| Aiming at the problems that include the leaching of uranium from the high grade uraniferous alkalescence dreg and the recovery of nitric acid from the extractive water phase are produced during the production process of a nuclear fuel element company.The influence of screening test of the high grade uraniferous alkalescence dreg and chemical composition analysis,dissolution leaching condition selection process,cyclic dissolution and uranium content of different slag,washing leaching exploration,pretreatment of insoluble residue,different liquid to solid ratio on uranium leaching were carried out.And the vacuum distillation was carried out to recover nitric acid from the extracted liquid.The conclusion were as follows:(1)The experiment of crushing sieve analysis of uraniferous alkalescence dreg S1 was carried out,and the chemical composition analysis was carried out.The results showed that the distribution of uranium in different particle size was uniform.The uranium content in slag was 59.63%.In addition to the main element uranium,there were many elements such as Fe,Ca,p and so on,which affected the leaching of uranium and decreased the leaching rate of uranium.According to the characteristics of uraniferous alkalescence dreg,the selection test of the leaching process parameters was carried out.The influence of the leaching time,leaching temperature,leaching liquid-solid ratio on uranium leaching were investigated.The leaching process parameters were determined to be leaching time 3h,leaching temperature 80?,solid-liquid ratio 2.2:1.The leaching rate of uranium was 99.01%.(2)In this paper,16 cyclic dissolution of the uraniferous alkalescence dreg S1 were carried out,and finally the uranium content 0.181% of the insoluble slag was obtained.The remaining residue rate was 1.33%,indicating that the effect of cyclic dissolution was better.When the dissolution cycle reached a certain number of times,the uranium content of slag remained stable.Six cyclic dissolution tests of A,B and C with different uranium content were carried out.The leaching of uranium was emphatically investigated.Through the comparison of uranium grade and leaching rate in the final slag,the uranium content of A decreased from 8.66% to 0.901%,the uranium content of B decreased from 25.09% to 0.223%,and the uranium content of C decreased from 57.28% to 0.822%.The leaching rate of slag could reach more than 98%.The results showed that the leaching effect of uranium was closely related to the varieties of slag.The influence of the reuse of slag S1 leaching solution and leaching time on uranium leaching was investigated.The uranium content of insoluble residue decreased from 0.197% to 0.091%,which indicated that the effect of reducing uranium content was more obvious for two times leaching and one washing process.(3)Two times of high concentration nitric acid washing,dissolution leaching rewashing,after pretreatment and leaching washing were carried out on the insoluble slag with a uranium content of 7.76%.The results of two high concentration nitric acid leaching and two dilute nitric acid leaching tests showed that the content of uranium in the slag reached 0.059%.Calcium sulfate,calcium oxide,calcium carbonate,sodium hydroxide,sodium hydroxide,silica and sodium fluoride were mixed with the insoluble residue respectively.After calcination at 750 ?,leaching of uranium with dilute nitric acid was carried out.The content of uranium in the slag could reach 0.105%.The mixture of concentrated sulfuric acid and insoluble slag was cooled after curing at 200?.The content of uranium was reduced to 0.248% by leaching of dilute nitric acid.The exploration test of different liquid to solid ratio after eight times washing with the insoluble residue which the content of uranium was 7.76% was carried out.The two leaching and one washing methods were used.When the total liquid-solid ratio was 30:1,the uranium content in the slag was 0.080%.The liquid-solid ratio continued to increase,which had little effect on uranium leaching and would result in waste of resources.Taking into account that multiple washing and leaching could increase the leaching rate of uranium and reduced the uranium content of slag.The leaching effect of uranium could be improved by roasting or maturing pretreatment,and the uranium content of slag could also be reduced by choosing appropriate liquid-solid ratio after washing of insoluble slag.(4)The experiment was carried out to recover nitric acid by distilling the leftover liquid.The effects of the composition of the extractive solution and the first order distillation and the two stage distillation on the recovery of nitric acid were investigated.The results showed that the concentration of nitric acid increased with the increase of the distillation time and the volume of distilled liquid.And the nitric acid concentration could reach a maximum of 700 g/L.The highest rate of recovery of nitric acid could reach 90%.The higher impurities in the extractive liquid,the more difficult the distillation,the longer the time needed,the more residue at the bottom.To sum up,according to the process parameters selected by lytic leaching of alkali slag,the uranium in alkali residue could be leached effectively by the method of cyclic dissolution,washing and leaching.According to the refractory uranium containing insoluble slag,the method of high temperature roasting or concentrated sulfuric acid pretreatment followed by nitric acid leaching could effectively improve the leaching of uranium and reduce the uranium content in the slag.The recovery of nitric acid from waste liquid was carried out by two-stage decompression distillation for the residual liquid produced during the production process.It could not only reduce the waste of resources,but also recycled and reduced pollution.It would provide experimental basis for the recovery of nitric acid from distilled residual liquid after distillation. |
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