Liquid Membrane Process for the Selective Recovery of Uranium from

Feb 5, 2009 - Application of the emulsion liquid membrane (ELM) technique was tested for the efficient recovery of uranium from the leach liquor, in t...
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Ind. Eng. Chem. Res. 2009, 48, 3118–3125

Liquid Membrane Process for the Selective Recovery of Uranium from Industrial Leach Solutions P. S. Kulkarni,*,† S. Mukhopadhyay, and S. K. Ghosh Process Engineering Section, Chemical Engineering DiVision, Bhabha Atomic Research Center, Trombay, Mumbai 400 085, India

Leach liquor of uranium concentrates are generated at Uranium Corporation of India Limited (UCIL), Jaduguda, India. Application of the emulsion liquid membrane (ELM) technique was tested for the efficient recovery of uranium from the leach liquor, in the presence of metal ions such as Fe, Ca, Mg, and Mn. The liquid membrane employed consisted of a diluent (light and heavy paraffin), a surfactant (Span 80), and an extractant (Alamine 336), and sodium carbonate was used as the stripping solution. Initially, the ELM process parameters were optimized by using synthetic leach liquor as the feed phase. The role of the acidic feed-phase pH, which enhances the cotransport of H+ ions along with uranium inside the emulsion globules, was found to be significant, as it helped in achieving the complete extraction and stripping of uranium. The best optimized parameters were directly used for the separation and concentration of uranium from industrial leach solutions. In one step, in the presence of various metal ions, the selective permeation of uranyl ions through the liquid membrane was observed to be greater than 90%, and the concentration of uranium inside the strip phase was observed to be nearly 7 times higher. Separation factors of uranium with respect to Fe, Mg, Ca, and Mn were experimentally found to be 129, 781, 43, and 1462, respectively, in 12 min. Finally, application of the process in uranium ore processing is demonstrated. 1. Introduction A uranium metal production plant in India is located at Jaduguda, Singhbhum Shear Zone, Bihar.1 The ore consists of quartz, chlorite, and sericite, with negligible amounts of apatite and magnetite, having a uranium concentration ranging from 0.06% to 0.07% U3O8. Uraninite is the major uranium-bearing mineral, and uranium is extracted by leaching with sulfuric acid using pyrolusite as an oxidant.2 The uranium material is recovered by solvent extraction and/or ion exchange and eventually precipitated as a yellow cake of magnesium diuranate (MgU2O7).3-5 Although these conventional separation processes have been in use for many years, they suffer from limitations; for example, the solvent extraction process includes the need for dispersion and coalescence, problems of emulsification, the requirement of more steps, flooding and loading limits in continuous countercurrent devices, the need for density differences between the phases, phase disengagement difficulties, use of a scrubber, high solvent losses, and large solvent inventories. The ionexchange process suffers from the problem of resin fouling, capacity limitations, the requirement of more complexing material, and selectivity. Moreover, these separation processes are equilibrium-limited.6 Therefore, there is a need for the reorganization of the present process designs and the development of new process schemes having high selectivity for the strategic material, low energy consumption, moderate cost-to-performance ratios, and time effectiveness. Liquid-membrane-based separation processes are attracting many scientists and industrialists because of their superior properties over the conventional separation processes.7,8 One such emerging membrane process, the emulsion liquid * To whom correspondence should be addressed. Tel.: +351218417627. Fax: +351-218464455. E-mail: ps_kulkarni@ rediffmail.com. † Present address: CQFM, Departamento de Engenharia Quı´mica e Biolo´gica, IST, 1049-001 Lisboa, Portugal.

membrane (ELM) technique, has received considerable attention in the past three decades because of its attractive characteristics, such as ease of operation, low cost factors, high selectivity, large area for mass transfer, and high fluxes.9-12 The ELM process is known to be one of the most effective methods for separation and concentration when the target component being extracted is present at very low concentrations. On the contrary, solvent extraction isotherms are not feasible for very dilute solutions. In the ELM process, the target species can be transported across the membrane against their concentration gradient. This uphill transport will continue until all of the species that can permeate the liquid membrane have been transferred from the feed to the strip phase, provided that the driving force of the process is kept constant. This situation often occurs in practice, when very dilute solutions of metal ions species are involved or when, in the case of concentrated metal ion solutions, the concentrations of the chemicals that are responsible for the driving force are continuously adjusted to keep the driving force constant. For example, pH and counterions are often used to maintain the driving force or concentration gradient. Nevertheless, the instability of the liquid membrane is a cause of concern in ELM processes, but it can be overcome with the proper selection of membrane components and their concentrations.13 2. Overview of the Separation of a Target Component from an Industrial Solution by the ELM Process Many ELM studies have been carried out using synthetic laboratory feed solutions; however, applications of the ELM technique in the recovery of target components from industrial solutions have seldom been investigated. Apart from the three commercial applications of ELM processes on an industrial scale, viz., zinc removal at a textile plant in Austria, cyanide removal from waste liquors at a gold processing plant in China, and phenol removal from wastewater,14 other examples that use industrial solutions as the feed phase include the extraction of uranium from wet process phosphoric acid,6 the recovery of

10.1021/ie800819q CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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zinc from an industrial effluent, the recovery of nickel from an electroplating bath,16 the removal of copper from waste mine water and acidic leachates,17,18 the removal of ammonia and heavy metals from wastewater,19,20 the extraction of chromium and zinc from cooling tower blowdown,21 the separation of carboxylic and amino acids from fermentation broths,22,23 the removal of alkali metal cations from wastewater,24 the separation of gallium from acidic leach solutions,25 the separation of mercury from wastewater,26 the separation of silver from photographic wastes,27 the removal of lead from storage battery industry wastewater,28 the removal of nitrophenol from wastewater,29 and the selective recovery of Pd from industrial wastewater.30 More studies are required for the scaling-up of the equipment in the above examples, and process developmental work with respect to other industrial feed solutions is necessary, in order to have new ELM-based commercial processes in the future. The successful separation and concentration of uranium from leach liquor in the presence of various ions is a major task to be accomplished. It is always desirable to remove uranium from leach liquor not only for its strategic value as a fuel for nuclear reactors, but also to meet stringent discharge standards.31 Previously, it was demonstrated that uranium can be successfully recovered from nitric acid wastes of uranium processing plants using a tri-n-octylphosphine oxide (TOPO) and sodium carbonate based ELM process.32 The recovery of uranium from the sulfate leach liquor of Jaduguda ore is of specific interest in the present study. First, synthetically prepared leach liquor was used to evaluate the parameters of an ELM process. Subsequently, the best optimized parameters were tested for the recovery of uranium from the leach liquor obtained from a uranium mill. The findings of this investigation are described herein. 3. Transport of Uranium Inside Emulsion Globules Previous studies on the ELM extraction of uranium from sulfate solution highlight the importance of anionic extractants, such as tri-n-octyl amine,33 Aliquat 336,34 and N-alkylcaprolactums35 for the successful transport of uranium inside emulsion globules. In view of such findings and prior experience in the field of uranium recovery by ELM processes,32,36 a systematic study on the transport of uranium ions from leach liquor using Alamine 336 (a tertiary amine, with aliphatic chains of 8-10 carbon groups) as the extractant and sodium carbonate as the stripping agent was undertaken. The extractant, Alamine 336 (R3N), contains a basic nitrogen atom; therefore, it reacts with the acid present in the feed phase and becomes protonated to form amine salts, 4(R3NH)+(SO4)24. The amine salts are capable of undergoing an ion-exchange reaction with the host (uranyl sulfate). In the ELM process, Alamine 336 first extracts uranyl sulfate ions from the leach liquor, and then, it is regenerated (stripped) inside the strip phase for recovery of the uranium and reuse of the reagent. The overall extraction reaction in the ELM process can be represented as37,38 Extraction: UO2(SO4)34- + 4H+ + 4R3N(org) h (R3NH)4UO2(SO4)3(org) (1) Generally, the basic stripping agents that reverse the amine protonation reaction can be a variety of inorganic salt solutions, such as NaCl, (NH4)2SO4, Na2CO3, and so on. However, the type of stripping agent used can determine the overall recovery process of uranium. The use of Na2CO3 can be of considerable importance in the recovery of uranium from the ore because of

Figure 1. Schematic representation of the coupled cotransport mechanism of uranium inside an emulsion globule, where A ) H+, B ) UO2(SO4)34-, C ) R3N, ABC ) (R3NH)4UO2(SO4)3, D ) Na2CO3, and BD ) Na4UO2(CO3)3.

the formation of water-soluble sodium uranyl tricarbonate, Na4UO2(CO3)3, in contrast to many other heavy-metal carbonates. The stripping action of Na2CO3 on the amine salt is given by Stripping: (R3NH)4UO2(SO4)3(org) + 7Na2CO3 h Na4UO2(CO3)3 + 3Na2SO4 + 4NaHCO3 + 4R3N(org) (2) Equations 1 and 2 clearly show that H+ ions are also being transported along with uranyl sulfate inside the membrane. Consequently, a coupled cotransport mechanism might be involved in the formation of sodium uranyl tricarbonate inside the emulsion globules. Sodium uranyl tricarbonate is a complex molecule rather than a simple ion, and hence, the stripping reaction maintains essentially a zero concentration of uranyl ions inside the strip phase. Thus, a very high uranyl concentration gradient can be constantly maintained across the liquid membrane, thereby preventing equilibrium limitations. Therefore, the complete extraction and recovery of uranium should be possible in the present ELM system. In contrast, use of hydrochloric, sulfuric, and/or nitric acid as the stripping solution does not result in the formation of complex molecule and, hence, might have limitations on the recovery of uranium. A schematic of the mechanism of transport of uranium inside an emulsion globule is shown in Figure 1. The rate of extraction of uranium from an aqueous stream by ELM is a function of the rates of a series of different transport processes, namely, (a) diffusion of the uranyl sulfate and hydrogen ions from the bulk of the feed phase to the external interface of the emulsion globule, (b) interfacial complexation reaction between uranyl sulfate and hydrogen ions and Alamine 336, (c) diffusion of the uranium complex into the membrane phase, (d) stripping reaction between the uranium complex and the internal sodium carbonate at the interface of the stripping phase, and (e) diffusion of sodium uranyl tricarbonate in the aqueous strip/membrane boundary layer. The performance of ELM extraction technique can be simply described in terms of the extent of extraction (%) and the stripping or enrichment factor (EF) extent of extraction ) of raffinate × 100% (3) (1 - concentration concentration of feed ) EF )

concentration of uranium in the strip phase concentration of uranium in the feed phase

(4)

4. Experimental Section 4.1. Chemicals. Alamine 336 (97% purity; Morarjee Chemicals, Ambarnath, India) was used as received without further purification. The surfactant, sorbitan monooleate (Span 80), was obtained from Mohini Organics (P) Ltd., Mumbai, India. Diluent

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paraffin (µ ) 1.28 mPa · s, F ) 0.83 g/mL) and heavy paraffin (HP) (µ ) 5.76 mPa · s, F ) 0.89 g/mL) were used in the membrane preparation. Synthetic leach liquor with a uranium concentration of 0.5 g/L (pH ) 2.7) was prepared by diluting uranyl sulfate stock solution, and a typical batch of leach liquor had the approximate composition 0.32 g/L U(VI), 0.47 g/L Fe(III), 0.062 g/L Ca(II), 0.45 g/L Mg(II), and 2.55 g/L Mn, with a pH of 1.8 and a density (F) of 1.005 g/mL; these solutions were directly used as the feed phase. All other chemical reagents were of analytical grade and were used as received. 4.2. Synthesis of Liquid Membrane. Liquid membranes were prepared by emulsifying an aqueous solution of strip phase (0.75 M Na2CO3) with an organic phase. The organic phase itself constitutes a liquid membrane consisting of surfactant, Span 80 (3% v/v), and Alamine 336 (3-5% v/v) in the diluent paraffin. The internal strip phase was added dropwise into a glass reactor containing the organic phase. A predetermined volume ratio of 1:1 was maintained between the organic and strip phases. The contents were stirred at 5000 rpm for 15 min with a four-blade turbine impeller of 40-mm diameter in a glass vessel of 90-mm diameter. Finally, a stable white emulsion was obtained that contained encapsulated droplets with mean diameters varying from 2 to 15 µm.12 4.3. Separation of Uranium from Leach Liquor. The obtained stable emulsion was dispersed in the feed phase of leach liquor from which uranium was to be selectively separated and concentrated. Extraction runs (contact between the emulsion and the feed phase) were performed in a glass vessel having an 80-mm internal diameter and a six-flat-blade turbine impeller of 40-mm diameter rotating at 300 rpm. A predetermined treatment ratio (emulsion to feed phase) of 1:5 was used for extraction purposes. The individual sizes of the emulsion globules could vary from 2 to 4 mm depending on the concentration of the reagents used and the hydrodynamics of the system. Samples of about 5 cm3 were withdrawn from the extractor at different intervals of time and were filtered through Whatman filter paper (grade 540) to separate the emulsion from the aqueous feed phase. This typical filter paper has a pore size of 8 µm and features a high wet strength and chemical resistance. It was observed that the tough surface of the filter paper allowed selective passage of the raffinate and retained the emulsion globules. Moreover, it exhibited a convenient combination of filtration speed and particle retention that helped in the easy analysis of uranium. A loaded emulsion was broken down by heating in a closed vessel to about 80 °C for recovery and analysis of the stripphase uranium concentration. Details of the range of parameters used for the present study are listed in Table 1. Emulsion swelling and breakage were calculated as per the reported method36 and were found to be less than 30% and 5%, respectively. ELM extraction experiments were carried out at 25 °C, and the reproducibility of the experiments was checked at least twice. 4.4. Analytical Details. Concentrations of uranium in the aqueous feed and strip phases were determined spectrophotometrically [model CHEMITO 2000, Toshniwal Instruments (P) Ltd., Mumbai, India] using the alkaline peroxide method39 and tributyl phosphate (TBP)-thiocyanate method.40 The concentration of uranium in the organic phase was calculated by mass balance. Measurements of the viscosity and pH were carried out using an Ostwald viscometer and a pH meter (Equip-tronics, Mumbai, India), respectively. Concentrations of other metal ions (Fe, Ca, Mg, Mn) were determined using an atomic absorption

Table 1. Experimental Conditions Used for the Separation and Concentration of Uranium by an ELM Process parameter

value Strip Phase (Phase I) VI ) 25 mL Na2CO3 (0.75 M)

volume base

Organic or Membrane Phase (Phase II) volume carrier surfactant diluent treat ratio (strip/organic)

VO ) 25 mL Alamine 336 (3-5% v/v) Span 80 (3% v/v) light and heavy paraffins 1:1

Feed Phase (Phase III) volume synthetic leach liquor actual leach liquor pH of the feed phase treat ratio (emulsion/feed) speed of agitation temperature

VF ) 250 mL uranyl sulfate (0.5 g/L) uranyl sulfate (0.32 g/L) 1.4-2.8 1:5 300 rpm 298 K

spectrophotometer (model AVANTA-PM, GBC Scientific Equipment Pvt. Ltd., Dandenong, Victoria, Australia). The error in the analysis of all metal ions was within (3%. 5. Results and Discussion 5.1. Optimization of ELM Process Parameters Using Synthetic Leach Liquor. Major parameters that influence the performance of ELM process are the viscosity of the membrane phase, the extractant and stripping concentrations, and the pH of the external feed phase. These parameters were optimized using synthetically prepared leach liquor. Extraction runs were performed for up to 16 min of time. Considering past experience, in the present work, it was decided to use an agitation speed of 300 rpm, a treatment ratio of 1:5 (emulsion to feed phase), and a strip concentration of 0.75 M, because it was observed that these parameters generate lower swelling and breakage of the emulsion.36 5.1.1. Role of Membrane-Phase Viscosity. The membranephase viscosity determines the rate of transport of uranium and the residence or contact time of the emulsion with the feed phase. It is important to note that residence time is systemspecific and varies for each metal ion under the given conditions used. It has been reported that the residence times for V, Ag, Mo, and Cr are 4, 7.5, 3, and 5 min, respectively. 9,10,12,13 It was observed that the degree of emulsion swelling was approximately linearly related to the residence time of the emulsion. In general, a longer residence time results in the transfer of more water into the internal phase, which causes the membrane to swell and ultimately starts the breakdown of the emulsion phase.41 In the present ELM system, preparation of a stable membrane was therefore thought to be necessary to achieve the maximum extraction and stripping of uranium from the leach liquor. Figure 2a shows that the use of light paraffin as a diluent increases the extent of extraction and stripping of uranium up to 8 min. At this stage, 50% extraction and 4.4 times of stripping can be achieved using the reported parameters. However, a further increase in the extraction time results in a slow decrease. This decrease in the extraction and stripping of uranium is attributed to the instability of the emulsion. The instability of the emulsion instigated osmotic swelling, which was observed to be 29%, and further led to the breakage of emulsion (5%). These events make the ELM process unselective and can release uranium to the external feed phase.

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Figure 2. Role of membrane phase viscosity in extraction and stripping of uranium: (a) light paraffin and (b) light paraffin with 30% heavy paraffin. (Experimental conditions: feed-phase uranium 0.5 g/L; pH 2.7; 3% v/v Span 80; 3% extractant, 0.75 M Na2CO3.) Table 2. Dependence of Viscosity of Membrane Phase Used for Emulsion Preparation on the Use of Heavy Paraffin (HP)a organic diluent (% HP)

viscosity (µ) at 25 °C (mPa · s)

0 10 20 30 40

1.62 2.07 2.52 2.96 3.41

a Alamine 336 concentration: 3% (v/v); Span 80 concentration: 3% (v/v); Diluent: Paraffin

Therefore, to improve the stability of the emulsion, it was decided to add 30% heavy paraffin during the preparation of the emulsion, keeping other experimental conditions constant. The stable emulsions were further mixed with the feed phase to extract uranium. Figure 2b shows that residence time of extraction was shifted from 8 to 12 min. Overall, 60% extraction of uranium was achieved, and the strip phase was found to contain 2.8 g/L of uranium, i.e., 5.6 times more concentrated than the initial leach liquor. Additionally, the emulsion swelling was significantly reduced to 12%. These results indicate that the stability of emulsion improves with the addition of heavy paraffin, which subsequently yielded less swelling and breakage of the emulsion. The data on the viscosity of the membrane phase containing 3% (v/v) extractant and surfactant and paraffin as the diluent with varying concentrations of heavy paraffin are presented in Table 2. In general, Figure 2a,b shows that increasing the concentration of heavy paraffin from 0% to 30% increases the performance of the membrane and, hence, also increases the

extraction and enrichment of uranium. This can be attributed to an increase in the viscosity of the organic phase. However, the internal diffusion of the uranium complex with the extractant was further affected by the viscosity of the membrane. This is because, with an increase in the addition of heavy paraffin to 40%, only the stability of the emulsion was enhanced, whereas the extent of extraction was decreased to 56.5%. These findings demonstrate that increasing the viscosity of the membrane phase beyond 3 mPa · s resulted in a reduction of diffusivity of the metal complex in the organic phase. Thus, there exists a tradeoff between the membrane stability and the mass-transfer rate in the present ELM process parameters. Therefore, to achieve maximum extraction and stripping of uranium, it was decided to design a new membrane formulation with the addition of 30% of heavy paraffin in further experiments. 5.1.2. Role of Extractant/Carrier Concentration. In the past, depending on the nature of the feed phase, several extractants were tested for the conventional extraction of uranium from aqueous solutions, mainly including tri-n-octylamine (TOA),38 dihexyl-N,N-diethyl carbamoyl methyl phosphonate (DHDECMP),42 di(2-ethylhexyl) phosphoric acid (D2EHPA),43 N-benzoyl-N-phenylhydroxyl amine (PC-88A), TOPO, and TBP.44 In the present work, Alamine 336 was used as the extractant because it has been used for liquid-liquid extraction processes and/or the resins used for industrial uranium extraction are of the strong basic anion-exchange type. It is important to note that, in an ELM process, because the inventory of extractant is very low, it is possible to use a highly selective and expensive extractant. Nevertheless, the concentration of carrier in the formulation of the emulsion can be a very important factor in the final process of recovering uranium. The effect of the extractant was studied by varying the concentration of Alamine 336 and maintaining other parameters constant. Figure 3a shows that, when a 4% carrier concentration in the membrane phase was used, a uranium extraction extent of 68% and nearly 5.9 times of stripping can be achieved inside the emulsion globules, which is higher than the values given in Figure 2b. A further increase in the concentration of carrier to 5% was found to increase the extent of extraction and stripping. A maximum of 78% extraction and 6.4 times of stripping of uranium was obtained using a 5% concentration of carrier (Figure 3b). This is mainly due to the increased mass transfer of uranium inside the emulsion globules with increasing concentration of carrier. However, a further increase in the concentration of Alamine 336 beyond 5% decreased the stripping of uranium. Here, it seems that the internal phase concentration of 0.75 M sodium carbonate is not sufficient to strip all of the uranium from the membrane phase in the given period of time. As a result, a maximum percentage of uranium remained in the organic form, which was confirmed by the material balance. Because the recovery of uranium inside the strip phase is more important than mere extraction, a carrier concentration of 5% is recommended. It was also observed that a change in the concentration of Alamine 336 did not affect the stability of the emulsion; that is, there was no effect of the extractant on the swelling and breakage of the emulsion. Throughout these experiments, the swelling and breakage were less than 12% and 2%, respectively. Conversely, in the past, the extraction of molybdenum by Aliquat 336 was found to exhibit a significant increase in emulsion swelling with increasing Aliquat 336 concentration.12

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Figure 3. Role of extractant concentration in extraction and stripping of uranium: (a) 4% and (b) 5%. (Experimental conditions: feed-phase uranium 0.5 g/L; pH 2.7; 3% v/v Span 80; 0.75 M Na2CO3; 30% heavy paraffin in light paraffin.)

Therefore, these results signify that the selection of the carrier and its concentration is very important in ELM processes. 5.1.3. Role of Feed-Phase pH. In the preceding experiments, the pH of the feed solution was 2.7; however, actual leach liquor contains a high amount of acidity. Therefore, to simulate the actual conditions of the leach liquor, it was decided to conduct the experiments in which the feed-phase pH was adjusted to the acidic scale. Figure 4a shows that, when the feed-phase pH was adjusted to 1.8, the extraction and stripping of uranium were remarkably increased. An extent of extraction of about 88% and 8.4 times of stripping can be obtained using an acidic feed-phase pH of 1.8. These results led to the temptation to further decrease the feed-phase pH. Accordingly, experiments were performed with the feed-phase pH adjusted to 1.4 (Figure 4b). Surprisingly, a complete extraction of uranium was achieved at pH 1.4; in fact, a visibly clear and colorless solution of the feed phase was observed after the extraction run, and the concentration of uranium inside the strip phase was found to be more than 10 times. However, at this stage, the overall swelling of the emulsion was found to have increased to 18%, but the breakage remained constant (less than 2%). As the complete extraction and stripping of uranium was achieved, further experiments at pH values of less than 1.4 were not conducted. This impressive recovery of uranium by external addition of H+ ions confirms the transport of H+ ions along with the uranyl sulfate ions, in the same direction inside the emulsion globules. The decrease in feed-phase pH increased the H+ ion concentration of the feed phase, which, in turn, result in the transport of more uranium into the emulsion globules. Figure 1 considers

Figure 4. Role of feed-phase pH in extraction and stripping of uranium: (a) 1.8 and (b) 1.4. (Experimental conditions: feed-phase uranium 0.5 g/L; pH 2.75; 3% v/v Span 80; 5% extractant, 0.75 M Na2CO3; 30% heavy paraffin in light paraffin.)

the coupled transport of uranium and H+ ions by Alamine 336. The cotransport was confirmed by measuring the feed-phase pH after completion of the experiment, which was found to be greater than 2. This process offers the possibility of transporting uranium against its own concentration gradient, i.e., from a low concentration to high concentration, as the real driving force is the chemical potential of uranium, which is a function of the pH in the different phases.45 These findings highlight the importance of coupled transport phenomena in the field of liquid membrane technology. 5.2. Application of the Optimized Parameters for the Selective Recovery of Uranium from the Leach Liquor of Jaduguda Ore. Recently, the ELM technique was tested for the extraction of uranium from gattar sulfate leach liquor of Egypt, and it was observed that a 0.02 M extractant (Aliquat 336) concentration and 1 M Na2CO3 as the strip concentration gave complete transfer of uranium inside the emulsion.34 However, the effect of co-ions, the degree of uranium stripping, and the emulsion behavior during the experiments were not reported. In the present study, the best parameters obtained were directly applied for the study of leach liquor from Jaduguda ore. Initial analysis of the leach liquor showed a uranium concentration of 0.32 g/L along with other co-ions, including 0.47 g/L Fe(III), 0.062 g/L Ca(II), 0.45 g/L Mg(II), and 2.55 g/L Mn, with a pH of 1.8 and a density of 1.005 g/mL. The leach liquor might also contain other metal ions in trace amounts. The results of applying ELM treatment to the actual leach liquor are shown in Figure 5, which illustrates an extraction extent of 81% and 5.6 times of stripping of uranium.

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Figure 5. ELM extraction of uranium from leach liquor at (a) original pH (1.8) and (b) pH 1.4 using best parametric composition. (Experimental conditions: feed-phase uranium 0.32 g/L; 3% v/v Span 80; 5% extractant, 0.75 M Na2CO3; 30% heavy paraffin in light paraffin.)

To improve the recovery of uranium further, it was decided to adjust the pH of the leach liquor to 1.4. As expected, this resulted in an enhancement of the extraction and stripping of uranium from 81% to 92% and from 5.6 to 6.8, respectively (Figure 5b). The difference in the results for synthetic leach liquor and actual leach liquor can be attributed to the presence of various co-ions in the latter. Nevertheless, the final emulsion swelling and breakage were observed to be similar to those observed in the experiments on the former. For this specific experiment, a detailed analysis of all of the metal ions was performed after the completion of the extraction run, and it showed extraction values of 3.8% for Fe, 4.6% for Ca, 2% for Mg, and 1.6% for Mn. The final value of stripping for each metal ion presenting in the leach liquor is given in Table 3, which clearly demonstrates that uranium has a very high stripping value as compared to the other metal ions. The selectivity of uranium is very important in the manufacturing process of uranium. Leach liquor usually contains several types of cations, and hence, the ability of the membrane to provide maximum transport for the target component is

Figure 6. Proposed ELM-based scheme for the separation and concentration of uranium from the leach liquor of Jaduguda ore.

essential. In the present ELM process, the selectivity was governed by the carrier and stripping agent. The selectivity of uranium over other metal ions can be calculated by measuring the separation factor (SF), which is defined as SF )

enrichment factor for uranium enrichment factor for co-ion

(5)

The greater the separation factor, the better the separation of uranium from the other metal ions. It was observed that uranium has the highest separation factor of 1462 over manganese from the given composition of leach liquor (Table 3). The selectivity of uranium over other metal ions was found to increase in the order Ca < Fe < Mg < Mn. Thus, a very high amount of separation factors can be achieved in a single stage of the ELM process. These observations indicate that the selections of

Table 3. ELM Extraction of Uranium from the Leach Liquor of Jaduguda Ore initial metal ion concentration in leach liquor (g/L)

final metal ion concentration in stripping solution (g/L)

stripping or enrichment factor

selectivity with respect to uranium

U(VI) ) 0.32 Fe(III) ) 0.47 Mg(II) ) 0.45 Ca(II) ) 0.062 Mn ) 2.55

U(VI) ) 2.2 Fe(III) ) 0.025 Mg(II) ) 0.004 Ca(II) ) 0.01 Mn ) 0.012

6.9 0.05 0.009 0.16 0.005

129 781 43 1462

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Alamine 336 and sodium carbonate are decisive factors for the separation and concentration of uranium from the leach liquor. 5.3. Potential Application. Laboratory-scale experimental studies have shown that the ELM technique could be used in the large-scale separation of uranium from leach liquor. This could have enormous potential in the industrial-scale separation of uranium from Jaduguda ore. Therefore, an integrated ELMbased scheme was proposed that combines all of the techniques of uranium ore processing. As shown in Figure 6, the overall steps involve initial acid leaching of the ore, followed by liquid-solid separation, which ultimately forms a leach liquor having a pH in the range of 1.4-1.8 and a uranium concentration in the range of 0.3-1.0 g/L. An ELM process has the ability to selectively extract uranium from lean leach liquor and does not require any pretreatment step. The process parameters determined in this work, namely, 3% w/v Span 80, 5% v/v Alamine 336, 0.75 M Na2CO3, and a feed-phase pH of 1.4, can be used in the ELM application. With these parameters, more than 90% extraction and nearly 7 times of uranium can be obtained in one step within 12 min. Moreover, a very high separation factor can be achieved, and the strip solution would contain less than 25 ppm concentration of the co-cations. These findings reveal that the ELM technique might have a better selectivity than the ion-exchange and/or solvent extraction processes that are currently in use and also can be economically profitable. After the application of the ELM process, the concentrated strip phase containing uranyl carbonate can be precipitated using Ca(OH)2 and MgO. Subsequently, it can be filtered and dried to form a yellow cake (MgU2O7). In this way, leach liquor can be effectively treated for the selective recovery of uranium. 6. Conclusions An ELM process was investigated for the separation and concentration of dilute uranyl ions from leach liquor using Alamine 336 and sodium carbonate. The use of 30% heavy paraffin in the composition of the membrane markedly improved the stability of the emulsion and the recovery of uranium. It was observed that the feed-phase pH plays a vital role among the other parameters studied. The experimental findings suggest that, in one step, nearly 7 times uranium and less than 0.2 times other metal ions were concentrated inside the strip phase. Selective permeation of uranium from various co-ions suggests potential applications of this method in the nuclear industries. Acknowledgment The authors are grateful to UCIL, Jaduguda, India, for the sample of leach liquor and to Professor Carlos A. M. Afonso (IST, Lisbon, Portugal) for his kind support. Literature Cited (1) Uranium Corporation of India Limited, UCIL Home Page, http:// uclindia.nic.in/, 2007. (2) Rao, N. K.; Rao, G. V. U. Uranium mineralization in Singhbhum shear zone, Bihar. Nature of occurrences of uranium in apatite-magnetite rocks(III). J. Geol. Soc. India 1983, 24, 555–556. (3) Beri, K. K. Jaduguda Uranium Mill. Rich experiences for future challenges. In National Symposium Uranium Technology, BARC (II); 1989; pp 431-462. (4) Mathur, A. K.; Viswamohan, K.; Mohanty, K. B.; Murthy, V. K.; Seshadrinath, S. T. Uranium extraction using biogenic ferric sulfate a case study on quartz chlorite ore from Jaduguda, Singhbhum Thrust Belt (STB), Bihar, India. Min. Eng. 2000, 13, 575–579. (5) Beri, K. K. Uranium ore processing and environmental implications. Met. Mater. Processes 1998, 10, 99–108.

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ReceiVed for reView May 22, 2008 ReVised manuscript receiVed November 18, 2008 Accepted December 19, 2008 IE800819Q