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A kinetic study on solvent extraction of samarium from nitrate solution with D2EHPA and Cyanex 301 by the single drop technique
Hydrometallurgy 150 (2014) 123–129
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A kinetic study on solvent extraction of samarium from nitrate solution with D2EHPA and Cyanex 301 by the single drop technique R. Torkaman a,b, J. Safdari b, M. Torab-Mostaedi b,⁎, M.A. Moosavian a a b
Oil and Gas Centre of Excellence, School of Chemical Engineering, University College of Engineering, University of Tehran, Tehran, Iran Nuclear Fuel Cycle Research School, Nuclear Science & Technology Research Institute, P.O. Box 14155-1339, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 10 December 2013 Received in revised form 2 October 2014 Accepted 7 October 2014 Available online 20 October 2014 Keywords: Reactive extraction Kinetics Single drop column D2EHPA Cyanex 301
a b s t r a c t The reactive extraction of Sm(III) from aqueous nitrate solution with D2EHPA and Cyanex 301 in kerosene was investigated by single drop column method. Various parameters affecting the extraction process including the column height, nozzle diameter and concentration of extractant, hydrogen ions and Sm(III) were studied. The rate equations for the extraction of Sm(III) with two reagents were obtained on the basis of the slope analysis data. The rate of samarium extraction was found to be directly proportional to the concentration of Sm(III), D2EHPA and Cyanex 301 and inversely proportional to aqueous phase acidity. The extraction of Sm(III) increases with the increase in rising drop diameter and indicate that the extraction is mainly controlled by diffusion mechanism. The extraction rate of samarium with Cyanex 301 was lower than that of D2EHPA, therefore D2EHPA extractant provided faster extraction rate and it would be useful in industrial continuous extraction process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The rare earths have an ever growing variety of applications in the modern technology. Industrial applications of rare earths have been developed in metallurgy, magnets, ceramics, electronics, chemical, optical, medical and nuclear technologies. Samarium, a rare earth element, specifically applied to samarium–cobalt permanent magnets, is also used in electronic watches, aerospace equipment, microwave technology and servomotors. The specific applications of samarium in various fields of technology have turned it into a crucial element from an industrial point of view (Gupta and Krishnamurthy, 2005). Solvent extraction at present is one of the major techniques on industrial scale for the extraction and separation of rare earth metals. This technique is characterized by its great effectiveness in the mutual separation of metals as well as its smooth and continuous operation. Solvent extraction has been operated by means of many types of devices such as mixer-settlers and pulsed extraction columns. Whatever equipment is applied, a comprehensive knowledge of extraction chemistry and kinetics is essential to design a satisfactory contactor (Kislik, 2013). The kinetics of metal extraction is complicated, and involves both diffusion and chemical reaction in a heterogeneous system. However, it is important to elucidate the kinetic behavior of an extraction system because it influences such important economic parameters as resident
⁎ Corresponding author. Tel.: +98 21 88221117; fax: +98 21 88221116. E-mail address: [email protected] (M. Torab-Mostaedi).
http://dx.doi.org/10.1016/j.hydromet.2014.10.002 0304-386X/© 2014 Elsevier B.V. All rights reserved.
time and hence the size of plant and volume of reagent (Gupta and Krishnamurthy, 2005). Several techniques for studying metal extraction kinetics have been developed over the years, which include the shake-out method (Islam and Biswas, 1980), Lewis cell (El-Hefny, 2010; Javanshir et al., 2011), Hahn cell (Biswas and Mondal, 2003; Biswas et al., 2004), the stirred tank AKUFVE apparatus (Flett et al., 1975; Miller and Atwood, 1975), the single drop technique (Awwad and Ibrahium, 2013; Biswas and Hayat, 2002; Durrani et al., 1976) and the rotating diffusion cell (RDC) (Hughes and Biswas, 1991; MacLean and Dreisinger, 1993). The comparison between several techniques for measurements of extraction kinetics is shown in Table 1. Measuring the kinetic of extraction and mass transfer coefficients by single drop experiments is a promising method. Bart, 2003 showed that pilot-plant experiments can be replaced by lab-scale measuring with single drops and appropriate simulation of the column behavior based on models. The literature data to determine rate equations by single drop technique is shown in Table 2. Saleh et al., 2002 surveyed the extraction of La(III) from nitrate-acetato medium by bis-(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) by single drop column and reported the rate expression based on the effective parameters such as time, temperature and concentration of H+, La(III), and Cyanex 272. The effect of different parameters such as temperature, column height, concentration of Fe(III), HCl, D2EHPA, Cl− and H+ on the kinetics of extraction and stripping of iron(III) was investigated by Biswas and Begum, 1999. The kinetic rate equations were reported for extraction and stripping of Fe(III) by D2EHPA extractant.
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Table 1 Several techniques for the measurement of metal extraction kinetics. Apparatus
Description of method
Advantage
Disadvantage
Reference
AKUFVE
Stirred cell with dispersing one phase to the other phase Cylindrical transfer cell without stirrer
Sampling at different times
Unknown interfacial area and hydrodynamic of bulk phases Not suitable for slow extraction
(Rydberg et al., 2004)
Hahn cell (constant interfacial area non-stirred Cell) Rotating diffusion cell
Lewis cell
Single drop technique
Thin porous membrane with hollow rotating cylinder containing the organic phase which sits inside Cylindrical transfer cell with a central paddle stirring both phases separately and simultaneously Traveling drops during the vertical column with extraction
Known interfacial area suitable for very fast extraction
(Biswas and Mondal, 2003)
Known hydrodynamics of both phases
Complicated construction and expensive equipment
(Stevens and Perera, 1997)
Known interfacial area
Maintaining a quiescent interface while stirring both bulk phases
(Rydberg et al., 2004; Stevens and Perera, 1997)
Simple in construction no moving parts flexible in operation responding rapidly to changes
Extraction during drop formation for very rapid reaction very long column for slow reaction
(Rydberg et al., 2004)
Tomita et al., 2000 discussed the process of extraction of europium and samarium by D2EHPA. The rate of extraction was measured using the funnel type cylindrical extraction cell. The solid film formed at the organic–aqueous interface and the formation of the solid films occurred in the specific zones of different concentrations of hydrogen ions and extractant. This phenomenon was not observed in other studies for extraction of rare earth elements by organophosphorus acidic extractants. The extraction reactions which take place at the liquid–liquid interface without solid film and the rate equations were reported (Geist et al., 1999; Wu et al., 2007). According to Table 2, a limited number of investigations have been devoted to the extraction of rare earth elements using the single drop technique. The present paper reports the kinetics of samarium extraction from an aqueous nitrate solution using two extractants (Cyanex 301 and D2EHPA) dissolved in kerosene. The aim of this work is to investigate systematically the effect of various parameters such as aqueous phase acidity, extractant concentration, metal content, nozzle diameter and column height on the extraction kinetics. The rate equation for the system obtained from this study would be useful in the design of extraction columns. 2. Experimental 2.1. Materials and chemicals The commercial extractants, 2-ethylhexyl phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) were supplied from Aldrich. The extractants were dissolved in kerosene to achieve the required concentration.
The stock standard solution of 500 mg/L for Sm(III) was obtained by dissolving samarium nitrate hexahydrate (Sm(NO3)3 · 6H2O, Middle East Ferro Alloy Company, 99.9% purity) in nitrate media at an ionic strength of 0.5 M with NaNO3 and diluting it with deionized water. All other reagents such as formic acid, ascorbic acid and Arsenazo III were used for the analysis of the Sm(III) concentration in the aqueous phase by UV–Visible spectrophotometer.
2.2. Experimental setup Fig. 1 shows the experimental setup. It consists of a glass column with an inner diameter of 50 mm, which was completely filled with the aqueous phase from a reservoir. The lengths of the columns used were 30, 40, 50 and 60 cm; changing the length of the column varied the contact time. The organic phase contained in the reservoir was allowed to flow through a burette column and passed into the mass transfer section with an inlet nozzle. Various drop sizes of the organic phase can be obtained by altering the inlet nozzle. Drops are produced from the nozzle at a fixed rate, usually 70 per min. The drops rose up the column and collected in a previously weighed dry beaker and the volume of the collected organic phase was determined by volumetric pipettes. The interface level between the two phases at outlet section was maintained constant so that the end-effects could be assumed to be constant. For a particular height of the column, the time of contact between the aqueous phase and organic drops was found by averaging the rise time required for at least 20 drops. The temperature of all experiments was constant at 25 °C.
Table 2 The literature data to determine kinetic equations by single drop technique. Metal
Extractant
Diluent
Reference
Fe(III)
D2EHPA
Kerosene
Ni(II) U(VI)a Zr(IV) Cu(II) Mn(II) Ti(IV) Gd(III) La (III) Cu(II)
D2EHPA D2EHPA-Cyanex-921 Mixture D2EHPA Cyanex 272 D2EHPA D2EHPA D2EHPA Cyanex 272 LIX 64N
U(VI)a Ti(VI)a
TPPO D2EHPA
Toluene Kerosene Kerosene Kerosene Kerosene Kerosene Kerosene Toluene Decahydronaphthalene 1,2,3,4-tetrahydronaphthalen Cyclohexane chloroform Kerosene
(Biswas and Begum, 1999) (Biswas and Begum, 2001) (Durrani et al., 1976) (El-Reefy et al., 1997) (Biswas and Hayat, 2002) (Biswas et al., 2007) (Biswas et al., 1996) (Biswas and Begum, 2000) (El-Hefny and El-Dessouky, 2006) (Saleh et al., 2002) (Pazos et al., 1986)
a
M(VI) represents MO2+ 2 .
(Awwad, 2004) (Awwad and Ibrahium, 2013)
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to be applied in the kinetic studies using the single drop technique. To investigate the effect of the samarium transfer rate, higher concentration range of Cyanex 301 was chosen to achieve the maximum extraction efficiency in comparison to lower concentration range of D2EHPA. These results and the stoichiometry of reactive extraction of samarium with D2EHPA and Cyanex 301 were reported in the previous work (Torkaman et al., 2013). 2.4. Reaction orders Based on the batch experimental results reported by Torkaman et al., 2013, the extraction reaction of Sm(III) by D2EHPA and Cyanex 301 can be represented by Eq. (1): K eq
þ
SmðIIIÞðaÞ þ 3H 2 A2ðoÞ ⇔ SmH3 A6ðoÞ þ 3HðaÞ
ð1Þ
where H2A2 represents the dimeric forms of D2EHPA or Cyanex 301. The pKa values, dimerization constants and equilibrium constants of Cyanex 301 and D2EHPA are shown in Table 3 (Torkaman et al., 2013; Wang et al., 2006). At a constant temperature, the extraction rate of Sm(III) from nitrate solution by an extractant (H2A2) may be expressed by Eq. (2): Rf
h ib kmol a þ c ¼ k f ½SmðIIIÞ H ½H 2 A 2 2 m :s
ð2Þ
where Rf and kf represent the rate of extraction per unit surface area and rate constant, respectively; and a, b and c represent the corresponding reaction orders. The rate of the reverse (stripping) reaction has been ignored in the Eq. (2) due to low concentration of Sm(III) in the organic phase during loading. On taking logarithms of the both sides, Eq. (2) takes the form: Fig. 1. Schematic diagram of the single drop apparatus used in the experiments.
2.3. Procedures The solubility of kerosene and water in each other is practically insignificant. However, to eliminate this effect, water and kerosene were saturated into each other before starting the single drop experiments. For stripping studies, the experiments were carried out by contacting equal volumes (200 mL) of the aqueous phase of samarium (500 mg/L) and organic phases (D2EHPA (0.06 M) or Cyanex 301 (0.4 M) in kerosene) by utilizing a water bath shaker adjusted to 25 °C. After equilibrium time between the two phases, the phases were separated by means of a separation funnel and the organic phase was employed in single drop experiments. Different stripping agents were tested for stripping of samarium with the two extractants. The results showed that nitric acid solution is suitable for stripping samarium from D2EHPA or Cyanex 301 in kerosene (Torkaman et al., 2013). It is also assumed that the conditions of the aqueous phase do not change with time. Experiments for the determination of the extraction rate were carried out three times and the aqueous phase in the column was replaced after each run. The amount of samarium transferred to the organic phase during extraction was estimated after stripping with 3 M HNO3. The samarium concentration in the aqueous phase was analyzed by UV–Visible Cary Model spectrophotometer and Arsenazo-III methods at 653 nm (Marczenko and Balcerzak, 2000). The desired concentrations of samarium and nitric acid in the aqueous phase and D2EHPA or Cyanex 301 in the dispersed phase were generated according to the previous experiments. Batch extraction experiments were first implemented to find the suitable conditions
h i þ log R f ¼ log k f þ a log ½SmðIIIÞ þ b log H þ c log ½H 2 A2 :
ð3Þ
The reaction orders for different variables can be obtained by plotting log Rf against log variable under investigation while keeping other variables constant. The value of kf can be determined with multiple linear regression from experimental results. The value of Rf was calculated from the experimental results using the following equation (Awwad, 2004): Rf
kmol V dC ¼ 2 A dt m :s
ð4Þ
V¼
Nπd3 6
ð5Þ
A ¼ Nπd Rf
2
ð6Þ
1=3 kmol V dC ¼ 2 36πN dt m :s
ð7Þ
Table 3 Physicochemical constants. Constants
D2EHPA
Cyanex 301
pKa Dimerization constant (m3/kmol) Equilibrium constant (log Keq)
3.24 3.1 × 104 0.9451
2.61 2.2 × 102 −4.295
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where V is the volume of organic phase collected, N is corresponding to number of collected drops, dC/dt is the change in the concentration of samarium with time per unit area. With unit conversion, the Eq. (7) can be rewritten as follows:
Rf
kmol ½Smðmg=LÞ V ðmlÞ1=3 ¼ : m2 :s 7:2 107 N1=3 t
ð8Þ
2.5. Calculation of overall dispersed phase mass transfer coefficient The mass transfer coefficients are measured experimentally. Considering the mass balance for a single drop at the interface (Marcus and SenGupta, 2004), we get: −d ln ð1−EÞ kd ¼ 6t
ð9Þ
Fig. 3. Farbu plot for the determination of end effect correction term. (Experimental conditions: pH = 3, 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, Nozzle diameter = 1 mm,T = 298.15 K).
3.1. Effect of different parameters on extraction rate C −C E¼ o C o −C
ð10Þ
kd, Co, C and C⁎ are mass transfer coefficients, solute concentration in primary drop (before contact), concentration in the specific position and the concentration in equilibrium with continuous phase, respectively. It is assumed that C⁎ is constant in the extraction process and the effects of continuous phase resistance are insignificant. 3. Results and discussions The effects of different parameters on the mass transfer coefficient and extraction rate of samarium from nitric acid medium using D2EHPA or Cyanex 301 diluted with kerosene were investigated. In all experiments, the formation of solid film at the droplet interface was not observed. A sample image of experiment is shown in Fig. 2.
3.1.1. Column height Fig. 3 shows the Farbu plots using the amount of mass transferred per drop versus drop rise time (Farbu et al., 1974). It can be observed that the samarium mass transfer during the drop rise is proportional to the drop rise time. But the straight lines do not pass through the origin; as a result, the intercept on the time (s) axis of 0.75 s with D2EHPA extractant and 0.56 s with Cyanex 301 extractant is obtained. This time is regarded as the end correction term. This is equivalent to the amount of mass transfer occurring during drop formation and coalescence. In carrying out the calculation of the correct extraction rates, these correction times have been added up to the drop rise time. The plot of log Rf as a function of column height without and with end corrections show that log Rf is independent of column height when the end correction effects are considered. The experimental results suggest that for applying the single drop technique in kinetic measurements of the extraction of Sm(III) from nitrate solution by D2EHPA or Cyanex 301, the selected column may be of any height. However, in calculating the flux values, the end effect correction must be considered. In subsequent experiments, a 40 cm column was used and the end effect corrections were incorporated in calculating the extraction rates. 3.1.2. Inlet nozzle The effect of the rising drop diameter on the extraction rate of samarium was investigated by using different inlet nozzles ranging
Fig. 2. Sample image of experiments without solid film formation.
Fig. 4. Dependence of extraction rate (Rf) on the inlet nozzle diameter (Experimental conditions: pH = 3, 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, Column Height = 40 cm, T = 298.15 K).
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from 0.5 to 3.0 mm. The logarithmic values of the extraction rate of samarium (Rf) are plotted against the inlet nozzle diameter in Fig. 4. It is clear that the extraction rate of samarium from aqueous to organic phase increases with an increase in the inlet nozzle diameter. However, the results indicate that the inlet nozzle diameter had no significant effect on the extraction rate of samarium at diameters in the range 2– 3 mm. Therefore, studies at a nozzle diameter selected at 2–3 mm would be useful in the future work. 3.1.3. Aqueous phase acidity The influence of HNO3 concentration on the rate of extraction by D2EHPA and Cyanex 301 is shown in Fig. 5. It was found that the plot of log Rf vs. log [H+] gives slopes of − 0.8 and − 0.5 for D2EHPA and Cyanex 301, respectively. The rate is inversely proportional to the H+ concentration in the aqueous phase. The low extraction rate is achieved in high acidity because the reaction is shifted preferentially in the back extraction reaction which is equivalent to stripping of the loaded organic. The rate of extraction with both extractants decreased with increasing pH (N 3). Therefore, further experiments with two extractants were carried out at pH 3 which gave the highest rate of extraction (Fig. 5). It was observed that the change in nitrate ion concentration had a negligible effect on the samarium extraction by D2EHPA or Cyanex 301 (Torkaman et al., 2013), therefore the effect of this parameter was not investigated on the extraction rate of Sm(III) with the both extractants. The nitrate ion concentration [NO− 3 ] was kept constant at 0.5 M. 3.1.4. Concentration of D2EHPA and Cyanex 301 The variation of the rate of extraction with D2EHPA or Cyanex 301 concentration in the organic phase is shown in Fig. 6. The slopes indicate that the rate of extraction to organic phase by D2EHPA or Cyanex 301 is directly proportional to the D2EHPA or Cyanex 301 concentration in the organic phase. The results in this figure show that the extraction rate of samarium with D2EHPA in lower concentration range is higher than that of Cyanex 301.
Fig. 6. Dependence of extraction rate (Rf) on the extractant concentration (experimental conditions: pH = 3, [Sm(III)] = 500 mg/L, nozzle diameter = 1 mm, column height = 40 cm, T = 298.15 K).
mechanism appear to be the predominant at high concentration of Sm(III). 3.2. Rate equation for extraction From the results in Figs. 5 and 7, the rate equations were obtained with multiple linear regression. These equations could be summarized by Eqs. (11) and (12): ½Sm−D2EHPA
R f −Sm−DE
h ih i−0:8 kmol −4:035 3þ þ H Sm ½H 2 A2 ¼ 10 2 m :s
ð11Þ
½Sm−Cyanex 301
R f −Sm−CYA
h ih i−0:5 kmol −5:168 3þ þ 0:65 Sm ½H2 A2 : H ¼ 10 m2 :s
ð12Þ
3.1.5. Concentration of Sm(III) The variation of the extraction rates with different samarium concentrations in the aqueous phase is given in Fig. 7. The slope of almost unity with the two extractants indicates that the rate of samarium extraction from aqueous to organic phase is directly proportional to the samarium concentration. The zero slope was observed at high concentration of Sm(III) in Fig. 7. It can be concluded that the surface of drop was saturated with absorbed complex and the influence of diffusion
3.3. Effect of different parameters on the overall dispersed phase mass transfer coefficient and drop diameter
Fig. 5. Dependence of extraction rate (Rf) on the aqueous phase acidity (experimental conditions: 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, nozzle diameter = 1 mm, column height = 40 cm, T = 298.15 K).
Fig. 7. Dependence of extraction rate (Rf) on the samarium concentration (experimental conditions: pH = 3, 0.4 M of Cyanex 301, 0.06 M of D2EHPA, nozzle diameter = 1 mm, column height = 40 cm, T = 298.15 K).
Table 4 shows the effect of aqueous phase acidity on the overall dispersed phase mass transfer coefficient. It is observed that the mass
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Table 4 Effect of different parameters on droplet diameter and mass transfer coefficient. Parameters
Nozzle diameter (mm)
HNO3 (M)
Samarium (mg/L)
D2EHPA concentration (M)
Cyanex 301 concentration (M)
0.5 1.0 2.0 3.0 0.5 0.1 0.05 0.01 0.001 0.0001 250 375 500 750 875 1000 0.015 0.03 0.045 0.06 0.09 0.12 0.1 0.2 0.3 0.4 0.5
Drop diameter (mm)
Mass transfer coefficient (kd) × 10−6
D2EHPA Cyanex 301
D2EHPA Cyanex 301
4.83 5.03 5.19 6.12 4.77 4.86 4.91 4.99 5.01 5.03 5.00 5.06 5.03 5.01 4.97 4.96 5.30 5.22 5.14 5.03 4.99 4.95 – – – – –
11.74 14.62 16.63 18.20 0.16 0.29 0.62 2.69 14.25 14.65 12.99 13.02 14.06 15.74 18.16 18.92 2.93 6.81 10.36 13.22 14.42 15.36 – – – – –
4.86 5.06 5.41 5.89 4.56 4.73 4.76 4.86 4.86 4.85 4.83 4.86 4.86 4.88 4.89 4.89 – – – – – – 5.12 5.09 5.06 4.86 4.53
1.59 2.07 2.78 3.78 0.09 0.19 0.42 1.04 3.07 3.16 2.04 2.31 2.71 3.08 3.29 3.67 – – – – – – 1.06 1.64 2.29 2.67 2.86
transfer coefficient and drop diameter decrease with the increase in the aqueous phase acidity. Also, the effect of extractant concentration on the mass transfer is shown in Table 4. The drop diameter decreases and the mass transfer coefficient increases with the increase in concentration of D2EHPA and Cyanex 301. The results show that high concentration of extractant is required for the formation of D2EHPA-Sm or Cyanex 301-Sm complex. Thus, the mass transfer coefficients increase with an increase in the extractant concentration. The mass transfer coefficients with D2EHPA are larger than those with Cyanex 301. The anionic oxygen donor atom of D2EHPA is a hard ligand and the anionic sulfur donor atom of Cyanex 301 fits into the soft category. According to the hard and soft ligand complex theory, the rare earth ions (hard acids) such as samarium preferably form a complex with D2EHPA (hard bases) than Cyanex 301 (soft base) (Vandegrift and Horwitz, 1980). It is observed from Table 4 that kd increases with increasing samarium concentration but the metal content has no effect on the droplet diameter. The influence of samarium loading in the organic phase on the interfacial tension was shown in the previous work (Torkaman et al., 2013). The results show that the samarium content in the aqueous phase has minimal effect on the interfacial tension. Therefore, the change of the droplet diameter with samarium content is not significant. From Fig. 7, it was found that the value of Rf is constant at higher concentration region, but the diffusion of the absorbed complex increased continuously based on the results in Table 4. Therefore, the system gradually becomes diffusion controlled. The relationship between nozzle diameter and mass transfer coefficient is also shown in Table 4. The mass transfer coefficient and drop diameter increase with increasing nozzle diameter. But the results in Fig. 4 describes that the rate of extraction reaches a constant value with increasing nozzle diameter. The reactive system contains a surface active component such as Cyanex 301 or D2EHPA which takes part in the reaction with samarium in aqueous phase. When the reaction occurs in the droplet interface, the
concentration of D2EHPA and Cyanex 301 will be reduced and consequently a larger part of the interface will be free to transfer momentum from the aqueous phase to the organic phase. Therefore, an inner circulation is induced in a larger part of the droplet and the system gradually becomes diffusion controlled. 3.4. Kinetics of stripping A nitric acid solution in different concentrations was used in the kinetic studies on the stripping of samarium with the two extractants. Fig. 8 shows the logarithmic values of stripping rate of samarium versus log [HNO3]. The rate of stripping increases with the increase in acid concentration with both extractants. Based on the results obtained, the stripping rates for samarium stripped by HNO3 solution with D2EHPA or Cyanex 301 can be represented as: ½Sm−D2EHPA
½Sm−Cyanex 301
Rs−Sm−DE
kmol 0:62 ¼ ks−sm−DE ½HNO3 m2 :s
Rs−Sm−CYA
ð13Þ
kmol 0:5 ¼ ks−sm−CYA ½HNO3 : ð14Þ m2 :s
4. Conclusion In this research, the effect of operating variables on the extraction rate of samarium and the overall dispersed phase mass transfer coefficient from nitrate solution with D2EHPA and Cyanex 301 in the single drop column was studied. The rate equations for the two extractants were obtained by changing different parameters such as [Sm(III)], [D2EHPA], [Cyanex 301] and [H+], while keeping other variables constant. The value of aqueous pH was optimized at 3. This value of pH is the droplet interfacial pH for the extraction to organic phase. It was observed that the forward extraction rate of samarium with D2EHPA was higher than that of Cyanex 301. The interfacial reactive system becomes diffusion controlled with an increase in the concentration of samarium in the aqueous phase and an increase in the droplet diameter. The stripping rate increases with the increase in acid concentration and the direct proportionality was obtained between log Rs and log HNO3.
Fig. 8. Dependence of stripping rate (Rs) on the HNO3 concentration (experimental conditions: 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, column height = 40 cm, T = 298.15 K).
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Geist, A., Nitsch, W., Kim, J.I., 1999. On the kinetics of rare-earth extraction into D2EHPA. Chem. Eng. Sci. 54, 1903–1907. Gupta, C.K., Krishnamurthy, N., 2005. Extractive metallurgy of rare-earths. CRC Press. Hughes, M.A., Biswas, R.K., 1991. The kinetics of vanadium (IV) extraction in the acidic sulphate-D2EHPA-n-hexane system using the rotating diffusion cell technique. Hydrometallurgy 26 (3), 281–297. Islam, F., Biswas, R.K., 1980. Kinetics and mechanism of solvent extraction of V(IV) from acidic aqueous solutions with bis-(2-ethylhexyl)phosphoric acid in benzene. J. Inorg. Nucl. Chem. 42, 421–429. Javanshir, S., Abdollahy, M., Abolghasemi, H., Darban, A.K., 2011. Kinetics of Au (III) extraction by DBC from hydrochloric solution using Lewis cell. Int. J. Miner. Process. 98 (1–2), 42–47. Kislik, V.S., 2013. Solvent Extraction: Classical and Novel Approaches. Elsevier. MacLean, D.W.J., Dreisinger, D.B., 1993. The kinetics of zinc extraction in the di(2ethylhexyl) phosphoric acid, n-heptane-Zn(ClO4)2, HClO4, H2O system using the rotating diffusion cell. Hydrometallurgy 33 (1–2), 107–136. Marcus, Y., SenGupta, A.K., 2004. Ion Exchange and Solvent Extraction: A Series of Advances. CRC Press. Marczenko, Z., Balcerzak, M., 2000. Separation, Preconcentration, and Spectrophotometry in Inorganic Analysis. Elsevier. Miller, J.D., Atwood, R.L., 1975. Discussion of the kinetics of copper solvent extraction with hydroxy oximes. J. Inorg. Nucl. Chem. 32 (12), 2539–2542. Pazos, C., Diaz, R.M., Coca, J., 1986. Extraction of copper from dilute solutions by LIX 64 N. Effect of dekalin and tetralin as diluents on the equilibrium and rate of mass transfer. J. Chem. Technol. Biotechnol. 36, 79–87. Rydberg, J., Cox, M., Musikas, C., Choppin, G.R., 2004. Solvent Extraction Principles and Practice. Taylor & Francis Group, New York. Saleh, M.I., Bari, M.F., Jab, M.S., Saad, B., 2002. Kinetics of lanthanum(III) extraction from nitrate-acetato medium by Cyanex 272 in toluene using the single drop technique. Hydrometallurgy 67, 45–52. Stevens, W.G., Perera, J.M., 1997. Kinetics of solvent extraction processes. Miner. Process. Extr. Metall. Rev. 17, 205–226. Tomita, A., Kanki, T., Asano, T., Sano, N., 2000. Formation of crystal film at interface in process of extraction of rare earth metals by D2EHPA. J. Chem. Eng. Jpn 33, 661–664. Torkaman, R., Moosavian, M.A., Torab-Mostaedi, M., Safdari, J., 2013. Solvent extraction of samarium from aqueous nitrate solution by Cyanex 301 and D2EHPA. Hydrometallurgy 137, 101–107. Vandegrift, G.F., Horwitz, E.P., 1980. Interfacial activity of liquid–liquid extractant reagents. J. Inorg. Nucl. Chem. 42, 119–125. Wang, X., Li, W., Meng, S., Li, D., 2006. The extraction of rare earths using mixtures of acidic phosphorus-based reagents or their thio-analogues. J. Chem. Technol. Biotechnol. 81, 761–766. Wu, D., Wang, X., Li, D., 2007. Extraction kinetics of Sc(III), Y(III), La(III) and Gd(III) from chloride medium by Cyanex 302 in heptane using the constant interfacial cell with laminar flow. Chem. Eng. Process. 46, 17–24.
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A kinetic study on solvent extraction of samarium from nitrate solution with D2EHPA and Cyanex 301 by the single drop technique R. Torkaman a,b, J. Safdari b, M. Torab-Mostaedi b,⁎, M.A. Moosavian a a b
Oil and Gas Centre of Excellence, School of Chemical Engineering, University College of Engineering, University of Tehran, Tehran, Iran Nuclear Fuel Cycle Research School, Nuclear Science & Technology Research Institute, P.O. Box 14155-1339, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 10 December 2013 Received in revised form 2 October 2014 Accepted 7 October 2014 Available online 20 October 2014 Keywords: Reactive extraction Kinetics Single drop column D2EHPA Cyanex 301
a b s t r a c t The reactive extraction of Sm(III) from aqueous nitrate solution with D2EHPA and Cyanex 301 in kerosene was investigated by single drop column method. Various parameters affecting the extraction process including the column height, nozzle diameter and concentration of extractant, hydrogen ions and Sm(III) were studied. The rate equations for the extraction of Sm(III) with two reagents were obtained on the basis of the slope analysis data. The rate of samarium extraction was found to be directly proportional to the concentration of Sm(III), D2EHPA and Cyanex 301 and inversely proportional to aqueous phase acidity. The extraction of Sm(III) increases with the increase in rising drop diameter and indicate that the extraction is mainly controlled by diffusion mechanism. The extraction rate of samarium with Cyanex 301 was lower than that of D2EHPA, therefore D2EHPA extractant provided faster extraction rate and it would be useful in industrial continuous extraction process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The rare earths have an ever growing variety of applications in the modern technology. Industrial applications of rare earths have been developed in metallurgy, magnets, ceramics, electronics, chemical, optical, medical and nuclear technologies. Samarium, a rare earth element, specifically applied to samarium–cobalt permanent magnets, is also used in electronic watches, aerospace equipment, microwave technology and servomotors. The specific applications of samarium in various fields of technology have turned it into a crucial element from an industrial point of view (Gupta and Krishnamurthy, 2005). Solvent extraction at present is one of the major techniques on industrial scale for the extraction and separation of rare earth metals. This technique is characterized by its great effectiveness in the mutual separation of metals as well as its smooth and continuous operation. Solvent extraction has been operated by means of many types of devices such as mixer-settlers and pulsed extraction columns. Whatever equipment is applied, a comprehensive knowledge of extraction chemistry and kinetics is essential to design a satisfactory contactor (Kislik, 2013). The kinetics of metal extraction is complicated, and involves both diffusion and chemical reaction in a heterogeneous system. However, it is important to elucidate the kinetic behavior of an extraction system because it influences such important economic parameters as resident
⁎ Corresponding author. Tel.: +98 21 88221117; fax: +98 21 88221116. E-mail address: [email protected] (M. Torab-Mostaedi).
http://dx.doi.org/10.1016/j.hydromet.2014.10.002 0304-386X/© 2014 Elsevier B.V. All rights reserved.
time and hence the size of plant and volume of reagent (Gupta and Krishnamurthy, 2005). Several techniques for studying metal extraction kinetics have been developed over the years, which include the shake-out method (Islam and Biswas, 1980), Lewis cell (El-Hefny, 2010; Javanshir et al., 2011), Hahn cell (Biswas and Mondal, 2003; Biswas et al., 2004), the stirred tank AKUFVE apparatus (Flett et al., 1975; Miller and Atwood, 1975), the single drop technique (Awwad and Ibrahium, 2013; Biswas and Hayat, 2002; Durrani et al., 1976) and the rotating diffusion cell (RDC) (Hughes and Biswas, 1991; MacLean and Dreisinger, 1993). The comparison between several techniques for measurements of extraction kinetics is shown in Table 1. Measuring the kinetic of extraction and mass transfer coefficients by single drop experiments is a promising method. Bart, 2003 showed that pilot-plant experiments can be replaced by lab-scale measuring with single drops and appropriate simulation of the column behavior based on models. The literature data to determine rate equations by single drop technique is shown in Table 2. Saleh et al., 2002 surveyed the extraction of La(III) from nitrate-acetato medium by bis-(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272) by single drop column and reported the rate expression based on the effective parameters such as time, temperature and concentration of H+, La(III), and Cyanex 272. The effect of different parameters such as temperature, column height, concentration of Fe(III), HCl, D2EHPA, Cl− and H+ on the kinetics of extraction and stripping of iron(III) was investigated by Biswas and Begum, 1999. The kinetic rate equations were reported for extraction and stripping of Fe(III) by D2EHPA extractant.
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Table 1 Several techniques for the measurement of metal extraction kinetics. Apparatus
Description of method
Advantage
Disadvantage
Reference
AKUFVE
Stirred cell with dispersing one phase to the other phase Cylindrical transfer cell without stirrer
Sampling at different times
Unknown interfacial area and hydrodynamic of bulk phases Not suitable for slow extraction
(Rydberg et al., 2004)
Hahn cell (constant interfacial area non-stirred Cell) Rotating diffusion cell
Lewis cell
Single drop technique
Thin porous membrane with hollow rotating cylinder containing the organic phase which sits inside Cylindrical transfer cell with a central paddle stirring both phases separately and simultaneously Traveling drops during the vertical column with extraction
Known interfacial area suitable for very fast extraction
(Biswas and Mondal, 2003)
Known hydrodynamics of both phases
Complicated construction and expensive equipment
(Stevens and Perera, 1997)
Known interfacial area
Maintaining a quiescent interface while stirring both bulk phases
(Rydberg et al., 2004; Stevens and Perera, 1997)
Simple in construction no moving parts flexible in operation responding rapidly to changes
Extraction during drop formation for very rapid reaction very long column for slow reaction
(Rydberg et al., 2004)
Tomita et al., 2000 discussed the process of extraction of europium and samarium by D2EHPA. The rate of extraction was measured using the funnel type cylindrical extraction cell. The solid film formed at the organic–aqueous interface and the formation of the solid films occurred in the specific zones of different concentrations of hydrogen ions and extractant. This phenomenon was not observed in other studies for extraction of rare earth elements by organophosphorus acidic extractants. The extraction reactions which take place at the liquid–liquid interface without solid film and the rate equations were reported (Geist et al., 1999; Wu et al., 2007). According to Table 2, a limited number of investigations have been devoted to the extraction of rare earth elements using the single drop technique. The present paper reports the kinetics of samarium extraction from an aqueous nitrate solution using two extractants (Cyanex 301 and D2EHPA) dissolved in kerosene. The aim of this work is to investigate systematically the effect of various parameters such as aqueous phase acidity, extractant concentration, metal content, nozzle diameter and column height on the extraction kinetics. The rate equation for the system obtained from this study would be useful in the design of extraction columns. 2. Experimental 2.1. Materials and chemicals The commercial extractants, 2-ethylhexyl phosphoric acid (D2EHPA) and bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) were supplied from Aldrich. The extractants were dissolved in kerosene to achieve the required concentration.
The stock standard solution of 500 mg/L for Sm(III) was obtained by dissolving samarium nitrate hexahydrate (Sm(NO3)3 · 6H2O, Middle East Ferro Alloy Company, 99.9% purity) in nitrate media at an ionic strength of 0.5 M with NaNO3 and diluting it with deionized water. All other reagents such as formic acid, ascorbic acid and Arsenazo III were used for the analysis of the Sm(III) concentration in the aqueous phase by UV–Visible spectrophotometer.
2.2. Experimental setup Fig. 1 shows the experimental setup. It consists of a glass column with an inner diameter of 50 mm, which was completely filled with the aqueous phase from a reservoir. The lengths of the columns used were 30, 40, 50 and 60 cm; changing the length of the column varied the contact time. The organic phase contained in the reservoir was allowed to flow through a burette column and passed into the mass transfer section with an inlet nozzle. Various drop sizes of the organic phase can be obtained by altering the inlet nozzle. Drops are produced from the nozzle at a fixed rate, usually 70 per min. The drops rose up the column and collected in a previously weighed dry beaker and the volume of the collected organic phase was determined by volumetric pipettes. The interface level between the two phases at outlet section was maintained constant so that the end-effects could be assumed to be constant. For a particular height of the column, the time of contact between the aqueous phase and organic drops was found by averaging the rise time required for at least 20 drops. The temperature of all experiments was constant at 25 °C.
Table 2 The literature data to determine kinetic equations by single drop technique. Metal
Extractant
Diluent
Reference
Fe(III)
D2EHPA
Kerosene
Ni(II) U(VI)a Zr(IV) Cu(II) Mn(II) Ti(IV) Gd(III) La (III) Cu(II)
D2EHPA D2EHPA-Cyanex-921 Mixture D2EHPA Cyanex 272 D2EHPA D2EHPA D2EHPA Cyanex 272 LIX 64N
U(VI)a Ti(VI)a
TPPO D2EHPA
Toluene Kerosene Kerosene Kerosene Kerosene Kerosene Kerosene Toluene Decahydronaphthalene 1,2,3,4-tetrahydronaphthalen Cyclohexane chloroform Kerosene
(Biswas and Begum, 1999) (Biswas and Begum, 2001) (Durrani et al., 1976) (El-Reefy et al., 1997) (Biswas and Hayat, 2002) (Biswas et al., 2007) (Biswas et al., 1996) (Biswas and Begum, 2000) (El-Hefny and El-Dessouky, 2006) (Saleh et al., 2002) (Pazos et al., 1986)
a
M(VI) represents MO2+ 2 .
(Awwad, 2004) (Awwad and Ibrahium, 2013)
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to be applied in the kinetic studies using the single drop technique. To investigate the effect of the samarium transfer rate, higher concentration range of Cyanex 301 was chosen to achieve the maximum extraction efficiency in comparison to lower concentration range of D2EHPA. These results and the stoichiometry of reactive extraction of samarium with D2EHPA and Cyanex 301 were reported in the previous work (Torkaman et al., 2013). 2.4. Reaction orders Based on the batch experimental results reported by Torkaman et al., 2013, the extraction reaction of Sm(III) by D2EHPA and Cyanex 301 can be represented by Eq. (1): K eq
þ
SmðIIIÞðaÞ þ 3H 2 A2ðoÞ ⇔ SmH3 A6ðoÞ þ 3HðaÞ
ð1Þ
where H2A2 represents the dimeric forms of D2EHPA or Cyanex 301. The pKa values, dimerization constants and equilibrium constants of Cyanex 301 and D2EHPA are shown in Table 3 (Torkaman et al., 2013; Wang et al., 2006). At a constant temperature, the extraction rate of Sm(III) from nitrate solution by an extractant (H2A2) may be expressed by Eq. (2): Rf
h ib kmol a þ c ¼ k f ½SmðIIIÞ H ½H 2 A 2 2 m :s
ð2Þ
where Rf and kf represent the rate of extraction per unit surface area and rate constant, respectively; and a, b and c represent the corresponding reaction orders. The rate of the reverse (stripping) reaction has been ignored in the Eq. (2) due to low concentration of Sm(III) in the organic phase during loading. On taking logarithms of the both sides, Eq. (2) takes the form: Fig. 1. Schematic diagram of the single drop apparatus used in the experiments.
2.3. Procedures The solubility of kerosene and water in each other is practically insignificant. However, to eliminate this effect, water and kerosene were saturated into each other before starting the single drop experiments. For stripping studies, the experiments were carried out by contacting equal volumes (200 mL) of the aqueous phase of samarium (500 mg/L) and organic phases (D2EHPA (0.06 M) or Cyanex 301 (0.4 M) in kerosene) by utilizing a water bath shaker adjusted to 25 °C. After equilibrium time between the two phases, the phases were separated by means of a separation funnel and the organic phase was employed in single drop experiments. Different stripping agents were tested for stripping of samarium with the two extractants. The results showed that nitric acid solution is suitable for stripping samarium from D2EHPA or Cyanex 301 in kerosene (Torkaman et al., 2013). It is also assumed that the conditions of the aqueous phase do not change with time. Experiments for the determination of the extraction rate were carried out three times and the aqueous phase in the column was replaced after each run. The amount of samarium transferred to the organic phase during extraction was estimated after stripping with 3 M HNO3. The samarium concentration in the aqueous phase was analyzed by UV–Visible Cary Model spectrophotometer and Arsenazo-III methods at 653 nm (Marczenko and Balcerzak, 2000). The desired concentrations of samarium and nitric acid in the aqueous phase and D2EHPA or Cyanex 301 in the dispersed phase were generated according to the previous experiments. Batch extraction experiments were first implemented to find the suitable conditions
h i þ log R f ¼ log k f þ a log ½SmðIIIÞ þ b log H þ c log ½H 2 A2 :
ð3Þ
The reaction orders for different variables can be obtained by plotting log Rf against log variable under investigation while keeping other variables constant. The value of kf can be determined with multiple linear regression from experimental results. The value of Rf was calculated from the experimental results using the following equation (Awwad, 2004): Rf
kmol V dC ¼ 2 A dt m :s
ð4Þ
V¼
Nπd3 6
ð5Þ
A ¼ Nπd Rf
2
ð6Þ
1=3 kmol V dC ¼ 2 36πN dt m :s
ð7Þ
Table 3 Physicochemical constants. Constants
D2EHPA
Cyanex 301
pKa Dimerization constant (m3/kmol) Equilibrium constant (log Keq)
3.24 3.1 × 104 0.9451
2.61 2.2 × 102 −4.295
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where V is the volume of organic phase collected, N is corresponding to number of collected drops, dC/dt is the change in the concentration of samarium with time per unit area. With unit conversion, the Eq. (7) can be rewritten as follows:
Rf
kmol ½Smðmg=LÞ V ðmlÞ1=3 ¼ : m2 :s 7:2 107 N1=3 t
ð8Þ
2.5. Calculation of overall dispersed phase mass transfer coefficient The mass transfer coefficients are measured experimentally. Considering the mass balance for a single drop at the interface (Marcus and SenGupta, 2004), we get: −d ln ð1−EÞ kd ¼ 6t
ð9Þ
Fig. 3. Farbu plot for the determination of end effect correction term. (Experimental conditions: pH = 3, 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, Nozzle diameter = 1 mm,T = 298.15 K).
3.1. Effect of different parameters on extraction rate C −C E¼ o C o −C
ð10Þ
kd, Co, C and C⁎ are mass transfer coefficients, solute concentration in primary drop (before contact), concentration in the specific position and the concentration in equilibrium with continuous phase, respectively. It is assumed that C⁎ is constant in the extraction process and the effects of continuous phase resistance are insignificant. 3. Results and discussions The effects of different parameters on the mass transfer coefficient and extraction rate of samarium from nitric acid medium using D2EHPA or Cyanex 301 diluted with kerosene were investigated. In all experiments, the formation of solid film at the droplet interface was not observed. A sample image of experiment is shown in Fig. 2.
3.1.1. Column height Fig. 3 shows the Farbu plots using the amount of mass transferred per drop versus drop rise time (Farbu et al., 1974). It can be observed that the samarium mass transfer during the drop rise is proportional to the drop rise time. But the straight lines do not pass through the origin; as a result, the intercept on the time (s) axis of 0.75 s with D2EHPA extractant and 0.56 s with Cyanex 301 extractant is obtained. This time is regarded as the end correction term. This is equivalent to the amount of mass transfer occurring during drop formation and coalescence. In carrying out the calculation of the correct extraction rates, these correction times have been added up to the drop rise time. The plot of log Rf as a function of column height without and with end corrections show that log Rf is independent of column height when the end correction effects are considered. The experimental results suggest that for applying the single drop technique in kinetic measurements of the extraction of Sm(III) from nitrate solution by D2EHPA or Cyanex 301, the selected column may be of any height. However, in calculating the flux values, the end effect correction must be considered. In subsequent experiments, a 40 cm column was used and the end effect corrections were incorporated in calculating the extraction rates. 3.1.2. Inlet nozzle The effect of the rising drop diameter on the extraction rate of samarium was investigated by using different inlet nozzles ranging
Fig. 2. Sample image of experiments without solid film formation.
Fig. 4. Dependence of extraction rate (Rf) on the inlet nozzle diameter (Experimental conditions: pH = 3, 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, Column Height = 40 cm, T = 298.15 K).
R. Torkaman et al. / Hydrometallurgy 150 (2014) 123–129
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from 0.5 to 3.0 mm. The logarithmic values of the extraction rate of samarium (Rf) are plotted against the inlet nozzle diameter in Fig. 4. It is clear that the extraction rate of samarium from aqueous to organic phase increases with an increase in the inlet nozzle diameter. However, the results indicate that the inlet nozzle diameter had no significant effect on the extraction rate of samarium at diameters in the range 2– 3 mm. Therefore, studies at a nozzle diameter selected at 2–3 mm would be useful in the future work. 3.1.3. Aqueous phase acidity The influence of HNO3 concentration on the rate of extraction by D2EHPA and Cyanex 301 is shown in Fig. 5. It was found that the plot of log Rf vs. log [H+] gives slopes of − 0.8 and − 0.5 for D2EHPA and Cyanex 301, respectively. The rate is inversely proportional to the H+ concentration in the aqueous phase. The low extraction rate is achieved in high acidity because the reaction is shifted preferentially in the back extraction reaction which is equivalent to stripping of the loaded organic. The rate of extraction with both extractants decreased with increasing pH (N 3). Therefore, further experiments with two extractants were carried out at pH 3 which gave the highest rate of extraction (Fig. 5). It was observed that the change in nitrate ion concentration had a negligible effect on the samarium extraction by D2EHPA or Cyanex 301 (Torkaman et al., 2013), therefore the effect of this parameter was not investigated on the extraction rate of Sm(III) with the both extractants. The nitrate ion concentration [NO− 3 ] was kept constant at 0.5 M. 3.1.4. Concentration of D2EHPA and Cyanex 301 The variation of the rate of extraction with D2EHPA or Cyanex 301 concentration in the organic phase is shown in Fig. 6. The slopes indicate that the rate of extraction to organic phase by D2EHPA or Cyanex 301 is directly proportional to the D2EHPA or Cyanex 301 concentration in the organic phase. The results in this figure show that the extraction rate of samarium with D2EHPA in lower concentration range is higher than that of Cyanex 301.
Fig. 6. Dependence of extraction rate (Rf) on the extractant concentration (experimental conditions: pH = 3, [Sm(III)] = 500 mg/L, nozzle diameter = 1 mm, column height = 40 cm, T = 298.15 K).
mechanism appear to be the predominant at high concentration of Sm(III). 3.2. Rate equation for extraction From the results in Figs. 5 and 7, the rate equations were obtained with multiple linear regression. These equations could be summarized by Eqs. (11) and (12): ½Sm−D2EHPA
R f −Sm−DE
h ih i−0:8 kmol −4:035 3þ þ H Sm ½H 2 A2 ¼ 10 2 m :s
ð11Þ
½Sm−Cyanex 301
R f −Sm−CYA
h ih i−0:5 kmol −5:168 3þ þ 0:65 Sm ½H2 A2 : H ¼ 10 m2 :s
ð12Þ
3.1.5. Concentration of Sm(III) The variation of the extraction rates with different samarium concentrations in the aqueous phase is given in Fig. 7. The slope of almost unity with the two extractants indicates that the rate of samarium extraction from aqueous to organic phase is directly proportional to the samarium concentration. The zero slope was observed at high concentration of Sm(III) in Fig. 7. It can be concluded that the surface of drop was saturated with absorbed complex and the influence of diffusion
3.3. Effect of different parameters on the overall dispersed phase mass transfer coefficient and drop diameter
Fig. 5. Dependence of extraction rate (Rf) on the aqueous phase acidity (experimental conditions: 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, nozzle diameter = 1 mm, column height = 40 cm, T = 298.15 K).
Fig. 7. Dependence of extraction rate (Rf) on the samarium concentration (experimental conditions: pH = 3, 0.4 M of Cyanex 301, 0.06 M of D2EHPA, nozzle diameter = 1 mm, column height = 40 cm, T = 298.15 K).
Table 4 shows the effect of aqueous phase acidity on the overall dispersed phase mass transfer coefficient. It is observed that the mass
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R. Torkaman et al. / Hydrometallurgy 150 (2014) 123–129
Table 4 Effect of different parameters on droplet diameter and mass transfer coefficient. Parameters
Nozzle diameter (mm)
HNO3 (M)
Samarium (mg/L)
D2EHPA concentration (M)
Cyanex 301 concentration (M)
0.5 1.0 2.0 3.0 0.5 0.1 0.05 0.01 0.001 0.0001 250 375 500 750 875 1000 0.015 0.03 0.045 0.06 0.09 0.12 0.1 0.2 0.3 0.4 0.5
Drop diameter (mm)
Mass transfer coefficient (kd) × 10−6
D2EHPA Cyanex 301
D2EHPA Cyanex 301
4.83 5.03 5.19 6.12 4.77 4.86 4.91 4.99 5.01 5.03 5.00 5.06 5.03 5.01 4.97 4.96 5.30 5.22 5.14 5.03 4.99 4.95 – – – – –
11.74 14.62 16.63 18.20 0.16 0.29 0.62 2.69 14.25 14.65 12.99 13.02 14.06 15.74 18.16 18.92 2.93 6.81 10.36 13.22 14.42 15.36 – – – – –
4.86 5.06 5.41 5.89 4.56 4.73 4.76 4.86 4.86 4.85 4.83 4.86 4.86 4.88 4.89 4.89 – – – – – – 5.12 5.09 5.06 4.86 4.53
1.59 2.07 2.78 3.78 0.09 0.19 0.42 1.04 3.07 3.16 2.04 2.31 2.71 3.08 3.29 3.67 – – – – – – 1.06 1.64 2.29 2.67 2.86
transfer coefficient and drop diameter decrease with the increase in the aqueous phase acidity. Also, the effect of extractant concentration on the mass transfer is shown in Table 4. The drop diameter decreases and the mass transfer coefficient increases with the increase in concentration of D2EHPA and Cyanex 301. The results show that high concentration of extractant is required for the formation of D2EHPA-Sm or Cyanex 301-Sm complex. Thus, the mass transfer coefficients increase with an increase in the extractant concentration. The mass transfer coefficients with D2EHPA are larger than those with Cyanex 301. The anionic oxygen donor atom of D2EHPA is a hard ligand and the anionic sulfur donor atom of Cyanex 301 fits into the soft category. According to the hard and soft ligand complex theory, the rare earth ions (hard acids) such as samarium preferably form a complex with D2EHPA (hard bases) than Cyanex 301 (soft base) (Vandegrift and Horwitz, 1980). It is observed from Table 4 that kd increases with increasing samarium concentration but the metal content has no effect on the droplet diameter. The influence of samarium loading in the organic phase on the interfacial tension was shown in the previous work (Torkaman et al., 2013). The results show that the samarium content in the aqueous phase has minimal effect on the interfacial tension. Therefore, the change of the droplet diameter with samarium content is not significant. From Fig. 7, it was found that the value of Rf is constant at higher concentration region, but the diffusion of the absorbed complex increased continuously based on the results in Table 4. Therefore, the system gradually becomes diffusion controlled. The relationship between nozzle diameter and mass transfer coefficient is also shown in Table 4. The mass transfer coefficient and drop diameter increase with increasing nozzle diameter. But the results in Fig. 4 describes that the rate of extraction reaches a constant value with increasing nozzle diameter. The reactive system contains a surface active component such as Cyanex 301 or D2EHPA which takes part in the reaction with samarium in aqueous phase. When the reaction occurs in the droplet interface, the
concentration of D2EHPA and Cyanex 301 will be reduced and consequently a larger part of the interface will be free to transfer momentum from the aqueous phase to the organic phase. Therefore, an inner circulation is induced in a larger part of the droplet and the system gradually becomes diffusion controlled. 3.4. Kinetics of stripping A nitric acid solution in different concentrations was used in the kinetic studies on the stripping of samarium with the two extractants. Fig. 8 shows the logarithmic values of stripping rate of samarium versus log [HNO3]. The rate of stripping increases with the increase in acid concentration with both extractants. Based on the results obtained, the stripping rates for samarium stripped by HNO3 solution with D2EHPA or Cyanex 301 can be represented as: ½Sm−D2EHPA
½Sm−Cyanex 301
Rs−Sm−DE
kmol 0:62 ¼ ks−sm−DE ½HNO3 m2 :s
Rs−Sm−CYA
ð13Þ
kmol 0:5 ¼ ks−sm−CYA ½HNO3 : ð14Þ m2 :s
4. Conclusion In this research, the effect of operating variables on the extraction rate of samarium and the overall dispersed phase mass transfer coefficient from nitrate solution with D2EHPA and Cyanex 301 in the single drop column was studied. The rate equations for the two extractants were obtained by changing different parameters such as [Sm(III)], [D2EHPA], [Cyanex 301] and [H+], while keeping other variables constant. The value of aqueous pH was optimized at 3. This value of pH is the droplet interfacial pH for the extraction to organic phase. It was observed that the forward extraction rate of samarium with D2EHPA was higher than that of Cyanex 301. The interfacial reactive system becomes diffusion controlled with an increase in the concentration of samarium in the aqueous phase and an increase in the droplet diameter. The stripping rate increases with the increase in acid concentration and the direct proportionality was obtained between log Rs and log HNO3.
Fig. 8. Dependence of stripping rate (Rs) on the HNO3 concentration (experimental conditions: 0.4 M of Cyanex 301, 0.06 M of D2EHPA, [Sm(III)] = 500 mg/L, column height = 40 cm, T = 298.15 K).
R. Torkaman et al. / Hydrometallurgy 150 (2014) 123–129
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