Research On The Process Of Cascade Recovery Of Rare Earths From Waste Phosphors

Rare-earth phosphors contain strategically rare earth elements such as Y,Eu,Ce,and Tb,which are dozens or even hundreds of times more valuable than natural ores.Therefore,it is of great significance to recover rare earth resources from used phosphors.At present,direct acid leaching and alkali roasting-acid leaching processes are mainly used for recovering rare earths from used phosphors.Although the above processes have been widely used,the direct acid leaching applications have a narrow range of applications and low yields,the alkali roasting-acid leaching costs are large and need more acid.In addition,there are few thermodynamic studies on the waste rare-earth phosphors in the acid leaching process,the main elements are unclear and the concentration of residual acid in the leachate is high.Therefore,this paper adopts the cascade method is acid leaching-caustic soda roasting-washing-two-stage acid leaching to recover the rare earth in waste phosphors.The main research contents and results are as follows:By thermodynamic calculation,potential-pH diagrams of Y-Eu-H₂O,Al-H₂O and Tb-Ce-H₂O series of spent phosphors in the process of rare earth recovery were plotted.The process of acid leaching-caustic soda roasting-washing-acid leaching and the direction of the main elements were studied.Aiming at the problem of high residual acid concentration after the acid leaching of the water-washed product,this paper proposed the second-stage acid leaching process of the washed product,and compared the effect of adding different alkaline substances to purify the leachate.The results show that under the acidic condition of pH<6 and temperature between 25 to 100?,the red powder of Y₂O₃ and Eu₂O₃ are stable in the form of Y³⁺and Eu³⁺in aqueous solution.The endpoint pH value of acid leaching solution was controlled between 3.5 to 4 to prevent the Fe³⁺and Al³⁺hydrolysis.When washing at room temperature 25?,ensure that the pH value of the final washing solution is greater than 12.46.When washing at 100?,ensure that the pH value of the final washing solution is greater than 10.41.The TbO₂ and CeO₂ were hard leaching directly in acid condition and the temperature between 25?to 100?,meanwhile,the Tb⁴⁺and Ce⁴⁺were exist in aqueous solution unstably,which could be reduced into Tb³⁺and Ce³⁺of stable formation in aqueous solution by adding appropriate reducing agent.The total leaching rate of rare earths during the acid leaching process is 57.95%,of which 90.31%of Y₂O₃,79.73%of Eu₂O₃ and traces of Tb₄O₇ are leached,and caustic soda roasting water is used to remove aluminum,and the rate is96.95%,the final rare earth leaching rate is 98%.The second stage acid leaching process is as follows:one stage of acid leaching with hydrochloric acid at a concentration of 2.04 mol·L^-1 HCl,the product at a liquid to solid ratio of 5:1,a leaching temperature of 60?,and leaching time 90 min,get leaching solution endpoint The pH value is 4.The two stages were leached for a period of leaching at a liquid-solid ratio of 6:1,a hydrochloric acid concentration of 6 mol·L^-1,a leaching temperature of 60?,a leaching time of 90 min,and a 10%?volume fraction?of hydrogen peroxide and 8%?mass fraction?of ascorbic acid,and the total rare earth leaching rate was 95.71%and 99.32%.In addition,adding ammonia or sodium hydroxide will adjust the end pH of the leachate to 5.The content of Fe and Si in the filtrate basically meets the following extraction conditions,but Al needs to be further removed by extraction or other methods.