Study On Reaction Processes And Mechanism Of Phosphogypsum Leachate And Karst Medium

A large amount of leachate will be produced during the process of phosphorus gypsum storage.If the anti-seepage measures are not perfect,leachate containing phosphorus and fluorine will contaminate surrounding water and threaten aquatic ecosystem.The carbonate rocks in Guizhou has a wide distribution and large thickness,and the leakage of acid phosphogypsum leachate will have more serious consequences.Fuquan is t is known as the“Asian Phosphorus Capital”.The Baiji gypsum stack has been polluted,and the Facai Cave is the main outlet for polluted groundwater.In this study,by combining the dynamic and static experiment and controlling the hydrodynamic conditions,p H value and reaction time,three kinds of carbonate rock formations that may be relatively easy to develop karst media were selected to explore the interaction process and mechanism.Main idea are as follows:1)The weight loss ratio of the rock samples was pretty high and showed a logarithmic growth trend,and the dissolution rate of the rock samples gradually decreased over time.The weight-loss-ratio trends of the dynamic and static experiments show a high consistency.In the group with a p H value of 1.86,the weight loss ratio of rock samples from high to low was Loushanguan Formation,Falang Formation and Qingyan Formation.But sequence in the group with a p H value of 2.71 was Falang Formation,Qingyan Formation and Loushanguan Formation.Dynamic test's weight loss rate was obviously higher than static test.2)The sequence of effect that test variables impact on the weight loss ratio of rock samples was water quality,dissolution time,hydrodynamic conditions,types of rock formations.The interaction that water quality and hydrodynamic conditions between other variables was strong.In the early stages of the test,water quality,hydrodynamic conditions and their interactions played a major role in the impact of dissolution,but their effects weakened in the later stages,while the effects of rock formation began to burgeon.The Loushanguan formation dolomite shows a great difference in weight loss rate among different test groups,which may be related to its dissolution characteristics.The effect of p H value and washout on dolomite may be greater than limestone.The concentration of part of ions(Ca²⁺,Mg²⁺and F^-)contributed more to weight-loss-rate change than the p H value,which can also indirectly affect the weight loss rate by affecting the concentration of water-soluble ions.The inhibition of weight loss rate can be achieved by reducing the concentration of Ca²⁺,Mg²⁺and F^-and increasing the p H value of the solution.3)The Qingyan Formation samples dissolve the calcite cement preferentially,which is characterized by the dissolution along the periphery of the bioclastic debris and gravel.The bioclastic debris forms the dissolution pore,calcite forms a large number of crystal cones,and a small amount of calcium sulfate crystal clusters are generated in the interstitial of the crystal plane bedding.The Falang Formation samples have the most uniform dissolution of the stone sample,smooth surface and relatively uniform silicon distribution.The stone samples of Loushanguan Formation reflect the difference of dissolution rate between calcite and dolomite.The intercrystalline pores are more developed.The Qingyan Formation is more likely to develop channels than the Falang Formation.It should attract enough attention.The p H value of the effluent from the Facai Cave varies with seasons change.The Laoshanguan Formation has the possibility of developing new dissolution channels.4)Combining the results of energy spectrum analysis,the microscopic corrosion characteristics of samples in electron microscopy are explained.The compositions of the three layers samples are similar,principal components are Ca F₂,Ca?H₂PO₄?₂,Ca HPO₄ and Ca₃?PO₄?₂.Ca F₂ is mainly distributed in the outer and middle layers of the product.Calcium phosphate accounts for the major proportion of the inner layer and calcite crystals.In the stone sample,there is a competition between calcium phosphate and calcium fluoride in precipitation.Calcium phosphate will be preferentially formed by calcium fluoride,and calcium phosphate also has the possibility of conversion to calcium fluoride.Calcium sulfate and magnesium fluoride are converted into calcium fluoride and calcium phosphate which are more difficult to dissolve.The metasomatism and dissolution of carbonate minerals occur simultaneously,and calcite,dolomite and calcium sulfate clusters may first be replaced by calcium phosphate and then continue to be replaced by calcium fluoride.The white deposits on the surface may be iron phosphate and ferrous phosphate.