Research On Solidification And Blocking Mechanism Of The Uranium Mill Tailings By Zeolite Minerals

Uranium tailings contain a large number of long life and high toxicity radionuclides, the stacking of uranium tailings has brought serious harm to human health and ecological environment. Zeolite minerals have good adsorption and thermal stability, and are excellent solidified substrate for the treatment of uranium tailings. In this paper, the Na₂ Si O3?9H2O, Na Al O2 and Na OH were used as raw materials to synthesize zeolite Na A and hydroxy sodalite in open normal pressure water system. T he removal efficiency of zeolite A and sodalite for Cs ions was researched. And the physical and chemical properties of uranium tailings were studied, the compound of z eolite minerals and uranium tailings were prepared. The leaching resistance properties of U, Mn and Sr in compound of zeolite minerals and uranium tailings were studied by simulating the groundwater of weak a cid and weak base environment, and the blocking mechanism of zeolite minerals to uranium tailings was revealed.These research results showed that the optimum conditions of synthetic zeolite Na A were that the molar ratio of Na 2O/SiO₂ was 2.0, crystallizati on temperature was 80℃, crystallization time was 10 h. And the optimum conditions of synthetic hydroxy sodalite were that the molar ratio of Na 2O/SiO₂ was 4.0, crystallization temperature was 90℃, crystallization time was 1 2 h. T he effect of Na₂O/SiO₂ molar ratio had a greater influence than crystallization temperature and time for the skeleton type and purity of products. And the formation of hydroxy sodalite mainly included four period, amorphous sodium aluminosilicate polymerization（5¹0min）, zeolite Na A nucleation（30⁴0min）, symplastic growth of zeolite Na A and hydroxy sodalite（1³h）, phase transformation of zeolite Na A into hydroxy sodalite（4¹0h）. T he growth of hydroxy sodalite consumed the Si and Al content in liquid phase, which conformed to the liquid phase transformation mechanism. T he thermal stability of zeolite Na A was relatively poor and the total energy of hydroxy sodalite was lower than that of zeolite Na A, which were the essential reason that zeolite Na A transformed into hydroxy sodalite. In the process of synthesis of zeolite Na A（or hydroxy sodalite） produced mixed crystal zeolite Na X and hydroxy sodalite（or Na A zeolite） depent on similar skeleton and structure total energy.The optimum conditions of synthetic zeolite A containing Cs were that the Cs initial concentration was 3.0 at.%, crystallization temperature was 80 ℃, crystallization time was 10 h. And the optimum conditions of synthetic sodalite containing Cs were that the Cs initial concentration was 3.0at.%, crystallization temperature was 90℃, crystallization time was 1 2h. Cs doped into zeolite A and sodalite lattice leaded to decrease of the diffraction peak intensity, and the lattice constant was increased due to the Cs doped into zeolite A lattice, while the lattice constant was decreased for doping into sodalite lattice. T he Cs removal efficiency of zeolite A and sodalite was above 86%, and the Cs removal efficiency could achieve 96.88% and 97.95% at optimum conditions, respectively, the Cs removal efficiency of sodalite was sli ghtly worse than that of zeolite A.The chemical composition of uranium tailings samples were mainly Si O 2, Al2O3, K2 O and so on, in which the content of Si O 2 and Al2O3 were 74.71% and 13.12% respectively. After digestion, the content of U, Sr and Mn conten t in uranium tailings samples were 30.4 μg/g, 122μg/g and 523μg/g, respectively. The main mineral phase of uranium tailings was composed of quartz, albite, microcline and gypsum. Zeolite A and sodalite had a good curing and sealing effect on uranium tailings, but the solidification and sealing effect of the sodalite on uranium tailings is better than that of zeolite A. T he leaching amount of U, Sr and Mn increased with increasing of the p H value and leaching time. And leaching amount remained at a relatively small and steady numerical when the leaching time was 21 days, compared to compound of zeolite A minerals and uranium tailings, the leaching amount of U and Mn in compound of sodalite and uranium tailings was more lower, and the leaching amount of Sr was slightly different, which indicated that the compound of sodalite and uranium tailings possessed better anti-leaching performance.