Huifang Li,Lijuan Li*,Xiaowu Peng3,Lianmin JiWu Li2,*

1Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes,Chinese Academy of Sciences,Xining 810008,China

2Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province,Xining 810008,China

3University of Chinese Academy of Sciences,Beijing 100049,China

4Qinghai University,Xining 810010,China

Keywords:Organic synergist Lithium extraction Mixed extraction system Mechanism

A B S T R A C T The organic synergists,including Octanol,ethyl acetate(EA),butyl acetate(BA),methyl isobutyl ketone(MIBK),diisobutyl ketone(DIBK),N,N-bis(2-ethylhexyl)acetamide(N523)and 8-hydroxylquiolate,were added to the TBP-FeCl3extraction system to extract lithium from brine.The effects of concentration of organic synergist and total organic extractant,molar ratio of Fe/Li,phase ratio,counter-current extraction and the acidity of stripping agent on lithium extraction were investigated to optimize the extraction conditions.Under the optimize conditions,the results of counter-current extraction showed the mixed extraction system was the preponderance on the lithium extraction.Especially the separation of lithium in organic phase and aqueous phase and the separation mass ratio of Mg/Li increased greatly.An extraction mechanism was proposed based on the analysis of FT-IR spectra and Raman spectra.

1.Introduction

Owingtotheparticularphysicochemicalandelectrochemicalproperties,lithiumanditscompoundsarequiteappealingtomanyfields,which include thermonuclear fusion reactors[1],metal additives,aerospace[2,3],medicinesandlithiumionbatteries[4,5].Especiallylithiumionbatteries have been broadly used as energy storage option due to its high capacity,long cycle life performance and excellent storage characteristic[6].This suggests the demand of lithium and its product has been intensively increased.There are two forms of lithium existing in nature:brine and lithium-containing ores.With the requirement of environmental protection and the depletion of ore resource,the recovery of lithium tends to the brine.Extracting lithium from brine can bridge the gap between the ever growing demand of lithium compounds and the supply because about 70%of lithium resources exist in brine[7].Furthermore,recovery of lithium from brine is simple and easy to industrialize compared to the hard-rock mining[8].The research of extraction of lithium frombrinehasfocusedonelectrodialysis[9-11],ionexchange,precipitation,nanofiltration[12],absorption[13]and solvent extraction[14-17].Solvent extraction has advantages of high-selectivity,easy-operation and environment-friendliness,which is attracting increasing attention.

The typical extraction system of recovery of lithium from brine by solvent extraction consists neutral organophosphorus extractant tributyl phosphate(TBP)used as extractant and FeCl3as co-extractant[18-20].Although the use of TBP can obtain a higher lithium extraction efficiency and a satisfactory separation of Mg/Li,the high concentration of TBP causes serious corrosion to the equipment and loss of reaction in the process of extraction[21].It is reported that the third phase appeared in the extraction process when the volume percentage of TBP was below 75%.Furthermore,Ji et al.[22]reported only 60 vol%TBP plays an important role in extracting lithium when 80 vol%TBP was used as extractant,and that 20 vol%TBP was considered as synergist.

Using organic synergists as modifiers can effectively improve the problem of the third phase and accelerate the speed of phase separation.Li et al.[23]reported that butyl acetate(BA)was used as synergist in the TBP-FeCl3traditionalextractionsystem.Methylisobutylketone[24]was considered as diluent in the TBP system,which also contributed to lithium extraction efficiency.N,N-bis(2-ethylhexyl)-3-oxobutanamide(NB2EHOTA)[21]was investigated to extract lithium from brine while TBPwasusedasthesecondcomponentandFeCl3asco-extractant.However,the comparison and extraction mechanism of organic synergist when TBP was treated as extractant was rarely reported.

In this study,octanol,EA,BA,MIBK,DIBK,N523 and 8-hydroxylquiolate were used as organic synergist adding to the TBPFeCl3extraction system to extract lithium from brine.The effects of these synergists in extracting lithium were compared and the factors on lithiumextractionwereinvestigated.Anextractionmechanismwasproposed based on the analysis of FT-IR spectra and Raman spectra.

2.Materials and Methods

2.1.Chemicals and instruments

The reagents used in this experiment were of analytical purity.LiCl,MgCl2,andFeCl3weresuppliedbyChiaPharmaceuticalGroupChemical Co.,LTD.and HCl was supplied by Silver ring Chemical Co.,LTD.EA and BA were supplied by Wing big Chemical Co.,LTD.,MIBK and DIBK were supplied by Aladdin Chemical Co.,LTD.,and TBP were supplied by Cologne Chemical Co.,LTD.

The instruments were:a shaker(SR-2DW;TAITEC),a centrifuge(TDL-40B-W;Shanghai Anting Scientific Instrument Factory),an inductively coupled plasma-atomic emission spectrometer(ICP-AES)(ICAP6500;DUO;America Thermo Scientific,USA),a Fourier Transform Infrared Spectroscopy(FT-IR)spectrophotometer(670;Thermo Nicolet Corporation,Madison,USA),and a Raman Spectrometer(DXR;America Thermo Scientific,USA).

2.2.Analytical method

All experiments were conducted in funnels at(25 ± 2)°C,the aqueous solution and organic phase were added to the funnel at a constant ratio.Then,the funnel was shaken for 10 min and left for 30 min to separate.Once the two phases were separated completely,the aqueous solution was diluted with proper folds for analyzing metal ion concentration.The loaded organic phase was backextracted with 6 mol·L?1HCl at the phase ratio of 1/2,and the stripping liquor was diluted to analyze metal ion concentration from the organic phase.

When the extraction reaction reaches equilibrium,the distribution ratio D,the separation factor β and the extraction efficiency E were expressed:

where Coand Ce(g·L?1)represent the initial aqueous phase and equilibrium aqueous phase concentration of metal ions respectively.Vaand Vo(ml)represent the volume of the aqueous phase and organic phase.

3.Results and Discussion

3.1.The test experiment

In order to detect the effect of organic synergists on the lithium extraction,the special organic phase and aqueous phase were made.The organic phase was made up of 20 vol%organic synergist and 60 vol%TBP treated as organic extractant and the aqueous phase was made up of 0.288 mol·L?1LiCl,4 mol·L?1MgCl2,0.375 mol·L?1FeCl3and 0.05 mol·L?1HCl.Fig.1 showed the effect of different organic synergists on the lithium extraction.It is obvious that EA,BA,MIBK and DIBK were superior to the others.It is possible that EA,BA,MIBK and DIBK replaced part of TBP which played the role of diluent,and the effectiveTBPwhichonlyplaystheroleofextractinglithiuminthisextraction system was relatively more.Furthermore,the time of phase separation was about 50 s when adding those synergists to the organic extractant,while the time was more than 3 min when no synergist was added.Those organic synergists helped to shorten the speed of phase separation greatly.Therefore,it is necessary to add those synergists to the extractant in the process of extraction.

Fig.1.The selectivity of lithium extraction by different organic synergists.

3.2.The effect of concentration of organic synergist

The effect of concentration of organic synergists on lithium extraction was investigated.The total volume of diluent kerosene was fixed at 20%and the organic extractant was 80%,which was composed of TBP and synergist.It is clear from Fig.2a that the lithium extraction effi ciencyincreasedfirstandthenitdescendedastheconcentrationoforganic synergist increased.Moreover,the lithium extraction efficiency was above 50%when only using synergist as extractant.Fig.2b showed the separation factor of Li/Mg appeared to have the same trend as lithium extraction efficiency as the concentration of organic synergist increases.The reason was that Mg2+competed with Li+in the process of extraction.When the dosage of organic synergist was less than 30%(EA,BA,MIBK)or 40%(DIBK),the Li+ions were extracted more than Mg2+ions.While the dosage of organic synergist was more than 30%(EA,BA,MIBK)or 40%(DIBK),the Mg2+ions were extracted more than Li+ions.Thelithium extraction showed the advantage on theconstant concentration of organic synergist,namely,30%EA,30%BA,30%MIBK and 40%DIBK.

3.3.The effect of concentration of total organic extractant

Considering the economy and environment protection,the use of total organic extractant including the organic extractant and organic synergist should be as little as possible.Under such a prerequisite,the concentration of total organic extractant ranged from 40 vol%to 80 vol%in the process of extraction.The results are shown in Fig.3.As the increase of concentration of total organic extractant increases,the lithium extraction efficiency also increased,the separation factor of Li/Mg presented first rise and then drop.The reason was that Mg2+competedwithLi+intheprocessofextraction.Inordertoguaranteetheextraction efficiency of lithium and the separation factor of Li/Mg,the concentration of total organic extractant was fixed at 60 vol%.

Fig.2.The effect of concentration of organic synergist.

Fig.3.The effect of concentration of total organic extractant.

3.4.The effect of molar ratio of Fe/Li

FeCl3was a more important co-extractant in the TBP extraction system,and it plays a crucial role in the extraction of lithium ion.The dosageofFeCl3hasinfluencedlithiumionextractiongreatly.Therefore,itis essentialtoinquire themolarratioof Fe/Li.Theuse of a small amountof FeCl3will weaken the lithium extraction efficiency,while the excessive useof FeCl3willleadtothehierarchyintheorganic phase.So,themolar ratio of Fe/Li ranging from 0.5 to 3 was implemented in the extraction process.It is clear from Fig.4 that when the molar ratio of Fe/Li was 1.3 the lithium extraction efficiency and the separation factor of Mg/Li were optimum.

3.5.The effect of phase ratio(O/A)

Fig.4.The effect of molar ratio of Fe/Li.

Fig.5.The effect of phase ratio.

Fig.5 showed the effect of phase ratio on the lithium extraction.The four extraction systems presented the same trend nearly in the lithium extraction efficiency,while the trend of the separation factor of Li/Mg was relatively complex.The reason was that Mg2+competed with Li+in the process of extraction as the phase ratio increases.The hierarchy occurred in the organic phase when phase ratios were 0.5 and 1.This phenomenon indicated the FeCl3was excessive to the organic extractant and it expended all extractant.It is the reason that lead to the loaded organic extractant was separated from diluent kerosene.When the phase ratio exceeded 2,the lithium extraction efficiency increased slightly.Therefore,the phase ratio was fixed at 2 for the four extraction system.

3.6.The counter-current extraction

Based on the equilibrium results,the conditions of extraction reaction were determined.In order to reach the equilibrium quickly,the countercurrent extraction was done.The operation art was shown in Fig.6;the phase ratio O/A was 2.During the extraction process of counter-current,thelossoforganicphaseandaqueousphasewasnegligible.Sevenparallel extraction reactions were implemented for the organic phase to reach equilibrium.The lithium extraction efficiency,the separation of lithium in organic phase and aqueous phase and the separation mass ratio of Mg/Li were listed in Table 1.It is obvious that the mixed organic extraction system presents the preponderance on the lithium extraction compared to the TBP system.Especially the separation of lithium in the organic phase and aqueous phase and the separation mass ratio of Li/Mg increased greatly which indicated that the addition of organic synergists improved the extraction environment.

Fig.6.The operation art of counter-current extraction.

3.7.The effect of acidity of stripping agent

The organic phase of counter-current was used in stripping study,andhydrochloric acidwasusedasstrippingagent.Toinvestigatetheeffect of acidity of stripping agent on lithium back-extraction,the acidity values of stripping agent were 0,0.5,1,3,6,8,and 10 mol·L?1.Fig.7a showed whether the acidity of the stripping agent,the lithium reextraction efficiency was above 95%nearly.While the concentration of Fe(III)inthestrippingagentafterre-extractiondecreaseduntiltheacidity of the stripping agent was 6 mol·L?1,the concentration of Fe(III)was nearly the same when the acidity ranged from 6 mol·L?1to 10 mol·L?1.Thephenomenon suggested thestripping agentwould degrade Fe(III)from the loaded organic phase when the acidity ranged from 0 to 3 mol·L?1,which was adverse to the lithium extraction and regeneration of organic extractant.Therefore,the acidity of the stripping agent was fixed at 6 mol·L?1.

3.8.Raman and FT-IR spectrum

The Raman spectrum was applied to analyze the inner information of the extracted complex.The loaded organic phase,the organic phase of counter-current,was detected by Raman.From Fig.8,the main characteristic peaks 108,333,and 380 cm?1of FeCl4?were shown.Therefore,it can be concluded that the complex of FeCl4?formed in the extraction process.The FR-IR spectrum of extracted complexes is showed in Fig.9.The results manifested the functional group P＝O of TBP appeared as rad shift,which shifted the lower wavenumbers 1263,1243,1262,and 1261 cm?1from 1283 cm?1.While the functional groups C＝O(1745,1743 cm?1)of EA and BA and C＝O(1718,1713 cm?1)of MIBK and DIBK had no change nearly,suggesting EA,BA,MIBK and DIBK mainly play the role in separating organic phase from aqueous phase and diluent.The major extractant of organic phase was still TBP.

Table 1 The effect of counter-current extraction

Fig.7.The effect of acidity of stripping agent.

Fig.8.Raman spectrum of extracted complexes.

Fig.9.FT-IR spectrum of extracted complexes.

4.Conclusions

In this study,octanol,EA,BA,MIBK,DIBK,N523 and 8-hydroxylquiolate were used as organic synergist being added to the TBP-FeCl3extraction system to extract lithium from brine.By comparing the lithium extraction efficiency,EA,BA,MIBK and DIBK were determined as the organic synergists.

The concentration of organic synergist was 30%EA,30%BA,30%MIBK and 40%DIBK in the organic phase respectively and the concentration of total organic extractant was 60%in the organic phase.The molar ratio of Fe/Li was 1.3 in the aqueous phase.In extraction process the phase ratio was 2 and the acidity of stripping agent was fixed at 6 mol·L?1.Under the optimize conditions,the results of countercurrent extraction showed the mixed extraction system was the preponderance on the lithium extraction.Especially the separation of lithium in the organic phase and aqueous phase and the separation mass ratio of Mg/Li increased greatly which indicated that the addition of organic synergists improved the extraction environment.

The Raman spectrum showed the complex of FeCl4?formed in the extraction process.The FT-IR spectrum suggested EA,BA,MIBK and DIBK mainly play the role in separating the organic phase from the aqueous phase and diluent.The major extractant of organic phase was still TBP.