Research On Leaching Of Uranium By Dilute Alkali-pretreatment Combined With Acid Leaching From Low-grade Uranium Ore

Uranium recovery from low-grade uranium ore is extremely difficult due to the encapsulation of uranium with high content of gangue minerals and complex mineralogical environment.The existing technology for uranium recovery is characterized by low efficiency,high energy consumption and serious secondary environmental pollution,which significantly hampered the green and efficient exploiting of low-grade uranium ore.Thus,the key to green development and utilization of low-grade uranium ore can be achieved through destruction of gangue and efficient dissociation of uranium from gangue minerals,improving the uranium extraction and reducing the secondary pollution during leaching period.Alkali pretreatment followed by H₂SO₄ leaching process was developed.The basic principle is using low concentration of alkali solution to comminute the gangue minerals and to promote the formation of micropores on the surface of particles,which providing a"path"for sulfuric acid to enter the interior of uranium minerals.The treated residues were then leached by H₂SO₄ acid to improve uranium exposure on leaching agent.In this paper,the effects of process parameters on uranium extraction and the mineralogical characterization and leaching kinetics behavior were investigated to show the leaching mechanism of uranium and mechanism of gangue comminution.The main conclusions obtained in this study are list as follows:The results show that the leaching efficiency of alkali pretreatment followed by H₂SO₄ leaching process is superior to conventional acid leaching.The lower concentration of alkali solution is more readily to the dissociate gangue minerals.The suitable pretreatment conditions for the alkali were:NaOH concentration 10 g/L,stirring speed 400 rpm,liquid-solid ratio 20:1,leaching time 2 h,uranium leaching rate reached 90.80%,and the leaching rate of uranium increased by 26.89%.The results of mineralogical characterization showed that the crystal structure of gangue minerals were selectively decomposed by low concentration alkali solution,forming micropores in the particles,which provided a‘path'for sulfuric acid to enter the interior of the uranium minerals.During the acid leaching process,the crystal structure of gangue minerals were further destroyed.Thus,a large number of mesopores in the residues formed to improve the exposure ratio of uranium to the leaching agent and increased the leachability of uranium.The uranium leaching efficiency of alkali pretreatment followed by H₂SO₄ leaching process is higher than that of single alkali leaching or acid leaching.In addition,the pH of alkaline solution has no significant changes during the pretreatment stage.Therefore,the alkali reagent could be used in circulation.The leaching kinetics behavior of low grade uranium ores showed that the leaching ratio of uranium increased with an increase in sulfuric acid concentration,leaching temperature and leaching time.Kinetic modelling has indicated that uranium leaching behavior does not conform to the shrinking core model,while the Avrami equation can well explain the leaching mechanism of uranium since the leaching considered as the reverse process of crystallization.And the Avrami equation showed that both the leaching process are controlled by internal diffusion.The activation energies value under the conventional acid leaching and two-step leaching were 8.75 kJ/mol and 2.71 kJ/mol,respectively,corresponding to a reaction order of 0.767 and0.419 based on the leaching experimental data.Besides,a higher temperature and acid concentration more readily destroy the crystal structure of the gangue mineral and the shape of the particles,further increasing the liberation rate of uranium.This study will is helpful for recovering valuable metals from low grade ore or high gangue content tailings,which provide a scientific basis for the utilization of a large number of low grade uranium resources and solve the problem of resource utilization of valuable metals in China's large solid waste.