Thermodynamical and catalytic aspects of zinc separation from aqueous solution

Sajjad Aghazadeh,Mahdi Gharabaghi*,Ziaedin Shafaei

School of Mining Engineering,College of Engineering,University of Tehran,Tehran,Iran

Keywords:Catalyst Ni-Raney D2EHPA Ionic strength Separation Solvent extraction Thermodynamic

A B S T R A C T The present research work examines extraction mechanism of zinc by D2EHPA(Di-2-ethyl hexyl phosphoric acid)and comprehensively studies the main effective parameters on the process.Results of thermodynamic experiments showed that zinc extraction by D2EHPA was endothermic and spontaneous,and thermodynamic parameters including entropy and enthalpy were+27.37 J·mol-1·K-1 and 25.21 kJ·mol-1,respectively.Gibbs free energy was varied between-7.21 kJ·mol-1 and-8.41 kJ·mol-1 with the variation of temperature from 20 °C to 70 °C.Solution ionic strength was increased by addition of potassium and lithium sulfate solution while addition of calcium sulfate decreased ionic strength whereby zinc extraction efficiency was also decreased.TBP showed positive synergism at concentration of 5%(v/v)and negative synergism effect at concentrations of 2%and 10%.Simultaneous addition of both TBP and salt caused extraction efficiency to drop significantly and lower both TBP and ionic strength efficiency.Results showed that a continuous addition of TBP tends to effectively improve the zinc extraction efficiency.Experiments in the presence of catalyst Ni-Raney demonstrated that zinc extraction kinetic increases remarkably and due to easy recycling of the catalyst,we can propose a novel idea in solvent extraction field.

1.Introduction

Pioneering works on the application of solvent extraction method in separation industries were done during years 1950–1970.The method of solvent extraction is commonly used for selective extraction of impurities from solutions[1].Di-2ethylhexyl phosphoric acid(D2EHPA),is a selective for zinc and some metals over most the species deleterious to electrowinning and is readily stripped by acids(mainly H2SO4)[2,3].It may be used alone or in combination with other extractants such as tributyl-phosphate(TBP)as an effective phase modifier in order to enhance the phase disengagement conditions,avoid third phase formation,reduce the viscidity of the loaded organic,and improve the compatibility of the extractant[4].

In 1983,Vazarlis and co-workers[5]used D2EHPA diluted by cyclohexane or kerosene for zinc extraction from copper and iron sulfide solution.Kinetic and mass transfer of zinc were studied to measure the rate of zinc extraction at different chemical composition by varying zinc,hydrogen ion and D2EHPA concentrations,also,the application of a newly developed Hancil cell[6]revealed that the use of carbon does not affect the kinetics of zinc extraction.The separation studies of cadmium from zinc by mixtures of carboxylic acid and trialkylphospine showed that D2EHPA still remains a strong,selective extractant for zinc[7].Zinc extraction by different extractants such as LIX groups,Aliquat 336 and etc.,has been thoroughly investigated[8].In this regard,D2EHPA and TBP have always been of paramount importance for zinc extraction and separation mechanism.Therefore,Tables 1 and 2 summarize the most prominent research done on the zinc extraction by TBP and D2EHPA.

Ionic strength is a measure of the total ion concentration in solution;however,ions with more charge are counted more due to stronger electrostatic interactions with other ions.The equation below describes ionic strength as:

where Ciis the concentration of i th specie,Ziis the charge on i th one and μ is the ionic strength.Over the last few decades,a lot of researches have been focused on ionic effects on the extraction yield of metals from aqueous solutions.In this regard,salting out effect plays a key role in solvent extraction of metals.It is a purification method which utilizes reduced solubility of certain molecules in a solution of very high ionic strength.Salting out strongly depends on the Hoffmeister series(i.e.the stronger ion strength in Hoffmeister series,the stronger effect of salting out)[22].The order of Hoffmeister series follows as:

Table 1A summary of zinc solvent extraction studies by TBP

Recently,Nickel Raney catalysts are widely used for a wide variety of applications and industries such as organic chemistry.It is commonly used to improve kinetic of reactions by activating its surface with reagents such as sodium hydroxide.The activation of A/Ni systems is the so-called Raney process,patented by the American engineer Murray Raney in 1925[23].

In this investigation,zinc extraction from solution by D2EHPA was investigated.In order to optimize results and decreasing reaction time,nickel Raney catalysts were used.Chemical and thermodynamic parameters were also calculated to determine reaction mechanism and ionic strength and various ion effects on the process were also determined.

2.Experimental

Zinc extraction from synthetic sulfate solution was investigated using D2EHPA(Merck Co.,Germany,95%purity)as extractant and TBP(Merck Co.,Germany,99%purity).D2EHPAwas diluted in Kerosene supplied by Sigma Aldrich,UK.Ionic strength experiments were carried out using salt solutions of K2SO4,CaSO4·2H2O,and Li2SO4·H2O(Merck Co.,Germany,99%purity).H2SO4and NaOH were used to adjust pH of aqueous phase.

2.1.Zinc extraction by D2EHPA

The effects of three important parameters including ionic strength of solution,thermodynamic conditions and catalyst addition were investigated.First,a standard zinc solution with concentration 20 g·L-1was prepared in a volumetric flask(capacity of 500 ml).Then,D2EHPA was diluted by kerosene to a required concentration.Finally,zinc extraction was continued by mixing aqueous and organic phases for a specified period of time.After each test,a sample from aqueous phase was prepared and sent for chemical analysis by atomic absorption spectroscopy(model:Varian 20,Australia)to measure zinc content.

2.2.Synergism experiment

Synergistic effect of D2EHPA with TBP was investigated in different concentrations of both extractants.TBP was added into D2EHPA/Kerosene system and after each test about 10 ml of aqueous solution was taken and analyzed for remained amount of Zn2+by Atomic Adsorption Spectrometry(Model 20,Varian Co.,Australia).

2.3.Ionic strength of solution

Ionic strength effects were examined using lithium,potassium,and calcium sulfate solutions in different concentrations.In each test,salt solution was added into aqueous phase and then mixing of two phases(organic-aqueous phases)was continued for a specified period of time.Finally,about 10 ml of aqueous solution was taken and analyzed to determine Zn2+concentration.

2.4.Catalyst experiment

Aluminum–nickel alloy(Ni–Al 50:50 wt%,Density 3.46 g·ml-1at 20°C,Sigma Aldrich co.)was used to produce Ni Raney catalyst.About 20%aq.NaOH was added to 1 g Ni–Al alloy powder and then stirred in an ice bath beaker for 1 h by Magnetic stirrer(KAYAGAKI KMS 1).After each test,mixed solution was washed with deionizedwater to reach a predetermined pH value,removing 80%aluminum,and activating Nickel.Finally,Nickel Raney catalyst was immediately added into zinc aqueous phase and then extraction process continued using D2EHPA as extractant.

Table 2A summary of zinc solvent extraction by D2EHPA

In this research work,in order to quantify the zinc extraction,the following equations were used:

3.Results and Discussion

3.1.Thermodynamic experiments

For thermodynamics,a thermodynamic state of D2EHPA/Kerosene is fully identified by examining zinc extraction efficiency in different temperature values[24].Then,thermodynamic parameters should be determined by using plots of lg D-pH,lg D-lg[D2EHPA],and equation below[25]:

X and Y are coefficient which should be determined using plots lg D-pH and lg D-lg[D2EHPA],respectively.

By considering plots 1 and 2,X and Y values in Eq.(1a)were obtained 1.83 and 1.67,respectively.Fig.1,gives a slope of1.83 for zinc extraction which indicates that the release of 2H+into aqueous phase leads to the extraction of zinc as ZnR2(org).Fig.2 gives a slope of 1.67 which indicates that consuming 2 mol of HR2(org)tends to extract 1 mol of Zn2+from aqueous phase.Therefore,Eq.(1a)could be completed as:

Zinc extraction mechanism also will be obtained as:

Fig.1.Plot of pH effect on zinc distribution coefficient(stirring speed:480 r·min-1,contact time:10 min,zinc initial concentration:1 g·L-1,O/A ratio:1.5,Temp:20 °C).

Fig.2.Plot of D2EHPA concentration effect on zinc distribution coefficient(stirring speed:480 r·min-1,contact time:10 min,zinc initial concentration:1 g/L,O/A ratio:1.5,Temp:20°C).

By obtaining Eqs.(1b)and(2),one could determine thermodynamic parameters by using plot of Eq.(1b).

Enthalpy ΔH is a thermodynamic state function.When it is positive,indicating that the reaction system is endothermic and negative means that the reaction system is exothermic[26].By considering slope of Fig.3 which is about-1.32 and Eq.(1b),enthalpy was calculated about ΔH=+25.21 kJ·mol-1which means that zinc extraction by D2EHPA is endothermic.Entropy is commonly known as a measure of system disorder[27].Based on y-intercept of Fig.3(i.e.+6.73)and Eq.(1b),entropy was calculated about ΔS=+27.3 kJ·mol-1which implies that disorder of the system increases because the number of particles increase as two aqueous particles(2H+)are produced.

Gibbs free energy is used to determine whether reaction is feasible or not[28].By considering equation below,Gibbs free energy was calculated and results were given in Table 3.

Table 3 describes that Gibbs free energy changes from-7.21 kJ·mol-1to-8.44 kJ·mol-1.Based on the thermodynamic results,zinc extraction by D2EHPA is endothermic,entropy favored and spontaneous.

Fig.3.Plot of(1000/T)versus lg D(stirring speed:480 r·min-1,contact time:10 min,zinc initial concentration:1 g·L-1,O/A ratio:1.5,pH=3,[D2EHPA]=15%).

Table 3Gibbs free energy in different temperature values

3.2.TBP synergistic effect

TBP shows a synergistic effect with D2EHPA extractant;however,it is not a selective and strong extractant for zinc when it is used alone[18].It shows a strong and positive synergistic effect within a specific range of concentration.As it appears from Fig.4,zinc extraction efficiency increases for all D2EHPA concentrations(5%,15%,and 20%)when TBP concentration is 5%.When TBP concentration is 2%,zinc extraction decreases because TBP concentration is very low and fails to disengage two phases.Therefore,it restricts D2EHPA from reaching zinc particles.Also,when TBP concentration is 10%due to the formation of hydrogen bonds between D2EHPA and TBP zinc extraction efficiency tends to decrease.

3.3.TBP spraying effect

In this study,the effect of TBP spraying effect was investigated for the first time.A specific amount of TBP was continuously added into the organic phase(i.e.D2EHPA/kerosene)until the end of each experiment.Results were given in Fig.5.It shows that adding TBP continuously improves the extraction efficiency of zinc for all the experiments.Because continuous addition of TBP gives a good opportunity for D2EHPA particles to effectively extract Zn2+from the aqueous solution.However,when there is no continuous addition of TBP(i.e.without spraying)into organic phase and it is added only in the beginning of each experiment,extraction efficiency is low.

3.4.Combined effect of TBP and ionic strength

Fig.4.Synergistic effect of TBP on zinc extraction by D2EHPA(stirring speed:480 r·min-1,contact time:10 min,zinc initial concentration:1 g·L-1,O/A ratio:1.5,pH=3,[D2EHPA]=15%).

Fig.5.Spraying effect of TBP on the zinc extraction efficiency(stirring speed:480 r·min-1,contact time:10 min,zinc initial concentration:1 g·L-1,O/A ratio:1.5,pH=3,[D2EHPA]=15%).

Fig.6.Combined effect of the TBP and ionic strength on zinc extraction efficiency(stirring speed:480 r·min-1,contact time:10 min,zinc initial concentration:1 g·L-1,O/A ratio:1.5,pH=3,[D2EHPA]=15%).

Lithium,potassium,and calcium sulfate were added into aqueous phase and TBP into organic phase in different concentrations in order to examine the combined effect of TBP and ionic strength.Results were given in Fig.6.As it can be seen from Fig.6,zinc extraction efficiency tends to decrease when TBP is added into lithium,potassium and calcium salts/D2EHPA/kerosene system.It is due to the fact that TBP addition neutralizes its positive synergist effect and ionic strength of sulfate salts by extracting calcium,lithium,and potassium from the solution.In other words,TBP functions as an extractant in the presence of impurities such as calcium,lithium,and potassium[29].

Fig.7.Nickel Raney catalyst effect on zinc extraction efficiency(stirring speed:480 r·min-1,zinc initial concentration:1 g·L-1,O/A ratio:1.5,pH=3,[D2EHPA]=15%).

3.5.Catalyst effect

Nickel Raney catalyst is widely used in many industries because of its stability in changing conditions and higher activity in room temperature[30].The effect of Nickel Raney catalyst on the solvent extraction of zinc by D2EHPA was investigated for the first time.Result was given in Fig.7.As it can be seen from Fig.7,Nickel Raney addition into aqueous phase strongly increases zinc extraction kinetics.When there is no catalyst in the aqueous phase,maximum extraction efficiency was 94.03%in 10 min;however,by adding Nickel Raney catalyst into aqueous solution zinc extraction efficiency of 93.39%was achieved only in 3 min.The reason is because Nickel Raney catalyst adsorbs hydrogen from D2EHPA structure and enhances zinc extraction rate.

Fig.8 shows a general mechanism which increases the kinetics of zinc extraction.In this regard,catalyst surface acts(i.e.Ni2+)as an accelerating place by adsorbing H+from D2EHPA structure and releasing H2into aqueous phase.This causes D2EHPA particles to quickly extract Zn2+from aqueous phase.Therefore,zinc extraction from aqueous phase takes place within short time.

4.Conclusions

Zinc extraction from synthetic sulfate solution was carried out using D2EHPA diluted with kerosene.Thermodynamic parameters were obtained+27.37 J·mol-1·K-1and 25.21 kJ·mol-1for entropy and enthalpy,respectively.It was also found that Gibbs free energy changes from-7.21 kJ·mol-1to-8.41 kJ·mol-1.Regarding thermodynamic results,it was concluded that zinc solvent extraction is endothermic and spontaneous.Potassium and lithium sulfate solution at concentration of 3 mol·L-1showed maximum zinc extraction efficiency while calcium sulfate solution decreased extraction efficiency for all of its concentrations.TBP demonstrated a positive synergistic effect at concentration of 5%for allD2EHPA concentrations;however,extraction efficiency was decreased at concentrations of 2%and 10%.Combined effect of TBP and ionic strength was investigated and it was found that TBP acts as a strong and selective extractant for ions including Ca2+,Li+,and K+decreasing zinc extraction efficiency.Continuous and steady addition of TBP into organic phase effectively improved zinc extraction from aqueous solution.Moreover,for the first time a unique catalyst(i.e.Nickel Raney)was used for solvent extraction of zinc.Adding Nickel Raney catalyst into aqueous phase strongly improved the kinetic of zinc extraction and because this catalyst is capable of recycling,it can propose a novel approach in the field of solvent extraction.