Leach Simulation Experiment On Uranium And Thorium In A Southern Uranium Tailing During The Precipitattion And Leaching Process

After being exposed to the open air for a long time,the uranium tailings will release radioactive elements and some heavy metal elements due to weathering,rainout washout and surface water scouring,which cause great harm to the environment.With a uranium tailing in a Southern uranium mine as the research object,in combination with field sampling,static leaching experiment and dynamic column simulation experiment,the release and migration characteristics of radioactive uranium and thorium elements in the leaching process are studied.The main research results are as follows:(1)The contents of uranium and thorium elements in the dump leaching are 147?g/g and 8.1?g/g,respectively.Among them,the content of uranium element is about 47 times higher than the average uranium content in China's ordinary soil;the contents of uranium and thorium elements in the uranium tailing are 41?g/g and 44.4?g/g,respectively.The content of uranium element is about 13 times higher than the average uranium content in China's ordinary soil,while the content of thorium element is about 3 times higher than the average thorium content in China's ordinary soil.The contents of nonradioactive elements like Pb,Ni and Cd are also several or dozens of times higher than their average contents in China's ordinary soil.(2)The uranium and thorium elements are mostly concentrated in the soil profiles of the mouths of gallery and pit near the pollution source,while the contents of uranium and thorium elements in the downward soil profiles away from the mouths of gallery and pit are relatively low.The nuclides are more affected by the pollution sources in terms of transversal migration.As for vertical migration,the retarding effect of soil medium exerts a great influence on the speed and quantity of nuclide migration.(3)Static leaching experiment shows that,the smaller the granularity and pH value of tailings are,the more the elements will be released,and the greater the amounts of the released uranium and thorium are.The influence of granularity of tailings has a greater influence on the release of uranium and thorium elements than that of pH value.Besides,under the same environmental conditions,it is more difficult for thorium to be leached than uranium,suggesting that thorium has unstable properties and will not easily migrate.In the leaching solution with a pH value of 4.0 and a granularity less than 0.45 mm,uranium and thorium elements have the greatest release amounts,which are 8.42mg/L and 0.151mg/L,respectively.(4)According to the dynamic leaching experiment,the nuclide release consists of three stages: rapid leaching stage,stable leaching stage and saturated leaching stage.The lower the pH value of leaching solution is,more easily the uranium element will be released;the influence of pH value of leaching solution on the release of thorium element is not significant.The pH values of the leaching-out solutions of different groups in different periods are finally stabilized within 7.5-8.0,which are higher than pH value of leaching solution.The uranium and thorium elements of Column 1 reach the highest release amounts,which are 6.86mg/L and 0.16mg/L,respectively.The leaching amount has a great influence in the rapid leaching stage of elements in the early period of uranium tailings,and it has a weak influence on the elements' leaching in the middle and later periods of uranium tailings.During the entire process,the cumulative leaching rates of uranium element for 4 columns of the uranium tailing in the five periods are 40.52% for Column 1,31.31% for Column 2,29.72% for Column 3 and 19.14% for Column 4.The cumulative leaching rates of thorium element are 0.14% for Column 1,0.13% for Column 2,0.13% for Column 3 and 1.08% for Column 4.The results show that acidic conditions promote the release of elements,and that the release of relevant elements from the mineral also heavily depends on their own components,forms,structures and other attributes.(5)Fick theoretical fitting results show that during the leaching process,uranium and thorium elements release and migrate into the water environment mainly through surface elution,proving that the pollution caused by tailings mainly come from water soluble ions and their complex compounds attached to the surface of tailings.Elovich equations are used to conduct fitting on the cumulative release amount and logarithm of time of uranium and thorium elements.The fitting results are significantly correlated,suggesting that the release and migration process of uranium and thorium elements is a heterogeneous phase diffusion process.