Dialkyl phosphinic acids:Synthesis and applications as extractant for nickel and cobalt separation

Three kinds of dialkylphosphinic acids(DAPAs),i.e.dihexylphophinic acid(DHPA),di-(2,4,4-trimethylpentyl) phosphinic acid(DTMPPA)and didecylphophinic acid(DDPA),were synthesized through free radical addition reaction.The influence of the types of initiator,reation time and reaction temperature on the yield of DAPAs were investigated.The products were characterized by NMR and MS.By using DHPA,DTMPPA and DDPA(10%in kerosene)as extractants,the extraction of Co2 +and Ni2 +in sulphate medium at different equilibrium pH values were measured.The results show that the maximum yield of DHPA, DTMPPA and DDPA can all be achieved at about 130℃under the initiation of di-tert-butyl peroxide(DTBP).All the extraction of cobalt with respect to DHPA,DDPA and DTMPPA precedes that of nickel.The difference in pH1/2 value(defined as the pH at which 50%metal extraction occurs)between cobalt and nickel increases in the following sequence from large to small:DHPA,DDPA and DTMPPA,which indicates that the separation ability for cobalt and nickel ascends from DHPA,DDPA to DTMPPA.

Loading capacity and emulsification phenomena of HREE extraction by dialkylphosphinic acids with different β,γ,δ-substituents

The relationship between dialkylphosphinic acid structure and their loading capacity for HREEs and emulsification phenomena has rarely been studied.In this paper,we took Lu extraction as an example to study the effect of β,γ,δ-substituents of dialkylphosphinic acids on their loading capacity and anti-emulsification performance for HREEs.To discuss conveniently,the dialkylphosphinic acids were classified into two groups:β-substituent group(P208,INET-1,P218,USTB-1 and P227)and γ,δ-substituent group(P218,P2132 and Cyanex 272).For β-substituent group extractants,their loading capacities are in the order P208(299.7 mg/L)≈INET-1(299.5 mg/L)>P218(270.8 mg/L)>>USTB-1(163.1 mg/L)>P227(151.4 mg/L),while their anti-emulsification/gelation performances are just in the opposite order P227>USTB-1>P218>P208≈INET-1 under the studied conditions.For γ,δ-substituent group extractants,their loading capacities are in the order P218(270.8 mg/L)>P2132(192.3 mg/L)>Cyanex272(131.7 mg/L),while the anti-emulsification performance of P218 is better than those of P2132 and Cyanex 272.The loading capacity data given in the parentheses are obtained through repeatedly extracting Lu from ~4×10^(-4)mol/L of Lu aqueous feed solution with initial pH of 2.40 by 0.01 mol/L extractant at phase ratio A/O of 1:1.

The role ofβ,γ,δ-substituent of dialkylphosphinic acids in HREE extraction and separation behaviors

Heavy rare earth element(HREE)separation has always been a great challenge.The kind of dialkylphosphinic acids is recognized as the third generation of organophosphorous acid extractants.However,detailed relationship betweenβ-substituent(especially when it changes from H to methyl,and further to ethyl)and their extraction behaviors and selectivity for HREEs have rarely been reported.Besides,there are also few reports on effect ofγ,δ-substituent on HREE extraction and separation.In this paper,we synthesized five and collected two dialkylphosphinic acids with differentβ,γ,δ-substituents(from H to ethyl).These extractants were divided into two groups to discuss their extraction behaviors and selectivity for HREEs:β-substituent group(P208,INET-1,P218,USTB-1 and P227)andγ,δ-substituent group(P218,P2132 and Cyanex 272).To evaluate and compare their extractabilities,P507 was used as a reference and Tm,Yb,Lu were extracted with 0.01 mol/L of these extractants from single Tm,Yb and Lu solutions(~4×10^(-4)mol/L)at different initial pH.Besides,pH_(0.5)(Tm),pH_(0.5)^(Yb)and pH_(0.5)^(Lu)values and their differences(ΔpH_(0.5)^(Tm-Yb),ΔpH_(0.5)^(Yb-Lu),ΔpH_(0.5)^(Tm-Lu))for each extractant were given to roughly evaluate their separation performance for Tm,Yb and Lu.To further investigate the selectivity of these extractants for HREEs,Tb,Dy,Ho,Er,Tm,Yb and Lu were extracted from their mixed solution with initial pH of 4.00 at phase ratio A/O of 2:1,1:1 or 1:2.Their HREE separation performance is mainly discussed based on the adjacent HREE separation factors(β_(N+1/N))and their average value(β_(Tb-Lu)).