efficiently recovering PG.Therefore,PCPB is considered to alleviate a series of problems caused by PG stockpiles and to achieve efficient recovery of underground mineral resources.However,the inherent toxic elements in PG,especially fluoride,are the main factors limiting the application of PCPB.Polyaluminum Chloride（PAC）is a water-soluble inorganic polymer,which is widely used as a coagulant in wastewater fluoridation.Accordingly,PG,PAC and cement were used to prepare PAC modified PCPB.The feasibility and environmental safety were evaluated by slump test,bleeding test,uniaxial compressive strength test and toxic leaching test.In addition,the mechanism of PAC affecting the macro properties of PCPB was further studied by micro detection.It provides a theoretical basis for the further application of PAC Solidification/Stabilization（S/S）of fluorine in PCPB.（1）The addition of PAC can promote the development of early strength（7d）by weakening the retarding effect of PG,and its later strength（28d）still increased with the addition of PAC.As the addition of PAC adversely affects the flow of slurries,slurry flow is one of the criteria to be considered in the application of PAC as an additive to PCPB.（2）The group with a PAC concentration of 2.0 g/L showed a greater reduction in fluoride ion concentration in both the bleeding and the toxic leachate compared to the control group,with decreases of 9.24%and 23.64%respectively.It indicates that PAC can contribute to fluoride reduction in the process of PCPB.The fuzzy comprehensive evaluation of the mixed water formed by the mixing of bleeding and groundwater shows that the water quality of the mixed water meets the Class III standard.（3）The 2.0 g/L PAC of PCPB,improved the UCS by homogenising the distribution of hydration reactant and densifying the pore structure.In addition,the SEM surface and line scan results showed that F was enriched in the area where the pyrite（Fe S₂）was located and that there was a correlation between the Al,Cl and the variation of the F content.The results obtained here imply that the S\S of F by PAC is achieved by electrostatic adsorption,ion exchange and chemical precipitation in the F-rich zone.（4）The results of the mercury injection pore experiment show that at the age of 14days,PAC hydrolysis forms polynuclear hydroxyl complexes,which act as skeletons to absorb flocs to form agglomerates,and agglomerates tend to form porous structures during accumulation.In addition,the slurry flowability decreases after the addition of PAC,which the introduction of air bubbles increasing total porosity.At the age of 28d,the hydration products such as C-S-H were generated and filled the pores to reduce the total porosity due to the facilitation of the hydration reaction of matrix by PAC.At the age of 14 days,the addition of PAC can increase the proportion of fine pores in the matrix by increasing the formation of hydration products.At the same time,the addition of PAC leads to a decrease in fluidity and an increase in the proportion of large pores caused by the bubbles introduced by the slurry preparation.（5）The PAC modified PCPB achieves fluoride ion S\S mainly through electrostatic adsorption and ion exchange of F^-by the PAC itself.In addition,PAC promotes the hydration of the matrix to generate more C-S-H,and the ion exchange between OH^-and F^-in C-S-H generates F-C-S-H to achieve the S/S of F^-.The PCPB can achieve the solidification of F^-by limiting the dissolution of ions.The 28d fillings after the addition of PAC all have a lower total porosity than the control group,thus enhancing the sealing effect on F^-.