| As an important strategic resource,rare earths are applied widely in the fields of military,aerospace,energy,medicine,agriculture,functional materials and so on.With the development of sophisticated technology,the demand for a single high-purity rare earth is increasing.Solvent extraction is an important methods in the process of separation and purification of rare earth.At present,the commonly used extractants are P204,P507 and C272.P204 is mainly used for group separation of rare earths because its selectivity for rare earths is very low.P507 is widely used in the separation and purification of single rare earth due to its higher average separation coefficient compared with P204 and excellent interface performance.But it is difficult to strip the rare earths loaded on P507,especially for heavy rare earth.Recently,it is found that C272 could be a potential good rare earth separative extractant because it has higher average separation coefficient and lower stripping acidity than P204 and P507.But its saturation capacity is small,and the loaded organic phase is is easier to be emulsified,which limits its practical application.In this thesis,the extraction behavior of di-(2-ethyl hexyl)phosphinic acid(P227)for heavy rare earth elements Er and Lu was studied.It is shown that the maximum extraction rate of Er and Lu(above 99%)occurs at pH = 2.0,phase ratio(O/A)= 1: 1,and contact time(t)= 10min;The optimal separation between Er and Lu(bEr/Lu = 4.71)was occurs at pH = 1.25,O/A = 1: 1,and t = 10 min.The highest stripping rate is achieved with 2mol/L HCl.The saturated capacity of P227 for Er and Lu were 0.137 and 0.175 mol/L,respectively.In summary,the comprehensive performance(separation coefficient,stripping acidity,saturated capacity)of P227 is better than that of P204,P507 and C272.The extraction behaviors of P227 for iron,aluminum and silicon which are common impurities in rare earth separation were studied in details.The effects of contact time,phase ratio(O/A)and pH value on the extraction rate,as well as the effect of stripping acid concentration,phase ratio,stripping stage numbers on the stripping rate were investigated.The extraction and stripping of Er(III)and Lu(III)by the organic phase based on P227 loaded with iron,aluminum and silicon were also studied,and the conclusions are as follows:(1)The maximum extraction rate(about 97%)of Fe(?)is achieved at pH = 1.5,t = 10 min,O/A = 1:1,and the extraction system start to emulsify when pH is above 1.5.The optimal single-stage stripping rate was obtained with 3mol/L HCl,and the stripping rate reaches 99.12% through 5-stage cross-flow stripping.The extraction and stripping rates of Er(III)and Lu(III)decrease obviously by using the Fe-loaded organic phase as an extractant,which is revealed that there is strong competition between Fe(?)and rare earths on P227.(2)The maximum extraction rate(15.69%)of Al(?)is achieved at pH =2.0,t = 10 min,O/A = 1:1,and the extraction system start to emulsify when pH is above 2.25.The optimal single-stage stripping rate was obtained with 2mol/L HCl,and the stripping rate reaches 94.32% through 8-stage cross-flow stripping.The stripping rate of Er(III)and Lu(III)decrease obviously by using the Al-loaded organic phase as an extractant,which is revealed that the coordination of rare earth and P227 is stronger greatly than that of Al(?)and P227.However,no significant change is found in the stripping rates of Er(III)and Lu(III).It is presumably that in HCl medium Al(?)can combine Cl-to form AlCln3-n which can associate with P=O group in P227 molecules.(3)The maximum extraction rate(13.26%)of Si((IV)is achieved at pH = 1.25,t = 10 min,O/A = 1:1,and the extraction system start to emulsify when pH is above 2.0.The optimal single-stage stripping rate was obtained with 3mol/L HCl,and the stripping rate reaches 97.95% through 6-stage cross-flow stripping.Both extraction rate and stripping rate of Er(III)and Lu(III)exhibit no significant change,which is shown that the coordination of Si((IV)and P272 is weaker than that of rare earth and P272,and no association exists between Si species and P227 in HCl medium.. |
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