Novel Method for Stripping Uranium from the Organic Phase in the

Mar 19, 2013 - ... continuous flow of loaded organic phase into this cell and stripped organic phase out of this electrolytic cell could be carried ou...
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Novel Method for Stripping Uranium from the Organic Phase in the Recovery of Uranium from Wet Process Phosphoric Acid (WPA) Kavitha Jayachandran,† I. C. Pius,*,† Chetty K. Venugopal,†,‡ V. A. Raman,#,‡ B. P. Dubey,§ G. K. Vithal,§ S. K. Mukerjee,† S. K. Aggarwal,† K. L. Ramakumar,∥ and V. Venugopal⊥ †

Fuel Chemistry Division, #Radioanalytical Chemistry Division, ∥Radiochemistry and Isotope Group, and ⊥Raja Ramanna Fellow, Bhabha Atomic Research Centre, Mumbai 400 085, India § Heavy Water Board, Department of Atomic Energy, Mumbai 400 094, India S Supporting Information *

ABSTRACT: The technique of recovering uranium from wet process phosphoric acid (WPA) using a synergistic mixture of di(2-ethylhexyl)phosphoric acid (D2EHPA) and tri-n-butyl phosphate (TBP) is being developed in India. In this respect, a novel procedure has been developed for stripping and concentrating uranium from the organic phase. Stripping of the loaded uranium from the organic phase is achieved by reductive stripping of U(VI) to U(IV) through contact with merchant-grade phosphoric acid (MGA) containing Fe2+, which is generated in situ by the electrolytic reduction of Fe3+ already present as an impurity. An electrolytic cell was designed such that the organic phase loaded with uranium could be continuously contacted with stripping medium within the cell. Provision was also made for the separation of the stripped organic phase within the cell, so that the continuous flow of loaded organic phase into this cell and stripped organic phase out of this electrolytic cell could be carried out conveniently. After optimization of the cell parameters, four electrolytic cells were connected in series, under countercurrent flow, and effective stripping of uranium from the organic phase was achieved.

1. INTRODUCTION Uranium is associated in significant quantities with phosphatic rocks, which are processed in large quantities for the production of phosphatic fertilizers. Such rocks are recognized as an important sustainable source of uranium.1 During the acidulation of uraniferous phosphatic rocks for the production of wet process phosphoric acid (WPA) (5−6 M H3PO4) under oxidizing conditions, the U3O8 fraction is dissolved to the extent of 0.1−0.2 mg/mL. Merchant-grade phosphoric acid (MGA), which contains ∼10 M H3PO4, is produced by concentrating WPA. Several solvent extraction methods have been reported for the recovery of uranium from WPA using combinations such as synergistic mixtures of di-(2-ethylhexyl)phosphoric acid (D2EHPA) and tri-n-octylphosphine oxide (TOPO),2 D2EHPA and tri-n-butyl phosphate (TBP),3−7 dinonyl phenyl phosphoric acid (DNPPA) and TBP,8,9 (2ethylhexyl)phosphonic acid mono(2-ethylhexyl) ester (PC88A) and TBP,10 dioctylphenylphosphoric acid (DOPPA) and TOPO,11 and octylphenylphosphoric acid (OPPA).12 Alternative methods such as ion exchange13,14 and liquid membrane extraction15,16 have also been investigated for the recovery of uranium from phosphoric acid. Among the solvent extraction methods, that based on D2EHPA and TOPO is the only one employed at various commercial plants in different countries. This method has many advantages such as good extraction and selectivity for uranium and low solvent loss, and it is a proven technology. An extraction process using a synergistic mixture of D2EHPA and TBP is an attractive alternative because of the easy availability and low cost of TBP as compared to TOPO. Efforts are being made to develop this method at the plant scale. Although this method has certain disadvantages because of its low selectivity, © 2013 American Chemical Society

leading to the extraction of iron and rare-earth elements, such problems could be overcome by introducing additional scrubbing steps using sulfuric acid before the final stripping of uranium with ammonium carbonate. The extraction and stripping profiles of uranium at various stages of recovery from WPA using D2EHPA/TBP mixtures have been reported.17 Removal of extracted Fe from the loaded organic phase at various stages of scrubbing with 20% sulfuric acid in pilot-plant tests has also been reported.17 An improved method for the removal of extracted Fe by scrubbing with oxalic acid has also been reported.3 Lower affinity to uranium of TBP as compared to TOPO could be overcome by introducing more extraction steps. A schematic diagram of the process is included in Figure S1 of the Supporting Information. The process comprises two cycles of solvent extraction. In the first cycle, uranium is extracted from WPA at an organic-to-aqueous phase ratio of 1:1 in six stages, and then uranium is stripped and concentrated using MGA containing 10−20 mg/mL Fe2+ at an organic-toaqueous phase ratio of 20:1 in six stages. A synergistic mechanism for the extraction of UO22+ into the D2EHPA + TOPO system based on addition, substitution, and solvation has been reported.18 The transfer of U(VI) from phosphoric acid to a solvent containing a synergistic mixture of D2EHPA (HA) and a neutral donor T can be represented as Received: Revised: Accepted: Published: 5418

November 18, 2012 March 14, 2013 March 19, 2013 March 19, 2013 dx.doi.org/10.1021/ie3031532 | Ind. Eng. Chem. Res. 2013, 52, 5418−5427

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∼0.005 mg/mL uranium, and all other constituents similar to those in WPA. Merchant-grade phosphoric acid was obtained by evaporating the WPA to 50% of its initial volume and also directly from the fertilizer plant; it contained 9−10 M H3PO4, 6−7 mg/mL Fe3+, and ∼0.12 mg/mL uranium. D2EHPA and TBP were obtained from Heavy Water Plant, Talcher, Orissa, India, and mixed with commercially available heavy normal paraffin (HNP) in the required proportion to obtain 1.25 M D2EHPA + 0.1 M TBP. Other reagents used were of AR grade. 2.2. Preparation of the Organic Phase Loaded with Uranium and Lean Phosphoric Acid (LPA). Initial extraction of uranium from WPA was carried out to simulate mixer settler conditions for six contacts as follows: About 1 mL of H2O2 (30%) was added to 1500 mL of WPA, and the mixture was warmed to convert all of the U to U(VI). Then, 1440 mL of this solution was brought into contact with 1440 mL of the organic phase in four stages with six contacts each (60 mL of the organic phase and 60 mL of the aqueous phase in each contact). The lean phosphoric acid (LPA) obtained after removal of uranium from WPA and the organic phase loaded with uranium were collected and assayed for U content. 2.3. Preparation of LPA Containing Titanium(IV). LPA containing Ti was prepared for reductive stripping studies of uranium from the loaded organic phase using Ti3+. AR-grade TiCl4 was hydrolyzed carefully to obtain TiOCl2 solution. The concentration of Ti was estimated by a gravimetric method to be 110 mg/mL. This solution of Ti was further diluted 10-fold with distilled water and added to a known weight of LPA solution with stirring. The solution was then warmed and evaporated to the original volume of LPA. This solution was electrolyzed in the cathode compartment of an electrolytic cell to obtain an LPA solution containing Ti3+. 2.4. Preparation of LPA Containing Fe 2+ . LPA containing Fe2+ was prepared for stripping uranium from the organic phase loaded with uranium. Two grams of iron powder was dissolved in 100 mL of lean phosphoric acid, obtained after the extraction of uranium from WPA, by holding the solution overnight at room temperature. The redox potential was measured using a cell prepared by the combination of a platinum electrode and a calomel electrode connected to a potentiometer. The concentration of Fe2+ was estimated by titration with standard dichromate, and the end point was determined potentiometrically. 2.5. Determination of Uranium in the Aqueous Phase. In experiments in which large volumes of solutions were handled, electrometric methods were used for the estimation of uranium, and the handling of large volumes of solution containing 233U tracer was difficult. For this purpose, 1−2 mL of the aqueous solution was pipetted into a 50 mL beaker and assayed for uranium content by the Davies and Gray method22 using standard dichromate solutions of two concentrations, 0.05 and 0.001 mequiv/g. End-point detection was carried out potentiometrically. 2.6. Determination of Uranium in the Organic Phase. In each experiment, 50 mL of organic phase was shaken with 40 mL of 1.5 M ammonium carbonate for 3 min. The aqueous phase was separated after being allowed to settle for 5 min, evaporated to dryness, and then treated repeatedly with a mixture of concentrated H2SO4 and concentrated HNO3 under hot conditions in a beaker. The U content was assayed by the Davies and Gray method22 by titrating against standard dichromate solutions.

UO2 (H 2PO4 )2 (aq) + 2HA(org) + nT(org) ↔ UO2 A 2 ·(H3PO4 )2 ·nT(org)

It was deduced from this equation that a neutral oxodonor such as TOPO could strongly promote the uranium extraction ability of D2EHPA without causing liberation of H+ ions. A similar mechanism might be operative in the case of extraction of U(VI) into a D2EHPA + TBP mixture. The stripping of uranium from the organic phase is carried out by the reduction of U(VI) to U(IV) through contact with MGA containing Fe2+. As the extraction of U4+, which is more highly charged than UO22+, involves a complex containing higher numbers of phosphoric acid molecules, further adduct formation and solvation by extractants would be hindered, which might result in less extraction of U4+, making reductive stripping effective. The second cycle is essentially a purification cycle, in which MGA is diluted, uranium is oxidized to U(VI) and extracted back to the organic phase, the organic phase is scrubbed with H2SO4 to remove iron and rare earths, and uranium is stripped using ammonium carbonate and then precipitated as the peroxide.4,5,19 When the impurity content is high, uranium first precipitated as sodium diuranate, which is purified by TBP extraction, and then reprecipitated as ammonium diuranate. In the first cycle, large quantities of scrap iron must be dissolved to produce MGA containing 20 mg/mL Fe2+. It is desirable to avoid addition of impurities and modification of the composition of phosphoric acid, which is utilized for the production of fertilizer. Moreover, addition of high concentrations of iron to MGA can lead to the precipitation of iron and cause difficulty in phase separation during the carbonate stripping step in the second cycle.3 The use of novel reducing agents has been reported for the reductive stripping of actinides and lanthanides.20,21 Some studies have reported the use of Ti3+ in place of Fe2+ for the reductive stripping of uranium.6 In the present study, a novel approach was investigated for stripping and concentrating uranium from the organic phase in the first cycle. In this approach, Fe2+ required for the reductive stripping of uranium is generated in MGA by the electrolytic reduction of Fe3+ already present in this acid and regeneration of consumed Fe2+ within the system. This eliminates the need for external addition of a large excess of scrap iron for the generation of Fe2+, which can precipitate at times, causing difficulty in phase separation during the ammonium carbonate stripping step. In the present study, the electrolytic reductive stripping method was also compared with the conventional method in which scrap iron is used for the generation Fe2+, as well as with the use of titanium(III) as a reducing agent. The majority of these experiments were carried out using wet process phosphoric acid obtained from a fertilizer plant and solvents prepared in the plant to simulate actual plant conditions.

2. EXPERIMENTAL SECTION 2.1. Reagents. Wet process phosphoric acid (WPA) obtained from a fertilizer plant in Mumbai, India, was analyzed for metallic and nonmetallic trace constituents and found to contain typically 4.85 M H3PO4; 3.25 mg/mL Fe3+; 0.068 mg/ mL U; and other impurities such as Al3+, Na+, K+, Ca2+, F−, SO42−, and SiO2. Details are included in Table S1 (Supporting Information). Lean phosphoric acid (LPA) was prepared from WPA by removing uranium from it by solvent extraction, and it was found to contain ∼4.85 M H3PO4, ∼3.25 mg/mL Fe3+, 5419

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2.7. Determination of Percentage Extraction of Uranium in Batch Experiments. In experiments in which small volumes of solutions were handled, 233U tracer was used because the estimation by liquid scintillation counting was fast and convenient. Around 25 μg of 233U was added to 2 mL of the aqueous phase and shaken with 2 mL of the organic phase for 5 min. After phase separation, 50 μL of the organic phase was pipetted into 5 mL of dioxane-based liquid scintillator solution, and 233U was measured by the alpha scintillation counting method. From the data, the percentage extraction was calculated. 2.8. Purification of 233U and Conversion to Phosphoric Acid Medium. 233U solution was purified from other major radioactive daughter products such as 228Th and 229Th by being loaded on a 5 mL bed of Dowex 1 × 4 anion-exchange resin in 6 M HCl. The column was washed with five bed volumes of 6 M HCl. Uranium was eluted using 0.1 M HCl, evaporated with a known amount of H3PO4, and made up with distilled water to obtain a final concentration of 5 M in H3PO4. The radiochemical purity of 233U was confirmed by the alphaspectrometric method. 2.9. Measurement of Redox Potential of Solutions. The redox potentials of MGA, WPA, and LPA were measured using the combination of a platinum electrode and a saturated calomel electrode (SCE). Before measurement, the electrode response was checked by Fe2+ versus dichromate titration in which the end point was observed between 600 mV to 640 mV vs SCE

provided in Figure S3 (Supporting Information). It consists of a PVC beaker having a height of 12 cm and a diameter of 7 cm in which a PVC tube fitted with two PVC perforated diaphragms is placed. A coiled 2-mm-diameter copper wire with a length of 300 cm (cathode) was introduced inside the tube, and a 1-mmdiameter platinum wire with a length of 7 cm (anode) was placed between the tube and the beaker. The space between the beaker and the tube acted as an anode compartment. The ∼2mm gap between the bottom of the tube and the beaker acted as an annular liquid junction that offered low resistance and did not becomme clogged by the organic phase during operation for extended period. Initially, 200 mL of MGA was added to the cell; thereafter, 200 mL of loaded organic phase was added to the cathode compartment, and electrolysis was carried out. During experiments involving the flow of the organic and aqueous phases, the loaded organic phase was introduced at the bottom of the cathode compartment through a side tube and was stirred using a 6 V dc motor-driven stirrer with a Perspex stirrer. After being stirred, the organic/aqueous mixture entered the settler compartment, which was the space above the upper perforated sheet. As the disturbance from the stirrer in this compartment was minimal because of the placement of the perforated sheet, the organic and aqueous phases separated in this compartment. The separated organic phase flowed out of this compartment through a side tube, whereas the separated aqueous phase from the settler compartment flowed by gravity back to the cathode compartment, where the used Fe2+ was regenerated by electrolysis. Fresh aqueous phase entered the cathode compartment through a tube at the bottom, and aqueous phase loaded with U(IV) flowed out through another side tube. The cell was designed with the following parameters in mind: (a) The distance between the anode and cathode was a minimum, the cross-sectional area of the liquid junction between the anode and cathode compartments was a maximum, and the thickness was a minimum, so as to obtain higher current at lower applied potential. (b) The compartment separators and electrodes were positioned such that, during vigorous mixing of the cathode solution, there was minimum intermixing of the solutions in the anode and cathode compartments. Also, there was no transport of the organic phase into the anode compartment, and there was minimal air oxidation of the Fe2+. (c) The surface area of the cathode was high. The salient features of the cell are as follows: (a) The organic and aqueous phases were mixed using a 6 V dc motordriven Perspex stirrer. Such mixing is much more effective than stirring using a magnetic stirrer. (b) Two perforated plastic sheets were placed below and above the tip of the stirrer, where the incoming organic phase mixed with the aqueous phase containing electrolytically generated Fe2+. These perforated sheets limited the vigorous stirring of the phases to the space between the sheets. The perforated sheet below the stirrer prevented the transport of organic-phase droplets to the anode compartment. The perforated plastic sheet placed above the stirrer head prevented stirring of the liquid above the sheet, where the organic and aqueous phases separated, and the stripped organic phase flowed out whereas the used aqueous phase settled to the bottom, where Fe2+ was regenerated electrolytically. As there was a layer of organic phase that was not stirred above the aqueous phase, there was less probability of air oxidation of Fe2+ in the aqueous phase. (c) The length of the Cu cathode was increased from 25 to 300 cm, which offered a higher cathode surface area, resulting in more efficient

Hg|Hg 2Cl 2, KCl saturated||Fe3 +/Fe 2 +|Pt

2.10. Electrolytic Stripping of Uranium from Loaded Organic Phase under Static Conditions. Preliminary electrolytic experiments were carried out using an electrolytic cell with a copper cathode and a platinum anode placed in a fritted glass tube. A schematic diagram of this setup is provided in Figure S2 (Supporting Information). It consists of a 100 mL plastic beaker into which a 2-cm-diameter glass tube with a G2 frit fitted at one end (anode compartment) was introduced. Inside the glass tube, a 1-mm-diameter Pt wire with a length of 7 cm was held to act as the anode. In the beaker, a coiled copper wire having a length of 25 cm and a diameter of 2 mm was introduced to act as the cathode. Thirty milliliters of the aqueous phase was added to the electrolytic cell and the solution was electrolyzed for 45 min at a constant current of 0.7 A and a potential of 8−10 V using a constant-current multioutput power supply (model MCPS-7, supplied by Gynac System, Mumbai, India). To this solution was added 40 mL of organic phase loaded with uranium, and the mixture was stirred using a Teflon-coated magnetic stirring bar. Electrolysis was continued for 1−2.5 h, after which the phases were allowed to separate. The organic phase was withdrawn and assayed for U concentration, and the percentage stripping was calculated as stripping (%) = (initial [U] in the organic phase − final [U] in the organic phase) × 100/initial [U] in the organic phase

2.11. Modified Electrolytic Cell with Motor-Driven Mixer. A modified electrolytic cell was prepared using a poly(vinyl chloride) (PVC) beaker and other PVC materials available in the laboratory that could be easily modified and shaped by heating. A schematic diagram of the setup is 5420

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conversion of Fe3+ to Fe2+. (d) A cathode/anode compartment separator such as a G2 frit or glass wool was completely eliminated in the present cell, avoiding clogging of the separator with the organic phase. The annular gap of ∼2-mm width formed between the plastic container separating the cathode and anode compartments and the bottom of the cell acted as a liquid junction. The thickness of the liquid junction was approximately equal to the thickness of the inner container, which was very low (∼1 mm), and it offered minimal resistance to the cell at the junction. At any point of time, a small volume of the organic phase was in contact with a larger volume of the aqueous phase, allowing faster stripping of uranium. An earlier study showed that, as the ratio of the organic to the aqueous phase decreases, the reduction of U(VI) becomes much more efficient.6 The effects of different cell parameters such as the surface areas of the copper cathode and platinum anode and the initial concentration of Fe3+ in MGA on the rate of generation Fe2+ were studied. The effects of cell operating parameters such as the flow rate of the organic phase and the initial concentration of uranium in the aqueous phase on stripping were studied using a single electrolytic cell. After optimization of the cell parameters, four electrolytic cells were connected in series, under countercurrent flow of the organic and aqueous phases, and effective stripping of uranium from the organic phase was achieved at an organic-phase flow rate of 30 mL/min. A schematic diagram of the setup used is provided in Figure S4 (Supporting Information).

3.3. Single-Contact Stripping of U from the Organic Phase Using 233U Tracer. To evaluate the reasons for the poor stripping of U from the organic phase, series of experiments were carried out using 233U tracer and small volumes of the organic and aqueous solutions in equilibration tubes. Assaying uranium by the liquid scintillation counting method is faster and more convenient, and it can be applied to experiments involving smaller volumes of solutions. The results showed that, by contacting loaded organic phase with 5 M phosphoric acid containing 10 mg/mL Fe2+, only 2% stripping of uranium could be obtained, whereas stripping with LPA containing a similar concentration of phosphoric acid in which 10 mg/mL Fe2+ was added (total Fe concentration of 13.25 mg/mL), yielded 22% stripping. LPA contains about 0.42 M fluoride. Fluoride ion can preferentially complex and stabilize U4+ in the aqueous phase as compared to UO22+ because of the higher ionic potential of U4+, and this might help the reductive stripping of uranium. The additional amount of Fe already present in LPA (3.25 mg/mL) also might add to the improvement in stripping. The experimental results also showed that a better stripping of 45% could be obtained using LPA in which 20 mg/mL Fe2+ was added (23.25 mg/mL total Fe). Details of the experimental conditions and results are included in Table S2 (Supporting Information). 3.4. Effect of Phosphoric Acid Concentration on the Stripping of Uranium. Table 1 reports the effects of the Table 1. Effect of the H3PO4 Concentration on the Stripping of U from the Organic Phase in a Single Contacta

3. RESULTS AND DISCUSSION 3.1. Extraction of Uranium from WPA. Extraction of uranium from WPA was carried out simulating mixer settler conditions with six contacts at an organic-to-aqueous phase ratio of 1:1, and the experimental conditions were maintained similar to plant conditions to confirm the extraction efficiency of this extractant for uranium from WPA. A higher organic-toaqueous phase ratio was not used because of the difficulties associated with handling larger volumes of the organic phase under plant conditions. Uranium assays were carried out by the potentiometric method in all experiments involving larger volumes of solutions because handling of large volumes of solutions containing radioactive 233U tracer is inconvenient. The results of uranium assays of the organic and aqueous phases showed 90% extraction with 0.063 mg/mL U in the organic phase and 0.0055 mg/mL U in lean phosphoric acid. These results confirm the efficiency of this extractant for the extraction of uranium from WPA.5 The loaded organic phase was used for stripping experiments, and the lean phosphoric acid was used for the preparation of aqueous solutions for stripping. 3.2. Stripping of U from the Organic Phase Using Lean Phosphoric Acid (LPA) Containing Fe2+. Batch countercurrent stripping was carried out simulating the mixer settler conditions for four contacts in a separating funnel, using lean phosphoric acid containing 10 mg/mL Fe2+ that was prepared by adding iron filings to lean phosphoric acid. Keeping an organic-to-aqueous phase ratio of 20:1 and taking 100 mL of organic phase and 5 mL of aqueous phase each time, four contacts and four-stage batch countercurrent stripping was carried out, and 80 mL of the stripped aqueous phase was collected and assayed for U content. The U content of the stripped solution was 8.7 mg, corresponding to a stripping of ∼12%.

no.

total [H3PO4] (M)

U stripped (%)

1 2 3 4 5

4.85 (WPA) 5.5 5.8 6.8 8.5

45 78 89 98 99

a

Conditions: aqueous phase, WPA + varying H3PO4 concentration; time of equilibration, 2 min; time of settling, 2 min; volume of organic phase, 2 mL; volume of aqueous phase, 2 mL; concentration of H3PO4 in WPA, 4.85 M; concentration of Fe in WPA, 3.25 mg/mL; concentration of U in the organic phase, 0.05 mg/mL.

addition of concentrated phosphoric acid to WPA on the stripping of U from the loaded organic phase in a single contact at an organic-to-aqueous phase ratio of 1:1. With an increase in concentration of H3PO4, there is significant improvement in the stripping of uranium. Using WPA containing a total of 8.5 M H3PO4 and 20 mg/mL Fe2+, near-quantitative stripping of U could be achieved at an organic-to-aqueous phase ratio of 1:1. 3.5. Stripping of Uranium from the Loaded Organic Phase Using Multiple Contacts of LPA Containing Fe2+. These experiments were carried out by varying different parameters such as the time of equilibration, number of contacts, organic-to-aqueous phase ratio, and H3PO4 and Fe concentrations, to identify favorable conditions for the stripping of uranium from the organic phase using LPA. Table 2 reports the uranium stripping percentages obtained in tracer experiments using equilibration tubes. 233U was stripped from the loaded organic phase using different compositions of LPA solutions containing Fe2+. LPA initially contained 4.85 M H3PO4, 3.25 mg/mL Fe, and 5 μg/mL uranium. Other conditions of stripping such as the composition of the stripping agent, number of contacts, time of equilibration, and organic5421

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Column b in Table 3 lists the percentage stripping of uranium, using LPA in which 2 mg/mL Ti4+ and then 5 mg/mL

Table 2. Stripping of U from the Organic Phase with Multiple Contacts Using LPA Containing Fe2+ U stripped (cumulative) (%) no.

a

b

c

d

e

1 2 3 4 5

54 76 87 91 −

3 6 9 12 18

11 21 34 45 50

8 33 54 72 79

30 67 87 96 98

Table 3. Stripping of U from the Organic Phase with Multiple Contacts Using LPA Containing Ti3+a U stripped (%)

a

LPA + 20 mg/mL Fe2+ (U, 0.005 mg/mL; total Fe, 23.25 mg/mL); organic/aqueous phase ratio, 1:1. Experiments 1−4 correspond to cumulative times of contact of 3, 9, 12, and 20 min, respectively. bLPA + 20 mg/mL Fe2+; organic/aqueous phase ratio, 20:1; time of contact, 3 min using fresh aqueous phase each time. cLPA + 20 mg/mL Fe2+; organic/aqueous phase ratio, 20:1; time of contact, 6 min using fresh aqueous phase each time. dLPA evaporated to 5.75 M in H3PO4 + 20 mg/mL Fe2+ (U, 0.005.9 mg/mL; total Fe, 23.85 mg/mL); organic/ aqueous phase ratio, 20:1; time of contact, 6 min using fresh aqueous phase each time. eLPA evaporated to 6.5 M in H3PO4 + 20 mg/mL Fe2+ (U, 0.007 mg/mL; total Fe, 24.6 mg/mL); organic/aqueous phase ratio, 20:1; time of contact, 6 min using fresh aqueous phase each time.

contact no.

b

c

d

e

1 2 3 4

9 20 35 49

23 60 85 92

55 81 90 −

40 83 95 −

a

Conditions: organic/aqueous phase ratio, 20:1; time of equilibration, 6 min. bLPA+ 2 mg/mL Ti + Fe 5 mg/mL. cLPA + 4 mg/mL Ti 3+ (generated by electrolysis). dLPA + 5 mg/mL Ti3+ (generated by electrolysis). eLPA + AR H3PO4 (total H3PO4 =5.75 M) + 4 mg/mL Ti3+ (generated by electrolysis).

Fe powder were added to generate Ti3+ and Fe2+. At an organic-to-aqueous phase ratio of 20:1, 49% stripping of U was achieved in four contacts. Column c in Table 3 reports the stripping of U by LPA containing 4 mg/mL Ti3+ that was generated by electrolysis. At an organic-to-aqueous phase ratio of 20:1, 92% stripping could be achieved in four contacts without addition of Fe to LPA. When the stripping of uranium was again carried out under similar conditions after the Ti3+ concentration had been increased to 5 mg/mL, 90% stripping was achieved in three contacts, as seen in column d in Table 3. At 5.75 M H3PO4 in LPA and 4 mg/mL Ti3+, 95% of stripping could be achieved in three contacts. These experiments show the effectiveness of Ti3+ as a reducing agent for the reductive stripping of uranium. Even though Ti3+ can act as a good reductive stripping agent, it might be difficult to utilize this costly material in large-scale plant operations. Moreover, higher concentrations of titanium in LPA can result in precipitation. 3.7. Decrease in Concentration of Fe2+ in MGA during Stripping of Uranium from the Organic Phase. Batch experiments were carried out to determine the amount of Fe2+ consumed by the organic phase loaded with 0.05 mg/mL uranium during stripping with MGA containing 6 mg/mL Fe2+ at an equilibration time of 30 min. This was carried out by estimating the difference in Fe2+ concentrations in MGA before and after coming into contact with loaded organic phase under identical conditions except that the first experiment was carried out in the presence of air whereas the other was carried out in the absence of air. A schematic diagram of the setup used for contacting the organic and aqueous phases in the absence of air is provided in Figure S5 (Supporting Information). It was observed that, when the stripping was carried out in the presence of air, a higher amount of Fe2+ was consumed from the aqueous phase (3.8 mg/mL for the organic phase) whereas, in the absence of air, the Fe2+ consumed from MGA was only 0.44 mg/mL for the organic phase. Details of the experimental conditions and results are included in Table S3 (Supporting Information). In these experiments, extraction of Fe2+ or Fe3+ into the organic phase was not studied, and therefore, its effects are not included. This experiments emphasize the need to maintain an air-free atmosphere during electrolytic stripping to avoid air oxidation of Fe2+ generated during electrolysis to achieve efficient stripping. 3.8. Electrolytic Stripping of Uranium from the Loaded Organic Phase. Preliminary electrolytic experiments were carried out using an electrolytic cell with a copper cathode

to-aqueous phase ratio for each experiment are also included in Table 2. In Table 2, column a lists the percentage stripping of U as a function of time of equilibration using a stripping solution prepared from 20 mg/mL Fe powder in LPA. The total concentration of Fe2+ was ∼23.25 mg/mL, because the Fe already present in LPA also would be reduced by Fe powder. The results show that even at an organic-to-aqueous phase ratio of 1:1, the stripping was slow. About 54% of the uranium was stripped in 3 min. However, as the contact time was increased to 20 min, more than 90% of the U could be stripped. Column b in Table 2 reports the percentage stripping of U at a 20:1 organic-to-aqueous phase ratio. As the organic-to-aqueous phase ratio was increased, the stripping became very slow. Even after five contacts with fresh aqueous phase, only 18% of the U could be stripped. Under similar conditions, as the time of contact was increased to 6 min, the cumulative percentage stripping increased to 50, as seen in column of Table 2. When the concentration of LPA was increased to 5.75 and 6.5 M in phosphoric acid by evaporation and when stripping was carried out under similar conditions, the cumulative stripping of uranium increased further to 79% and 98%, respectively, as seen in columns d and e in Table 2. During the evaporation of LPA, the concentration of Fe also increased, and the total Fe2+ concentrations were ∼23.85 and ∼24.62 mg/mL, respectively, in solutions d and e, and this also might positively influence the stripping process, whereas the increase in the initial concentration of U during evaporation might adversely affect the stripping. These studies show the important influence of the organic-to-aqueous phase ratio and concentration of phosphoric acid in LPA on the reductive stripping of uranium. 3.6. Reductive Stripping of Uranium by Ti3+. Ti3+ is known to be a stronger reducing agent than Fe2+. Therefore, experiments were carried out using Ti3+ or a mixture of Ti3+ and Fe2+ for the reductive stripping of uranium, so that the addition of iron to the stripping solution could be minimized. In these experiments, Ti3+ was generated in LPA by the electrolytic reduction method or by the dissolution of Fe powder in LPA containing Ti4+. 5422

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influenced by the rate of the electrolytic reaction at the anode, the rate of reaction at the cathode, and the conductance at the liquid junction. As the same current is passing through all three parts, the overall resistance of the cell is controlled by the process that is slowest among the three processes. In some of our preliminary experiments, a graphite electrode of 5-mm diameter and 50-mm length was tried as the anode. However, the applied potential to obtain 700 mA was high (>30 V). Replacement of the graphite rod by a Pt wire of 1-mm diameter and 10-cm length dipped in MGA was sufficient to bring the potential below 12 V, probably because the rate of oxygen generation at the Pt surface is much higher than that at the graphite surface. The effect of the geometric surface area of the platinum anode on the potential of the cell was studied, keeping other parameters constant. Details of the experimental conditions and results are included in Table S5 (Supporting Information). The results showed that a very small surface area of platinum, namely, 1.25 cm2, was sufficient to decrease the applied potential to 4.5 V at a constant current of 700 mA. When a cell having a glass tube of 5-cm length and 2-cm diameter fitted with a G2 frit connecting the cathode and anode compartments was used in another experiment, the liquid junction offered a high resistance, and a potential of >25 V was necessary to obtain a 700 mA current. When the liquid junction was replaced by an annular gap as in the present cell, the cell potential decreased to