Experimental Study On Engineering Characteristics And Leaching Of Pollutants From Landfilled Phosphogypsum

Storage is one of the main disposal methods of phosphogypsum in China,and phosphogypsum tailings pond is the main storage and disposal site of phosphogypsum.With the increasing stock of phosphogypsum,the safety,stability and pollution control of phosphogypsum pond have become the bottleneck restricting the development of phosphorus chemical enterprises.Study of geotechnical engineering characteristics such as consolidation creep,strength deformation law and pollutant leaching and migration law of landfilled phosphogypsum is helpful to improve the design and operation level of phosphogypsum pond and pollution prevention level.On the basis of previous studies,the primary compression and microscopic mechanism of phosphogypsum were studied by compression test,scanning electron microscope test and image processing technology.One-dimensional creep test was carried out to study the rheological properties of phosphogypsum and compare the applicability of different creep models.The large size triaxial test of undisturbed sample and the consolidation stress path triaxial test of remolded sample were carried out respectively to study the influence of depth,effective mean stress of consolidation,consolidation stress ratio and overconsolidation ratio on the strength deformation of phosphogypsum.Finally,based on leaching experiment and geochemical modeling,the leaching behavior and release and migration mechanism of multiple elements were explored.The main conclusions of this study are as follows:（1）The e-logp compression curve of undisturbed phosphogypsum is nonlinear,and the compression index C_c increases nonlinearly with the consolidation stress.The power function compression model can accurately reflect the stress-strain relationship of phosphogypsum.The compression deformation of phosphogypsum is a process of crystal structure breaking and fracture,collapse reduction of large and medium pores,rearrangement of skeleton particles and filling of pores with debris.（2）The relationship between the secondary consolidation coefficient,consolidation stress and initial porosity ratio of phosphogypsum was established.The isochronous stress-strain curve of phosphogypsum shows obvious nonlinear characteristics.The empirical rheological model with power function is suitable for describing the creep process of phosphogypsum under low stress level and stable strain rate,while the Merchant element model is suitable for describing the creep curve of samples with high porosity ratio.（3）Depth,consolidation mean stress,degree of consolidation anisotropy and overconsolidation ratio all affect peak strength,effective stress ratio,pore pressure response and effective stress path in phosphogypsum shear process.The peak strength and dilatancy of phosphogypsum can be improved with higher consolidation stress ratio and overconsolidation ratio.（4）Pressure and age will not change the mineral phase composition and main element content of phosphogypsum.The depth distribution of pH,conductivity and water content in leaching test has temporal and spatial variability and interlinked.The leaching behaviors of different elements are related,and the leaching concentration of elements is higher in the deep part of the pond,the dominant mineral phase diagram and the element morphology analysis model could predict the solubility control phase of each element,and it was suggested that phosphogypsum should be stored in hazardous waste landfill or Class II general industrial solid waste landfill.（5）The leachate with low liquid-solid ratio（L/S）is the most harmful,and the leaching toxicity of phosphogypsum in saturated area is greater than that in unsaturated area.It is suggested to take bottom drainage and horizontal drainage measures to reduce the leachate water level in the pond;In this study,the release amounts of each element in phosphogypsum from high to low were SO₄^2-,P,Zn,F,Mg,Al,Mn,Cu,As,Ni.Based on the cumulative release curve,the release mechanism of each element can be preliminarily determined.