Migration Of Harmful Elements In Underground Filling Of Phosphorous Tailings

In recent years,the filling mining method has been widely concerned because of its environmental protection and economic benefits,and has been applied in many mines.When tailings are used as filling materials to prepare a slurry for underground filling,the harmful elements contained in them will be dissolved and pollute the groundwater and surrounding environment of goaf.It is of great significance to make full use of phosphorous tailings resources,protect mine environment and carry out green mining.So,it is necessary to study the migration and transformation rules of the harmful elements.The physical and chemical characteristics of filling material phosphate tailings,fly ash,cement and the characteristics of leaching medium were analyzed.Based on the situations on filling site and the condition of the labaratory,a self-made leaching experimeantal device was used to simulate the process of permeation and migration after slurry formed into a filling body.Leaching test and immersion test were carried out to study the migration patterns of harmful elements in phosphate tailings filling,and the main conclusions are as follows:（1）According to the results of X-ray diffraction（XRD）analysis and X-ray fluorescence spectrum（XRF）analysis on the filling materials,the main minerals of phosphorus tailings are dolomite,fluoropatite and a small amount of calcite and quartz,the main chemical compositions are CaO and MgO.The main minerals of fly ash are quartz,gypsum and hematite,and the main chemical components are SiO₂,Fe₂O₃ and Al₂O₃.The main minerals of cement are calcite and plagioclase,and the main chemical components are CaO,SiO₂ and a small amount of Al₂O₃,Fe₂O₃,etc.The main minerals of leaching medium are dolomite,fluorapatite and quartz,and the main chemical components are CaO,SiO₂,P₂O₅,MgO.（2）When leaching test were carried out on ordinary pressure,the results show that the pH value,total phosphorus,fluoride（F^-）,total hardness（Ca⁺,Mg⁺）and sulfate(SO₄^2-)of the exudate after filling increased significantly in a short time,and migration along with the flow of leachate.（3）When leaching test were carried out on pressured condition,the results show that increasing pressure would accelerate the migration speed of the slurry’s bleeding,so that the contents of pH value,total phosphorus,fluoride（F^-）,total hardness(Ca²⁺,Mg²⁺)and sulfate(SO₄^2-)of the exudate are higher than those in the normal pressure test within a short time after the leaching began.With the leaching,the contents of harmful elements in the exudate of the pressure test group are gradually lower than those of the normal pressure test group.（4）Static immertion test were carried out on the filling body after leaching.The results show that the contents of harmful elements in the leaching solution of filling body after 60 days are significantly lower than that of slurry bleeding,and most harmful elements have migrated with the leaching solution.However,a certain amount of harmful elements are still contained in the filling body,and these harmful elements would dissolve.（5）By measuring the strength of filling body formed in the leaching test on ordinary pressure,it can be seen that after leaching continuously for 60 days,the filling body has a flexural strength of 3.2MPa and a compressive strength of 2.93MPa,meeting the requirements of filling.And,analyze the phase composition of the body show that,there is no significant changes.Fourier infrared spectroscopy（FTIR）analysis showed that the absorption peak of the filling body changed significantly before and after leaching,indicating that the internal structure of the compound changed during leaching.（6）Comparative analysis of mineral composition and chemical composition of before and after leaching,1#（60d,20cm）medium、5#（60d,100cm）medium are the same as the original media,but there is a certain change in the content.Fourier infrared spectroscopy（FTIR）analysis showed that the internal structure of the leaching medium did not change significantly during the leaching process,but contents of elements contained in leaching medium are changing constantly.With the prolongation of leaching time,the influence of leachate on the element content of leaching media increases gradually;with the increase of leaching depth,the influence of leachate on the element content of leaching media with different depths decreases gradually.