Immobilization And Removal Of Radionuclides From Uranium Tailings And Their Leachate

When uranium tailings was solidified by lime,problems such as acid reflux or excessive radionuclide concentration would occur in leachate of tailings after a certain period of time,leading to serious radionuclide pollution in soil and groundwater.In this study,three kinds of typical uranium tailings（tailing slag,sediment and associated mineral）were screened for good immobilization effect of radionuclide（uranium and thorium）in uranium tailings.Immobilized materials and schemes were optimized for the associated radioactive tailings with high concentration of radionuclide leaching.The effect of geopolymer curing to seal the associated radioactive slag was studied.Porous geopolymer was prepared and evaluated for its adsorption capacity of uranium in leachate.In turn,the safety guarantee technology of"sealing immobilization&adsorption of uranium in leachate"for uranium tailings was established.The main research results are as follows:（1）Suitable immobilized materials were selected for tailing slag,sediment and the associated mineral.The dosage of materials and applicable p H range were determined according to the discharge requirements of enterprises with uranium and thorium concentrations less than 0.3 mg/L in leachate.Zero-valent iron,ferrous sulfide and attapulgite were selected to immobile tailing slag;zero-valent iron,fly ash and biochar were selected to immobile the sediment;zero-valent iron,ferrous sulfide and apatite were selected to immobile the associated mineral.Among them zero-valent,attapulgite and biochar can be applied to p H ranges from acidic to neutral.Dosages of selected materials were not more than 10%.The leaching concentration of uranium and thorium in the associated mineral was higher than that of tailing slag and sediment,which had a higher leaching risk.The morphological changes of uranium and thorium in tailings after stabilization indicated that the soluble forms of uranium and thorium can be transformed into more stable ones by adding immobilized materials.（2）Optimizing and determining the immobilized adding scheme for the associated mineral:a combination of a small amount of lime+alkaline material/modified apatite to ensure that the leaching concentration of uranium and thorium always meets the discharge requirements in the range of acid to neutral.The alkaline materials that can replace lime are K₂CO₃and Na₂Si O₃.Two kinds of modified apatite materials,Fe-HAP and HAP-NH₂were prepared.The results showed that the immobilization efficiency was improved at p H 3,5 and 7.After the associated mineral was immobiled by modified apatite,XRD and FTIR analysis showed that the process of immobiling uranium and thorium was a reaction mechanism of adsorption,reduction,dissolution and precipitation and ion exchange.（3）The double guarantee technology of"sealing immobilization&adsorption of uranium in leachate"was proposed for the associated mineral that has not yet been landfill.The geopolymer solidified body was prepared by doping with fly ash and metakaolin as raw materials.Doing research on the compressive strength and radionuclide leaching properties of solidified body with different alkali activator and the associated mineral doping ratios.The alkali activator additive amount did not exceed 0.2 or the associated mineral doping ratio was lower than 40%when the solidified body compressive strength of up to 37.9 MPa,satisfying the requirement of safe landfill.Concentration of the leaching process of uranium and thorium also satisfied the requirement of discharge.Concentration of uranium and thorium in leachate of the solidified body in the range of acid to neutral leaching met the emission requirements.XRD and FTIR analysis of the associated mineral cured by geopolymer suggest that uranium and thorium are immobilized by entering the three-dimensional network structure of geopolymer.The porous geopolymer prepared with the same raw material has a remarkable adsorption effect on uranium in leachate,and the removal rate is up to 95%when the initial concentration was 20 mg/L.The double guarantee technology of"sealing immobilization&adsorption of uranium in leachate"developed by geopolymer can be used as an economical and effective way to immobile uranium tailings.