Synergistic Extraction Of Rare Earths By Quaternary Phosphonium Salt [P66614][Cl] And Neutral Organophosphates DEHEHP

Liquid-liquid extraction is the most commonly applied technology for the separation of rare earths(REs).However,the conventional liquid-liquid extraction process usually uses VOCs as diluents which normally result in emulsification and environmental pollution.In addition,the widely used conventional acidic organophosphorus extractants leads to the discharge of large amount of ammonia-nitrogen wastewater that is difficult to handle.Therefore,to develop innovative and green extraction separation technologies would be significant.Synergistic extraction is an extraction method that employs two or more extractants to extract target material.It has advantages including enhanced extractabilities and separation factor.And ionic liquids(ILs)as green solvent have received increasing attention in separation science,especially in hydrometallurgical industry.We have proposed an idea about developing IL-based synergistic extraction strategy without any diluent in our previous work.It has been found that the combination quaternary ammonium salt([A336][NO3])and Di(2-ethylhexyl)2-ethylhexyl phosphonate(DEHEHP)shows an obvious synergistic extraction effect for rare earths nitrates.Based on this,synergistic extraction behaviors without any diluent for rare earth nitrates based on tri(hexyl)tetradecylphosphonium chloride([P66614][Cl])and DEHEHP have been systematically investigated in this work.Although quaternary ammonium salts and quaternary phosphonium salts have the similar physical and chemical characteristics,the different anions and cations would lead different behaviors of synergistic extraction.Particularly,Pr(?)was used as a model RE,and the extraction mechanism and the separation selectivity of rare earths nitrates by [P66614][Cl]-DEHEHP have been studied.The influence factors on the synergistic extraction of Pr(?),such as the acidity of feed solutions,concentration of anions and metals,have been discussed.Thermodynamic and kinetic behaviors for the extraction of Pr(?)by [P66614][Cl]-DEHEHP has also been investigated.Finally,the synergistic enhancement coefficient and separation factor were compared between [P66614][Cl]-DEHEHP and [A336][NO3]-DEHEHP.In addition,the interactions between [P66614][Cl] and DEHEHP have been investigated via infrared spectroscopy and nuclear magnetic resonance technology.The following results can be obtained:1.The maximum synergistic coefficient(R = 2.15)was obtained at a volume ratio of 2:3(V[P66614][Cl]/V DEHEHP)for [P66614][Cl] and DEHEHP;2.The extraction behaviors of Pr(?)can be properly described by Langmuir isotherm equations and pseudo-second-order rate equations.The percentage extraction would decline and the extraction rate would increase with the increasing temperature;3.There is an obvious synergistic enhancement effect for light REs(La-Sm)and an anti-synergistic effect for heavy REs(Gd-Lu and Y);4.The extraction mechanism belongs to the typical ion-association reactions;5.The maximum loading capacity is 0.78 mol·L-1 Pr(?)at 298.15 K.The extraction ability of [P66614][Cl]-DEHEHP is superior to that of reported [A336][NO3]-DEHEHP(0.46 mol·L-1 298.15 K),and is also greater than that of conventional acidic organophosphate P507-kerosene extraction system(0.18-0.20 mol·L-1).6.The stripping of loaded metal ions can be realized by H2 O and the extraction system has a very excellent recyclability;7.The co-extraction system has higher separation factors between REs and non-REs compare with the single extraction system [P66614][Cl];8.It can be confirmed that there exists obvious interaction between [P66614][Cl] and DEHEHP.There also exists strong interaction between [P66614][Cl] and H2 O.The existence of H2 O can be enhance the interaction between [P66614][Cl] and DEHEHP.Furthermore,the strength of the hydrogen bonds can be enhanced by water.This may be the main reason for the existence of synergistic effect.