Study On The Leaching Technology Of Uranium From Alkali Residue Precipited From Wastewater In The Production Process Of Nuclear Fuel Element

For the low-enriched uranium alkali residue produced in the production of a fuel element plant,there is no mature method to recover the uranium due to the problems of complex sources and large differences in physical and chemical properties.By studying the main chemical composition and mineral composition of the alkali residue,the enhanced countercurrent dissolution leaching methods was used to study the leaching mechanism of the alkali residue.The uranium in the dissolving solution was extracted to separate by using a centrifugal extractor,and the uranium and impurity content of the obtained stripping solution was up to the mustard.A reliable way was proposed for industrial extraction of uranium from alkali residues.The specific conclusions are as follows:1)The physical and chemical properties of the alkali residues are vary greatly.U in the alkali residue mainly exists in the form of oxides,but a small amounts of U are encapsulated by oxide solid solutions or forms oxide solid solutions.It showed that most of the U in the alkali residues are easy to be leached,while a very small amount of U is difficult to be leached.2)Alkali residue samples are leached with the methods of three-stage and two-stage countercurrent dissolution and leaching.The results of the leaching rates of U and the U contents of insoluble residues are similar to the two methods.However,the three stage counter-current solution leaching has the shortcomings of long process and complex operation,so the two stage counter-current solution leaching is suitable for alkali slag treatment.The proposed countercurrent solution leaching method avoids the problem of low concentration uranium containing process wastewater.Therefore,the alkali residue can be mixed into materials with basically uniform U content according to different enrichment degrees and then treated.3)The results of the two-stage countercurrent dissolution and leaching conditions of homogeneous mixed alkali slag showed that the dissolution and leaching of U in alkali slag was most affected by temperature,followed by reaction time,dosage and concentration of nitric acid.Under the optimum conditions of atmospheric pressure,the leaching rate of U in alkali slag is 99.46% and the content of uranium in slag is 1.07%.4)The test results of the two-stage countercurrent pressure enhanced dissolution leaching method show that the leaching effect of U is the best under the condition of 130℃.The calculated leaching rate of U slag is99.75%,the uranium content of insoluble slag is 0.782%,and the residual rate of insoluble slag is 9.24%.Under the optimal conditions of two-stage countercurrent solution leaching under pressure and heat strengthening,when the water phase return ratio of raster is 50%,the leaching rate of U decreases by 0.2-0.3%,and the residual rate of insoluble slag increases to 15.41%.5)It has been verified by the expanded test that under the optimal conditions of autoclaving enhanced secondary countercurrent dissolution and leaching,when the return ratio of raffinate water phase is 50%,the primary dissolution temperature is increased to 120 °C,and the average U leaching rate is reduced to 99.32%,the average residual rate of insoluble slag is 21.63%,and the average uranium content of the slag is 0.83%,which are lower than the indicators when the raffinate water phase does not return to dissolve and leaching.It is confirmed that the main technical indicators will decrease when the raffinate water phase is returned to dissolve and leaching,and the advantage is that the consumption of nitric acid can be reduced.After mixing the solubilized liquids obtained in all levels of tests,centrifugal extraction and stripping of 30% TBP kerosene solution,the obtained stripping liquid components meet the standard quality requirements.