| The use of non-saponification extraction system,P507-N235 system,can avoid the emission of ammonia-nitrogen waste water that caused by the extraction of rare earth in saponified P507 system.However,the presence of non-rare earth impurities such as Fe,Al,Mg,Ca in the leach liquors of rare earths may interfere with the separation of rare earths in the extraction system containing P507,and even cause the phenomena of emulsification and third phase.In this paper,the extraction behaviors of non-rare earth impurity elements(Fe,Al,Mg,Ca)in P507,N235 and P507-N235 systems,as well as the effects of impurity concentration,pH value,O/A ratio were investigated on the extraction of rare earth elements.The conditions of iron removal were studied.The main conclusions were summarized as follows:1.The extraction behaviors of Fe3+ and Fe2+in P507 system were investigated.(1)Both P507 and saponified P507 had high extraction capacities to Fe3+.When the Fe3+concentration was 0.06～3g/L,the pH of the aqueous phase was in the range of 0～2,the extraction percentage of Fe3+was near to 100%.When O/A ratio was less than 1:3,and the concentration of Fe3+was 3 g/L,a part of the P507 would unevenly dispersed in water,resulting in the emulsification in the aqueous phase.When the pH of the aqueous phase was ≥1.5,the hydrolysis products of Fe3+adsorbed P507 that was dispersed in water,forming the oil-water mixture and leading to the emulsification of the aqueous phase.The degree of the emulsification increasedwith the decrease of Fe3+concentration.However,The emulsification during the Fe3+extraction in saponified P507 system was more serious than that in P507system.The time required for the phase separation in saponified P507 system was longer than that in P507 system,and O/W emulsion was formed.The reaction of P507 with Fe3+belonged to the cation exchange reaction,the molar ratio of P507 to Fe3+was about 1.5:1.The analysis of infrared spectroscopy showed that no water was introduced into the organic phase during the reaction,and thus no emulsification was observed in the organic phase.(2)The extraction capacity of Fe3+was greater than that of Fe2+in P507 system.When the pH of the aqueous phase was in the range of-1～4,the concentration of Fe2+was 0.04～3 g/L and the O/A ratio was 3:1,no emulsification occurred during the extraction of Fe2+.The extraction percentage of Fe2+ decreased with the decrease of pH value and the increase of Fe2+concentration.(3)In order to prevent the emulsification during extraction of Fe3+ by P507,the O/A ratio should be greater than 1:1,the pH of the liquid should be less than 1.5 and the concentration of Fe3+should be greater than 0.7 g/L.2.The extraction behaviors of Fe3+ and Fe2+ in N235 system was investigated.(1)When the concentration of Fe3+ was 0～2.3 g/L and the pH value of the aqueous phase was in the range of-0.5～3,the extraction percentage of Fe3+in N235 system decreased with the increase of the pH value and O/A ratio,and increased with the increase of Fe3+concentration.When the aqueous phase pH was≥1,the extraction percentage of Fe3+ was very low,Fe3+ was likely to be extracted in the form of Fe(OH)2+,and the milky W/O interfacial emulsion was observed.When the aqueous phase pH was≤0,the third phase appeared.The analysis of infrared spectroscopy,conductivity determination,chloride ion detection and Karl Fischer water analysis showed that the loaded organic phase was mainly concentrated on the third phase and the molar ratio of N235 to Fe3+ was about 1:1.The composition of the loaded organic phasemight be R3NH·FeCl4·nH2O,and the water might be extracted into the third phase by N235 according to the ratio of nN235:nH2O=3:2.The presence of Fe3+ may promote water to enter into the organic phase.(2)When the pH of the aqueous phase is-1～4 and the concentration of Fe2+ is 0.04～3 g/L,no emulsification or the third phase was observed when Fe2+ was extracted in N235 system.The extraction percentage of Fe2+ decreased first and then increased with the decrease of pH.When the solution pH was>2,the extraction capacity of N235 to Fe2+ was higher than that to Fe3+.3.The effect of iron ion on the extraction of rare earths in P507-N235 system was investigated.(1)When the Fe3+ concentration was 0.06～3 g/L,the pH of the aqueous phase was 0～3,and the O/A ratio was 3:1,the extraction percentage of Fe3+ was near to 100%in P507-N235 system.When the aqueous phase pH was≥1.5,the emulsification phenomenon appeared in the aqueous phase after the extraction of Fe3+,and the degree of emulsification gradually increased with the increase of Fe3+concentration.The main reason for the emulsification might be the Fe3+hydrolysis.The Emulsification,the third phase and the water entering into N235 were the main reasons for the increase of the time of the phase separation.The high amount of N235 and the high solution pH will aggravate the emulsion and the third phase.When the pH value was<1 and the concentration of Fe3+ was less than 0.1 g/L,the emulsification was not easy to occur when Fe3+was extracted.(2)When the concentration of Fe2+ was 0.04～3 g/L,the pH value of the aqueous phase was-1～4,the extraction percentage of Fe2+ decreased with the decrease of pH of the aqueous phase,and there was no emulsification and third phase,but the time of the phase separation increased with the increase of pH of the aqueous phase,with the decrease of Fe2+ concentration.(3)During extraction of rare earths containing Fe3+ in P507-N235 system,Fe3+ was preferentially extracted to the rare earths.Therefore the extraction of Fe3+ influenced the extraction percentages of rare earths and the time of phase separation.When the concentration of Fe3+ was>0.05 g/L in the solution containing light rare earths,and the concentration of Fe3+ was>0.5 g/L in the solution containingmiddle and heavy rare earths,the time of extraction was significantly increased.Therefore,Fe3+ should be removed before extraction of the rare earths.The organic phase composed of 10%N235+90%kerosene can be used to extract all the Fe3+ in the solutions containing rare earth and Fe3+.When the pH of the feed liquid was-0.5 and the extraction O/A ratio was 5:1,all of the Fe3+ was extracted by N235.When O/A ratio was 1:3,Fe3+ could be completely stripped into the water phase when using the water as the back-extractant.Therefore the complete removal of Fe3+ from the rare earth solution was realized.The organic phase composed of 10%N235+90%kerosene can be used to extract all the Fe2+ in the solutions containing rare earth and Fe2+.When the pH of the feed liquid was-1 and the extraction O/A ratio was 3:1,all of the Fe2+ was extracted by N235.The organic phase loaded Fe2+could be stripped when using 0.05 mol/L hydrochloric acid as the back-extractant.Therefore the complete removal of Fe2+ from the rare earth solution was realized.4.The effects of Ca2+,Mg2+ and Al3+on the extraction of rare earths in P507-N235 system were investigated.(1)In P507-N235 system,Al3+ was mainly extracted by P507.The organic phase loaded Al3+could be completely stripped with 0.5 mol/L hydrochloric acid.(2)In P507-N235 system,Ca2+was mainly extracted by N235.When the liquid pH was 0.5,Ca2+ could be stripped with 0.05 mol/L hydrochloric acid,and when the liquid pH was-0.5,Ca2+ could be stripped with water.(3)In P507-N235 extraction system,Mg2+was mainly extracted by N235.When the liquid pH was-0.5,Mg2+ could be stripped with water,and when the liquid pH was 0.5,Mg2+ could be stripped with 0.05 mol/L hydrochloric acid.(4)Impurities such as Al3+,Ca2+and Mg2+all affect the time of phase separation during the extraction of rare earths in P507-N235 system.The time of phase separation increased with the increase of impurity concentration.In order to reduce the time of phase separation,the concentration of Al3+ in the solution containing light or medium rare earths should be controlled within 0.5 g/L,and should be controlled within 0.1 g/L in the solution containing heavy rare earths.The concentration of Mg2+ in the solution containing light,medium or heavy rare earths should be controlled within 0.3 g/L,the concentration of Ca2+ in the solution containing light,medium or heavy rare earths should be controlled within 3 g/L.(5)When the concentration of Al3+ was 8 g/L,the extraction percentage of rare earths in the rare earth solution deceased,while the concentration of Ca2+and Mg2+ was 0.09 g/L and 2.7 g/L,the presence of Ca2+ or Mg2+had little influence on the extraction percentage of light rare earths.In summary,in P507-N235 system,the effect of iron on the extraction of rare earths is the most significant,and the iron should be removed before the extraction of rare earths.The influence of calcium,magnesium or aluminium is not as obvious as that of iron.If the concentrations of calcium,magnesium or aluminium are controlled within a certain range in the solution,the influence of calcium,magnesium or aluminiumon the extraction of rare earths can be eliminated. |
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