Chuanxu Xiao ,Kun Huang *,Huizhou Liu 3,*

1 CAS Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

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

3 Qingdao Institute of Bioenergy and Bioprocess Technology,Chinese Academy of Sciences,Qingdao 266101,China

Keywords:Solvent extraction Measurement Kinetics Constant interfacial area cell Er(III)extraction EHEHPA

A B S T R A C T A novel constant interfacial area cell(NCIAC),by spatially separating the agitation from liquid flow circulation of organic and aqueous two phases,was suggested to obtain detailed kinetic data for Er(III)extraction from chloride medium by 2-ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester(EHEHPA).Different from the traditional Lew is cell and the constant interfacial area cell with laminar flow,the concentrations of Er(III)in organic and aqueous two phases were uniform,and the stability of the interfacial area between the two phases could be controlled effectively.Therefore,the special requirements for the design of agitators in the traditional Lew is cell and the constant interfacial area cell for minimizing the influence of diffusion resistance could be avoided.Experimental results indicated that the extraction kinetics was mainly affected by the aqueous flow rate,interfacial area between organic and aqueous two phases,and the aqueous pH values.An extraction kinetic equation was suggested based on the experimental data.

1.Introduction

Solvent extraction is widely employed to separate or/and enrich target components in the field of chemical industry,hydrometallurgy,etc.The extractors provide the place where the extraction reaction occurs.The cost of extractor makes up a large proportion of the total investment in equipment.The study in extraction kinetics plays an important role in determining the type and size of the extractors[1–8].

Generally,extraction kinetics data can be determined by using a constant interfacial area cell.The design of a constant interfacial area cell is concerned with two key factors:the concentration of target components in each phase being uniform,and the interface between organic and aqueous phases being stable.The uniform concentration of target component means that its concentration may vary with the extraction time but keeps independent of position in each phase.The stable interface between two phases means that its area is fixed during an experiment.The uniformity of concentration in each phase is beneficial for minimizing the effect of diffusion resistance in individual phase.The rate of liquid–liquid extraction in extractor can be controlled by either diffusion or the chemical reactions at the interface,but sometimes both at the same time[9].When the kinetics of extraction is found independent of the agitating intensity in individual phase,it indicates that the effect of diffusion in the bulk phase is minimized.The activation energy is another criterion generally used to identify the ratedetermining step[10].In general,if the extraction rate is controlled by a chemical reaction at the interface,the activation energy will be more than 40 kJ·mol?1;however,if a diffusion process at the interface controls,the activation energy is often less than 20 kJ·mol?1[11].

To date,there are two basic prototypes of constant interfacial area cell.The first one proposed by Lew is[12],called Lew is cell(LC,as shown in Fig.1),was a cylindrical cell in which each phase was agitated separately by its own agitator to make the concentration uniform.The interface between two phases was an opening in the interfacial plate separating two phases.Organic extractant phase and aqueous phase contacted with each other through the interface.The area of the interface may be altered by replacing another interfacial plate with different opening areas.In LC,each liquid phase is driven by an agitator from the center to all around to form a disturbed circulation through the cylindrical grids and horizontal baffles.The diversified directional liquid flow makes both phases well agitated[13].Generally,there is a contradiction between how to achieve a more uniform concentration in each phase and a less disturbed interface in an experiment[9].These two features could not always be met simultaneously in LC,because excessive intensity of agitation would destroy a stable interface between two phases.Considering those reports,it was known that the hydrodynamics in the cell was still poorly defined,and therefore,many developments were made to improve the stability of the interface[14].

Fig.1.Lew is cell.(1:axis;2:cylindrical cell;3:organic phase;4:aqueous phase;5,6:agitator;7:interfacial plate).

The second prototype,called the constant interfacial area cell with laminar flow(CIACLF)as shown in Fig.2,was designed by Zheng et al.[15].CIACLF consists of two rectangular cells separately for two phases.Each cell has an agitating chamber with an agitator of special design and a flowing chamber.Organic extractant phase and aqueous phase are agitated separately by their own agitators to make the concentration uniform in their own agitating chambers.Different from LC,the agitating chamber and the flowing chamber are connected through two apertures,and each fluid was circulated within the cell with the help of the horizontal guideplatelaid in each flow chamber.In the CIACLF,the fluid circulates as Poiseuille flow with a parabolic velocity profile in parallel to the guide plate shown in Fig.2[15].The contact interface area between two phases is the opening in the interfacial plate separating two phases.The flow of two phases is both laminar near the interface between two phases because of low velocity of circulation near the interface.The uniform concentration of target component can be fulfilled by intensively agitating in respective agitating chamber,while stable area of the interface is ensured by the laminar flow of two phases near the interface.Researchers[16–18]obtained extraction kinetics data in several extraction systems by using CIACLF.

Fig.2.Constant interfacial area cell with laminar flow[15].(1:constant interfacial area cell with laminar flow;2,3:agitator;4:guide plate for organic phase;5,6:agitating chamber;7:guide plate for aqueous phase;8: flowing chamber for organic phase;9: flowing chamber for aqueous phase).

However,a slight change in agitator configuration,installation position or physiochemical characteristics of the two phases will result in the deviation from the requirements both for uniformity of concentration of target component in each phase and the stability of the interfacial area of contacting,because the flow conditions of two phases are susceptible to above three factors in the cell designed by Zheng et al.[15].

In this work,a novel constant interfacial area cell(NCIAC)by spatially separating the agitating operation and flow circulation was suggested for the purpose of acquiring the extraction kinetic data.The difficulty for special design of the agitator can be avoided.There liability of using the suggested NCIAC to obtain the extraction kinetics was also testified.Extraction kinetics of erbium ion from chloride medium by 2-ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester(EHEHPA)as extractant was studied in the NCIAC.The effects of the aqueous flow rate,interfacial area between two phases,and pH in aqueous phase on the extraction rate were investigated systematically.

2.Experimental

2.1.Materials

2-Ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester(EHEHPA,known by trade name “Ionquest 801”,“P507”and “PC-88A”),purchased from Shanghai Rare-earth Chemical Co.Ltd.,was used as the extractant.Kerosene,purchased from Xinhuancheng(Beijing)Co.Ltd.,was used as diluent for EHEHPA.Both EHEHPA and kerosene were used as received,without any further purification or modification.Er2O3of AR grade was supplied from Alfa Aesar(China).Stock solutions of Er(III)was prepared by dissolving Er2O3in hydrochloric acid solution.The other reagents used were of AR grade.Aqueous phase was prepared by diluenting stock solutions of Er(III)with deionized water.

2.2.Equipment

The NCIAC extraction system includes a NCIAC and other accessories as shown in Fig.3.The accessories refer to pump and agitating chamber for each phase.The pumps used in the experiment are circulation pump(TSD01–01,Lead fluid,China),which were used to maintain the flow rates precisely for each phase.Two 200 ml beakers were used as the agitating chambers for two phases.Uniform concentration of Er(III)in individual phase was fulfilled by respective self-made agitators in the agitating chambers.The agitator used is shown in Fig.4.The structure of NCIAC suggested in this work is shown in Fig.5.The NCIAC is made of polymethyl methacrylate(PMMA)and includes two rectangular cell for each phase.The volume of aqueous phase in the beaker was less than 50 ml,as well as the volume of the organic phase.There is an opening in the bottom of the rectangular cell for organic extractant phase.This is also the room for the interfacial plate,and the two phases contact is supposed to be within this opening.Several interfacial plates with different sizes of opening are made of stainless steel.The aqueous phase level was 10 mm,and the organic extractant phase level was about 12 mm in the NCIAC.

A pH meter(pH 211,Hanna,Italy)was used to measure the pH value of aqueous phase.The aqueous sample was taken from the sample location,which was labeled in Fig.3.The concentration of Er(III)in the sample of aqueous phase was measured using Optima 7000DV inductively coupled plasma-optical emission spectrometer(ICP-OES)(Perkin-Elmer,USA).

2.3.Experimental procedure

The NCIAC system was employed to measure extraction kinetics.The volume of aqueous phase containing Er(III)was285 ml and the volume of organic extractant phase was 170 ml.The pH value was 3.0 in aqueous phase unless stated.The 1.50 mol·L?1EHEHPA-kerosene was used as organic extractant phase unless stated.The concentration of Er(III)in aqueous phase prepared by stock solution was about 150 mg·L?1.The aqueous phase and the organic phase were introduced into NCIAC from respective agitating chambers.The agitating speed were both more than 400 r·min?1in order to maintained the uniform concentration of Er(III)in each phase.Flow rate of each phase was controlled carefully in order not to disturb the interface between two phases.After certain intervals of time(usually 20 min),0.1 ml of the sample for analysis was with drawn from the aqueousphase.All experiments were carried out at 298 K±2 K.In most experiments,the interfacial area was kept at 15 cm2,unless otherwise stated.

Fig.3.Sketch diagram of novel designed constant interfacial cell system.(1:NCIAC;2:rectanglecell for extractant phase;3:rectangle cell for aqueousphase;4:pump;5:aqueousagitating tank;6:extractant agitating tank;7:interfacial plate;8:sample location).

For the equilibrium experiments,equal volumes(10 ml)of two phases were mixed and shaken for 30 min at 298 K±2 K.The distribution coefficient(Do-a)was defined as the ratio of the concentration of Er(III)in the organic phase to that in the aqueous phase when equilibrium reached.

2.4.Data analysis

The experimental data were analyzed based on the approach proposed by Danesi and Vandegrift[19].The function of f(ca,t)

was plotted against extraction time t for each experiment.Here Vaand Vorepresent the volume of aqueous phase and extractant phase,respectively.ca,irepresents the initial concentration of Er(III)in aqueous phase.ca,trepresents the concentration of Er(III)in aqueous phase at time t.Do-ameans the o/a distribution coefficient of Er(III).All plots were straight lines indicating that the mass transfer process could be formally treated as a pseudo- first order reversible reaction[18,20]with respect to Er(III),that is,

In Eq.(2),ka-o(cm·s?1),and ko-a(cm·s?1)represent the observed forward mass transfer coefficient and the backward mass transfer coefficient,respectively.ka-o(cm·s?1),and ko-a(cm·s?1)are the determined values under known conditions.ca,tand co,trepresent the concentration of Er(III)in aqueous phase and organic extractant phase at time t,respectively.The ca,tcan be obtained by ICP-OES,while the co,tcan be calculated by mass balance.

The rate that the number of mole for Er(III)entered into the organic phase at time t,d n/d t,is given by

where S means the interfacial area between two phases.The S is the area of opening of interfacial plate.The S is a known value in each experiment.

At steady state,Eq.(3)equals to zero,and reads

w here ca,eand co,erepresent the equilibrium concentrations of Er(III)in aqueous phase and extractant phase,respectively.Then,Eq.(4)reads

With the definition of distribution coefficient(Do-a),

and the mass conservation of Er(III)during extraction,

substituted into Eq.(3)and simplifying,the following Eq.(8)is obtained:

The slopes of the plots of left hand against extraction time t from Eq.(8)can be used to determine the ka-o.

Wang et al.[17]and Xiong et al.[21]determined the ko-awhen the volume of aqueous phase equals to that of extractant phase according to

Compared with Eq.(9),Eq.(8)is more general because the volume of aqueous phase and the volume of extractant are not always the same.Eq.(9)is aparticular case of Eq.(8)when constant interfacial area cell is employed to determine the extraction kinetics.

Fig.4.Agitator.(1:sketch diagram;2:front view;3:top view).

Fig.5.The structure of NCIAC.(1:front view;2:top view).

For example,the experiment was conducted with the conditions as follow s.The initial concentration of Er(III)in aqueous phase was about 120 mg·L?1with pH at 3.0.The volume of aqueous phase was 285 ml.The interfacial area between phases was 15 cm2.The flow rate of aqueous phase was 6.34 ml·s?1.The organic phase was:concentration of EHEHPA of 1.50 mol·L?1,the volume 170 ml,and the flow rate of 2.39 ml·s?1.The sampling interval was 10 min.

Fig.6.Plots of c a,t with the increase of extraction time t.Aqueous phase:conc.of Er(III)120 mg·L?1;volume 285 ml;p H 3.0; flow rate 6.34 ml·s?1.Organic extractant phase:1.50 mol·L?1 EHEHPA-kerosene;volume 170 ml; flow rate 2.39 ml·s?1.The opening area of interfacial plate:15 cm2.

The ca,twas determined by ICP-OES,and the co,twas obtained from mass balance.The first sample of aqueous phase is called the initial sample,and the concentration of Er(III)in the first sample is defined as the initial concentration.Fig.6 showed the variation in ca,twith the increase of extraction time.The extraction rate was analyzed by fitting the Er(III)remaining concentration in the aqueous phase,ca,t(mg·L?1),verse the extraction time(t),with the pseudo- first order model equation ca,t=m·exp.(?ka-o·t)+b,where ka-o(cm·s?1)is the observed forward mass transfer coefficient,t is the extraction time(min),m is the Er(III)concentration decrease coefficient due to extraction,and b is the aqueous concentration of Er(III)after infinite extraction(ca,tat t→∞)[22].Then the f(ca,t)can be calculated according to Eq.(1).The experimental data was regressed according to the Eq.(8),which is shown in Fig.7.The preliminary finding from the Fig.7 is that f(ca,t)is linear with extraction time t,then the ka-ocan also be obtained as 0.00457 cm·s?1by the slope method[21].

Fig.7.Plots between f(c a,t)and extraction time t.Aqueous phase:conc.of Er(III)120 mg·L?1;volume 285 ml;p H 3.0; flow rate 6.34 ml·s?1.Organic extractant phase:1.50 mol·L?1 EHEHPA-kerosene;volume 170 ml; flow rate 2.39 ml·s?1.The opening area of interfacial plate:15 cm2.

Though,the extraction reaction takes place immediately when aqueous phase and extractant phase contact with each other,and it takes a few minutes to establish the steady recirculation of two phases.The concentrations of Er(III)in the aqueous phase with the steady recirculation is different to the concentration of Er(III)in the newly prepared aqueous solution.This initial change of concentration of Er(III)in aqueous phase has no effect on the extraction kinetics based on the pseudo- first order model with respect to Er(III),because of the linear relationship between the f(ca,t)and extraction time t,as commented in[17,18,20,21,23–25].

3.Results and Discussions

3.1.Effect of flow rates of two phases on the observed extraction rate constant

The dependence of the extraction rate on the agitating speed is the criterion to minimize the effect of diffusion resistance in the cell designed by both Lew is[26,27]and Wang et al.[25],and the observed extraction rate constant was used to justify the minimization of diffusion resistance when “plateau region”appeared in the plot of the observed extraction rate constant versus agitating speed.For the NCIAC,the criterion to minimize the effect of diffusion is turned to the dependence of the observed extraction rate constant on the flow rate of phases.At constant composition,the ka-ofor Er(III)at different flow rates of aqueous phase was investigated,and the results are shown in Fig.8.When the flow rate of aqueous phase ranges from 0.79 ml·s?1to 4.72 ml·s?1,the observed extraction rate constant increases gradually,which indicates that the diffusion resistance plays an important role in the extraction process.When the flow rate of aqueous phase is above 4.72 ml·s?1,the extraction rate is nearly constant,which indicates that the diffusion resistance is minimized to affect the extraction rate.The concentration of EHEHPA-kerosene in organic extractant phase is far more than that required for complete extraction of Er(III).Diffusion resistance of extracted complex in extractant phase can be ignored.The lower flow rate of extractant phase will not bring diffusion resistance when it is 1.62 ml·s?1.

Fig.8.Effect of aqueous flow rate on k a-o.Aqueous phase:Volume 285 ml,p H 3.0,initial concentration of Er(III)about 150 mg·s?1.Extractant phase:volume 170 ml,1.5 mol·L?1 EHEHPA-kerosene, flow rate 1.62 ml·s?1.Interfacial area between two phases:15 cm2.

3.2.Effect of specific interfacial area on the extraction rate

The rate-determining step that controls the extraction rate in a kinetic regime can occur either in the aqueous bulk phase or at the interface between two phases.One of the important criteria to differentiate whether the rate-determining step that controls the rate of extraction in a kinetic regime occurring in the bulk phase or at the interface is the relationship between the extraction rate variety and the specific interfacial area.If the slow chemical reaction occurs in the bulk phases,the extraction rate would be independent of the specific interfacial area.On the contrary,reaction occurring at the interfacial zone would show direct proportionality between the extraction rate and the specific interfacial area[25].Experiments were conducted with the interfacial areas of 2,4,5,8,10,15 cm2in the interfacial plate and other conditions unchanged.The results in Fig.9 show a linear relationship between the interfacial area and the ka-o,which proves that the rate-determining step of extraction is at the interface.

Fig.9.Effect of specific interfacial area on k a-o.Aqueous phase:Volume 285 ml,p H 3.0,Er(III)150 mg·L?1, flow rate 6.34 ml·s?1.Extractant phase:Volume 170 ml,1.5 mol·L?1 EHEHPA-kerosene, flow rate 1.62 ml·s?1.

A further criterion to differentiate whether the chemical reaction or diffusion controls the rate of extraction occurring at the interface is activation energy.In general,if the overall extraction rate is controlled by a chemical reaction,the activation energy is usually more than 40 k J·mol?1,on the contrary,if the overall extraction rate is controlled by a diffusion process,the activation energy is less than 20 kJ·mol?1.Otherwise,the overall extraction rate is controlled by both chemical reaction and diffusion process[11,25].

Fortunately,researchers have studied the activation energy of Er(III)extracted by 2-ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester in kerosene.For example,Yue[28]reported the activation energy of Er(III)extracted by 2-ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester in kerosene,and they thought the rate of extraction was controlled by mass transfer resistance.Considering the activation energy of Er(III)extracted by 2-ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester in kerosene,we presume that the rate of extraction was most probably controlled by the diffusion resistance at the interface.

According to traditional explanation,the solubility of EHEHPA at the aqueous phase plays an important role on the extraction rate.There is a slim chance that rate-determining reaction occurs in bulk aqueous phase because of the relative insolubility of the extractant in the aqueous phase.

3.3.Effect of contration of H+on the observed extraction rate constant

The H+has an effect on the extraction rate in many ways.More than 96%of EHEHPA exists as dimer in kerosene because the dimerization constant is very large[29].The dimer can only be absorbed onto the interface between two phases.Then the dimer is depolymerized into monomer at the interface,and the monomer hydrolyzes into H+and EHEHPA?.H+also influences the ion-atmosphere of Er(III).Er(III)is surrounded by several water molecules,for example,[Er(H2O)n]3+,but[Er(H2O)n]3+does not coordinate with Cl?because of the weak interaction between them.The mass transfer of Er(III)from aqueous bulk phase to the interface is affected by the concentration of H+,so does the formation rate of extracted complex of Er(III).Extraction experiments with different concentration of H+w ere conducted for elucidating the effect of H+.The concentration of H+was adjusted by hydrochloric acid and sodium hydroxide.The result of the observed extraction rate with different pH values is shown in Fig.10.

Fig.10.Effect of pH on lg k a-o.Aqueous phase:Volume 285 ml,Er(III)about 150 mg·L?1,flow rate 6.34 ml·s?1.Extractant phase:Volume 170 ml,1.5 mol·L?1 EHEHPA-kerosene, flow rate 1.62 ml·s?1.Interfacs:15 cm2.

It could be seen from Fig.10 that pH affects the observed extraction rate constant obviously in a linear way from 2 to 4 and the slope is about 0.1.The slope is defensibly zero and indicated the reaction was H+independent process[25].How ever,the observed extraction rate constant are almost the same from the pH 1 to 2.The result indicates that extraction mechanism changes when the concentration of H+is high.The Er(III)extraction with EHEHPA is a cation exchange at low concentration of H+,but the Er(III)extraction mechanism would change to solvation at higher acidity according to Sato and Ueda[30].In addition,dissociation of EHEHPA is inhibited at higher acidity,and electron donor is the oxygen atom on phosphine group in the molecule of EHEHPA[31].On the other hand,at higher acidity,ionic atmosphere of[Er(H2O)n]3+was relatively larger than that at lower acidity according to Debye–Hückel theory[32],therefore,the resistance of Er(III)mass transfer increases.And the distribution ratio of Er(III)(Do-a)changes,for example,its value is 180 at pH 3.0,while the value is 34 at pH 2.0.These reasons together affect the extract rate with EHEHPA in kerosene.

3.4.Effect of EHEHPA concentration on the observed extraction rate constant

EHEHPA can be concentrated at the interface between phases due to their interfacial activity and create a surface excess.Part of EHEHPA dissociates and diffuses,and an interface double layer forms.The polar group=P(OH)O of the EHEHPA forms and extends into the aqueous layer in the aqueous side of the interface[13].The concentration of EHEHPA affects the amount of EHEHPA which adsorbs on the interface,and then the amount of polar group=P(OH)O of EHEHPA.Experiments with EHEHPA concentration changed from 0.253 to 1.520 mol·L?1were conducted,and the result of the observed extraction rate constant with different concentration of EHEHPA is shown in Fig.11.

Fig.11.Effect of concentration of EHEHPA on k a-o.Aqueous phase:Volume 285 ml,Er(III)150 mg·L?1, flow rate 6.34 ml·s?1.Extractant phase:Volume 170 ml, flow rate 1.62 ml·s?1.Interface:15 cm2.

It can be observed from Fig.11 that the observed extraction rate is positively correlated with the square root of the concentration of EHEHPA in the bulk extractant phase.EHEHPA is mostly dimeric in kerosene as above mentioned.Interfacial tension and composition are affected by the concentration of EHEHPA in extractant phase according to the Gibbs adsorption equation.The relationship between the interfacial tension and the concentration of EHEHPA in n-heptane was studied by Kamio et al.[33],and they found that the interfacial tension decreased with the increase of concentration of EHEHPA in n-heptane from 0.1 mol·L?1to 1.5 mol·L?1.This phenomenon can be interpreted that the monomer of EHEHPA is adsorbed at the interface between two phases.In this study,the characteristics of kerosene is similar to n-heptane,then the behavior of EHEHPA in kerosene can be regarded as the same with that in n-heptane approximately.The monomer of EHEHPA dissociatesat the interface between two phases,and its anionic form extends to the side of aqueous phase.The relationship between the viscosity and the concentration of EHEHPA in n-heptane was also studied by Kamio et al.[33],and they found that viscosity of the system was almost the same when the concentration of EHEHPA in n-heptane changed from 0.1 to 1.5 mol·L?1.The diffusion resistance is correlated to the viscosity of the system according to Kislik[34].Therefore,the diffusion resistance of the extracted complex formed at the interface is almost the same despite of the changes in the concentration of EHEHPA in extractant phase.

Combining all the experimental results,the extraction rate was affected obviously both by pH of the aqueous phase and the concentration of EHEHPA in the extractant phase.The extraction can be regarded as a simple order reaction,and therefore:

Here,H2L2is the dimeric EHEHPA.k is the reaction rate constant,which can be calculated as 2.15 × 10?4(cm2.5·mol-0.5·s?1)according to the observed extraction rate constant.Then the extraction reaction rate equation of Er(III)from aqueous phase by EHEHPA in kerosene can be expressed as

which is applicable for the conditions when aqueous pH changes in the range from 2 to 4,concentration of Er(III)is less than 150 mg·L?1,and concentration of EHEHPA in the organic phase is ranged from 0.253 to 1.520 mol·L?1.

Previous experimental results reported by Yue[28]indicated that,the extraction reaction rate was inversely proportional to the concentration of H+.How ever,the effect of H+concentration on the extraction rate can be achieved through many ways.In present work,it was demonstrated that the extraction reaction rate was proportional to the p H.The extraction reaction rate varied with the pH other than the concentration of H+.The present work proved the feasibility of NCIAC used to measure the extraction rate.

4.Conclusions

A novel constant interfacial area cell(NCIAC),featured with spatially separation of the agitation and flow circulation,was demonstrated effective for determining the extraction kinetics of Er(III)from aqueous chloride medium by 2-ethyl-hexyl-phosphonic acid mono-(2-ethylhexyl)ester(EHEHPA).The extraction kinetics of Er(III)was mainly affected by the aqueous flow rate,interfacial area between organic and aqueous two phases,and the aqueous pH value.

The present work demonstrated that the suggested constant interfacial area cell could obtain satisfactory extraction kinetic data compared to the previous works reported in the literature.Compared with the previously reported Lewis cell and the early constant interfacial area cell,the uniformity of the concentration of target species in organic and aqueous phases as well as the stability of the interfacial area between the two phases could be controlled in the NCIAC.Since the agitation and recirculation of individual phase were fulfilled respectively by agitators and pumps,the requirements for special design of the agitators to eliminate the influence of uniformity could be avoided.The suggested NCIAC in this work is expected to find widely application in determining extraction kinetics.

Nomenclature

b concentration of target component after infinite extraction,mg·L?1

ca,econcentration of target component in aqueous phase when equilibrium,mg·L?1

co,econcentration of target component in extractant phase when equilibrium,mg·L?1

ca,iconcentration of target component in aqueous phase at time 0,mg·L?1

ca,tconcentration of target component in aqueous phase at time t,mg·L?1

co,tconcentration of target component in extractant phase at time t,mg·L?1

Do-ao/a distribution coefficient of Er(III)

H2L2dimeric EHEHPA,mol·L?1

k reaction rate constant,cm2.5mol-0.5s?1

ka-oobserved forward mass transfer coefficient,cm·s?1

ko-aobserved backward mass transfer coefficient,cm·s?1

m the target component concentration decrease coefficient due to extraction

n amount of substance,mol

R extracion reaction rate of Er(III),mg·s?1·cm?2

S interfacial area,cm2

t extraction time,min

V volume of aqueous phase or organic phase,ml

Vavolume of aqueous phase,ml

Vovolume of organic extractant phase,ml