Selective Extraction Of Rare Earth Elements From Polymetal Ions In Panbei Coal Fly Ash Leaching Solution

The over-exploitation of rare earths has resulted in rare earth resources becoming increasingly exhausted.The large amount of coal fly ash produced by coal-fired power plants is a secondary resource that can be recycled and reused,which is rich in rare earth elements.Extracting rare earth elements from coal fly ash can solve the problem of rare earth depletion.This paper mainly studies the leaching of rare earth elements and macroelements from coal fly ash,the selective extraction of rare earth elements yttrium and macroelements of aluminum and iron,the selective extraction of rare earth elements from coal fly ash leaching solution,the extraction mechanism and extraction thermodynamic and kinetic processes.The main research results obtained are as follows.The leaching rule of rare earth elements and macroelements from coal fly ash was studied.Using 3mol/L hydrochloric acid,leaching time 1.5h,liquid-solid ratio（v/m）is10,stirring speed 300r/min,leaching temperature 80℃,the leaching rates of La,Ce,Pr,Nd,Y,Al and Fe in coal fly ash are 88.45%,81.44%,89.57%,82.12%,32.13%,31.90%,52.01%.The element leaching rate increases with the increase of hydrochloric acid concentration,leaching time,liquid-solid ratio and leaching temperature and then remains unchanged,while the stirring speed has little effect on the element leaching rate,and the leaching rate hardly changes with the change of stirring speed.The leaching rate of Y is basically maintained in a stable range,because Y is similar as heavy rare earth elements and is not easily leached,and its leaching rate is lower than that of light rare earth elements.Through XRD analysis,most of the quartz and anhydrite in the coal fly ash leaching slag were not destroyed,and the crystals such as magnetite and mullite were destroyed.Through SEM and EDS analysis,the main elements in coal fly ash slag are oxygen,carbon,aluminum,iron,silicon and so on.The selective extraction of rare earth element Y and macroelements Al/Fe with extractants P204（di-2-ethylhexyl phosphoric acid）and P507（2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester）were studied.Both extractants showed good extraction effect on Y³⁺,and the separation factors of Y³⁺and Fe³⁺β_{Y/Fe（Ⅲ）}of the two extractants are much higher than Y³⁺and Fe²⁺β_{Y/Fe（Ⅱ）},indicating that Fe³⁺and Y³⁺are more easily separated.At the same time,the extractant P204 has better separation effect on Fe³⁺and Y³⁺than the extractant P507 on separation of Fe³⁺and Y³⁺.The separation factors of Y³⁺and Al³⁺,Fe³⁺,and Fe²⁺,β_{Y/Al（Ⅲ）},β_{Y/Fe（Ⅲ）},andβ_{Y/Fe（Ⅱ）},decrease in turn,indicating that the separation effect of Y³⁺and Al³⁺,Fe³⁺,and Fe²⁺decrease in turn.Rare earth elements and macroelements show good selective extraction.After treatment by physical and chemical means,most of the aluminum and iron are removed,and the rare earth elements in the leaching solution are initially enriched.The single extraction system P204 and the synergistic extraction system P204-P507 were used to selectively extract the rare earth elements La,Ce,Pr,Nd and Y from the coal fly ash leaching solution.Under a single extraction system,the solution of pH of 2.1,oil-water ratio（O/A）of 1,extraction time of 25 minutes,extractant volume concentration of 6%,and temperature of 30°C.The extraction rates of La,Ce,Pr,Nd,and Y reach 89.16%,94.11%,95.56%,96.33%,99.80%,respectively.Under the synergistic extraction system,the extraction rates of several rare earth elements reached almost 100%.The extraction rates of La,Ce,Pr,and Nd increase with the increase of the atomic number.The larger the atomic number,the smaller the ion radius,and the easier the extraction.Under the same environmental conditions,the separation factors of single extraction system P204 for Y and La,Ce,Pr,and Ndβ_Y/La,β_Y/Ce,β_Y/Pr,andβ_Y/Nd are higher than synergistic extraction system,indicating that Y and four light rare earth elements are easier to separate under a single extraction system.At the same time,the separation factorsβ_Y/La,β_Y/Ce,β_Y/Pr,andβ_Y/Nd show the order of decreasing order,and the difference between the rare earth element numbers,the easier to separate.The extraction mechanism of Y under single extraction system P204 and synergistic extraction system P204-P507 were studied.In the extraction system,the molecular formula of the extractant are Y（HA₂）Cl₂ and Y（HA₂）（HB₂）Cl,respectively.Through studying the thermodynamic process of extraction,it is found that both the single extraction system and the synergistic extraction system are endothermic processes,and theirΔH are 86.68kJ/mol and 11.11kJ/mol,respectively.The extraction kinetics of Y was studied.In the hydrochloric acid system,the extraction rate of Y³⁺increases with the increase of stirring rate,boundary area,temperature,extractant concentration,hydrochloric acid concentration,and Y³⁺concentration.The extraction process is controlled by chemical reaction and diffusion mixing.The energy Ea of the extraction reaction is 22.45 kJ/mol.In the hydrochloric acid extraction system,the reaction order of the extraction rate equation for Y³⁺,P204,and HCl is 1.04,0.93,and 0.42,respectively.The rate constant k is 6.65（mol/L）^-1.39?min^-1,and the chemical reaction rate equation isr（28）k^{[Y3（10）]1.04}[P204]^0.93[HCl]^0.42.