solvent extraction in production and processing of uranium and thorium

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Solvent Extraction in Production and Processing of Uranium and ThoriumHarvinderpal Singh a; C. K. Gupta a

a Materials Group, Bhabha Atomic Research Centre, Mumbai, India

Online Publication Date: 01 September 2000

To cite this Article Singh, Harvinderpal and Gupta, C. K.(2000)'Solvent Extraction in Production and Processing of Uranium andThorium',Mineral Processing and Extractive Metallurgy Review,21:1,307 — 349

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Solvent Extraction and Processing of and Thorium

in Production Uranium

HARVINDERPAL SlNGH and C. K. GUPTA

Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India

Solvent extraction plays a vital role in the production and processing of uranium and thorium for use as fuels in the front-end of the nuclear fuel cycle. The development of solvent extraction technology in the nuclear field in the last five decades has contributed to advances in the non-nuclear hydrometallurgy. In turn the large scale applications in the field of base metals such as copper have led to development of new equipment and techniques as well as better understanding of the process chemistry and hydrodynamics. Advances in the field of solvent extraction of relevance to the nuclear fuels, are reviewed in this paper. The significant results from the research and development work in India are also included. Various aspects discussed include chemistry of process flowsheets for uranium and thorium recovery and refining including recent improvements, diluents for use in the processes, thermal effects in extraction, process instrumentation including on-line measurements, solvent loss by entrainment, purification of feed streams prior to extraction, solvent-in-pulp processing, separation of uranium and thorium, binary extractants and application of solvent extraction in uranium enrichment.

Keyu'ords: Uranium; thorium; solvent extraction; nuclear fuel; processes; chemistry; instrumentation; loss; diluent; enrichment

INTRODUCTION

The high purity required for production and processing of uranium and thorium for use as fuels in the nuclear reactors, with concentration of some of the specific elements (called as nuclear poisons) limited to less than a fraction of ppm, can only be achieved by use of highly selective extracting agents used in the liquid or immobilised forms such as membranes o r impregnated resins. Developments in the nuclear field

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308 H. SlNGH AND C. K. GUPTA

have led to maturing of solvent extraction technology which finds application today in diverse fields such as hydrometallurgy for recovery of base metals, environmental engineering for removal of pollu- tants, analytical chemistry for separation and determination of metals in trace concentrations and organic industry, among others. There has been an inter-dependent growth among these areas in last few decades, whereby advances made in one field find application in others. In particular there has been a better understanding of process chemistry, hydrodynamics of processes operated in large scale equipment and the interface of solvent extraction with related fields. Some of the recent developments in these areas, of interest to the nuclear hydrometallurgy, are discussed in this paper. The data from the Indian research investigations has been highlighted. The discussion essentially pertains to the 'front-end' of the nuclear fuel cycle, but it is also recognised that there are related developments in the 'back-end' of the cycle which are equally relevant.

Uranium and thorium processing by solvent extraction essentially involves two steps. In the first step processing of leach solutions obtained by acid o r alkali leaching of lean ore/crude concentrate is carried out with the objective of produing a chemically pure concentrate that is free of gross impurities such as silica and iron. In the second step, referred to as refining, purification is carried out by separation of elements that are regarded as nuclear poisons. There is a related area where uranium and thorium are separated from the solutions where they occur simultaneously. The separation processes for the two steps are discussed first in this paper, and then specific fundamental phenomenon, common to all solvent extraction processes are discussed with reference to uranium and thorium.

URANIUM EXTRACTION AND REFINING PROCESSES

Extraction of uranium from ore leach solutions is carried out by solvent extraction, o r by ion exchange or by a combination of the two processes. The leach 'solutions are generally acidic in nature. Amine extraction has replaced the extraction by di-2-ethyl hexyl phosphoric acid (DZEHPA) which has poor selectivity with reference to iron.


URANIUM AND THORIUM SOLVENT EXTRACTION 309

Amine Extraction from Ore Leach ~olutions

High molecular weight amines are widely used for uranium recovery from sulphuric acid ore leach solutions on industrial scale [I]. Terti- ary amines are known to be efficient extractants [2], as are branched aliphatic amines. The mechanism of extraction has been a matter of study by several investigators [3-51. Reactions between sulphuric acid and amine, equilibria between amine sulphate/bisulphate and uranium complexes has been difficult to explain. In particular the observed exponential dependence of the uranium-VI distribution ratio on the amine sulphate concentration is difficult to correlate with the apparent ratio of amine to U-Vl in the extracted species [4]. Mechanism based on parallel competing reactions and the law of mass action yields results consistent with the observations [5]. The sulphuric acid extraction can be described by the reactions

The reactions involving uranyl sulphate extraction are:

UO, SO4 = uo;+ + so:- (6)

Parallel reactions (3) and (4) occur in the stoichiometric ratio, x, while (7) and (8) are described by the ratio, y. By mass balance the distribution ratio D can be expressed in terms of x and y. At high initial amine


310 H . SlNGH AND C. K. GUPTA

and free amine concentrations x - 1, while a t lower values x is lower and a proportion of amine bisulphate is formed. The D value for acid shows a maxima in the pH range 1.25-2.25, with the peak shifting to higher value as the initial amine concentration increases. The extraction of uranium shows similar trends, except that initial uranium concentration is too small to saturate the organic phase. Hence D falls only a t the lowest conditions, at pH - 1.25.

P r o c e s s Improvements in t h e Amine Solvent Extraction P r o c e s s

Improvements in amine process flowsheets include the use of strong acid for stripping of uranium, and precipitation from the strip solution by hydrogen peroxide, thereby completely eliminating the use of ammonia [6,7]. The conventional ammonium sulphate stripping of uranium can lead to crud problem and loss of solvent. Besides ammonia limits for effluent disposal are stringent and difficult to achieve continuously on plant scale. The chloride strip is free of the crud problem, but again there are environmental discharge limits on chloride ion. Strong sulphuric acid strip, - 400 g/l, yields a pregnant solution containing upto 125 g/l uranium. After peroxide precipitation, the solution can be recycled o r safely sent to the effluent treatment plant. Reactions involved are:

Free acid level of 275 g/l was found necessary for efficient stripping, with barren organic containing less than 0.1 g/l uranium. Degradation of the organic by strong acid becomes high a t high temperatures, and stripping must be done below 35°C. At this temperature the degradation is significantly lower than the entrainment loss rate. The acid concentration also needs to be kept lower than 450g/I. High oxidation potential of the leach solution and the presence of molybdenum in the feed are other causative factors for crud formation.



In India we have carried out batch and pilot plant solvent extraction tests on leach solution of Domiasiat uranium ore [8-131. The feed was obtained by two stage counter-current leaching. I t was purified by activated carbon. The feed analysis is given in Table 1. Batch kinetics were studied for extraction with 3.5% AIamine 336 and 1.75% iso-decanol in refined kerosene at 29°C. Results on kinetics show complete extraction in 10 seconds. A continuous mixer however requires more time, 30-60 seconds. Pilot plant flowsheet is shown in Figure I. Four stage counter-current extraction showed a loading of 1.7g/l in the extract with raffinate analysing 0.005g/l, showing over 99% extraction. Distribution ratio of 24 and saturation loading of the organic phase of 2.241 were determined. The mixer-settler used for test runs were operated at a feed rate of 1000-1500l/h. The mixer hydrodynamic data is given in Table 11. Stripping with ammoniacal ammonium sulphate as well as strong acid was equally effective. Acid strip results are given Table 111. During operation, slight crud formation was observed. The average rate of crud of generation was 39 g/m3 of feed processed. The crud accumulated in the settler solvent, which was processed periodically in a solid bowl centrifuge a t a feed rate of 1200 l/h. Crud recovered was 145 g/m3 or solvent. The amine entrainment losses were measured a t various points in the process. The raffinate was passed through a parallel plate separator and then through a flotation cell. Results are given in Table IV. The solvent recovered in the parallel plate separator was measured and found to be 40 rnl/rn3 of feed solution processed. Similarly the recovery from the flotation

TABLE1 Analysis of the Domiasiat uranium leach solution used as reed lor solvent extraction by alamine

Species d l

U3°8 0.93 Fe-Ill 0.85 Fe-I1 0.1 P A 0.13 Ca 0.27


H. SINGH AND C. K. GUPTA

pregnant feed raffiate

1 : AQUEOUS FEED TANK 2: SAND FILTER %CARBON COLUMN 4 : EXTRACTION UNIT 5: STRIPPING UNTI 6 : PARALLEL PLATE 7 : AIR FLOTATION CELL 8 : SOLVENT TANX 9 : SOLVENT 10 : mm FILTER 11:CARBON COLUMN

REGENERATOR 12 : STRIP FEED TANK

FIGURE 1 Schematic flowsheet for uranium extraction.

cell was 0.2 ml/m3. A final effluent sample was treated with carbon and it was round that complete removal was achieved. In a parallel test, the final effluent was neutralised with lime and filtered and in the filtrate the amine level was found to be 2 ppm, which is much below the toxicity limits. Air-borne hydrocarbon levels in the working environment were measured with Kitgawa gas detector tubes. I t was



TABLE 11 Mixer-settler for solvent extraction

I. MIXER

1.1 SlZE 1.2 VOLUME 1.3 IMPELLER (D) 1.4 NO. O F BLADES 1.5 WIDTH (W) 1.6 TYPE 1.7 SPEED (N) 1.8 N3D2 1.9 INFLOW ORIFICE 1.10 CLEARANCE 2. SEITLER

2.1 SlZE 2.2 INLET 2.3 RECYCLE

500 x 500 x 410 (mm) 100 Litres 240 mm, D,T = 0.48 6 40(W/D= 116, WIT= 1/12) T O P SHROUDED TURBlNE 250 RPM = 4.17RPS 45 (it2 rps3) 100 mm 15 mm

500 mm x 1200mm (0.6 m2) PICKET FENCE BAFFLES ORGANIC 1:4

TABLE 111 Strong acid stripping ofamine extract of uranium

Cf fPOJ Strip U30, S. no. M (910 % Stripping

TABLE IV Entrainment levels in pilot plant (ppm)

Floated S. no. Raffinare PPS efluent efluent

I 13 3 3 2 26 I I 6 3 22 13 6 4 2 1 12 6

PPS = Parallel Plate Separator. Levels given in ppm.

found that the concentrations were in the range 50-100 ppm near the unit and below detection limit at three metres distance. The concentration inside the vapour space below the mixer cover and above liquid level was about 200 ppm. All these are below the T.L.V. limits for hydrocarbon vapours.


314 H . SlNGH AND C. K. GUPTA

Recent developments, which are a matter of intense study, include the use of hollow fibre type liquid membranes for uranium extraction [14-161. A pilot plant plant was operated for extraction of uranyl nitrate Tri-n-octyl phosphate (TOP). T O P was preferred over T B P to cnsure low solubility in the water and increase its retention in the membrane pores. A counter-current flow of the feed and strip phases was maintained. Reactions at the feed-membrane interface are:

UO, (NO,), + 211 T O P = UO, (NO,), (n TOP), ( I 1)

HNO, + n T O P = HNO, (n TOP) (12)

The extracted complex diffuses across and at the membrane-strip interface are:

= Na,U02(C0,) + 4 n T O P + H,O + CO, + 4 N a N 0 , (13)

2 HNO, (n TOP) + N a z C 0 3 = 2n T O P

+ H 2 0 + CO, + 2 N a N 0 , (14)

Operating flux of 100 miro-gram U/mz.sec was observed. Studies have been reported [17] on uranium extraction from ground water using phosphinic acid carrier in a polypropylene membrane [16]. The process was found to be very erective in separating uranium at pH = 2. Pilot plant uranium recovery on extraction by tertiary amine in kerosene with a polysulfone membrane has shown [17] operational costs lower than the conventional extraction or ion exchange. Studies in BARC have shown uranium Rux value of 4.8 micro-moles/m2.s for TBP in silicone membranes, while the value for DZEHPA is lower to a third the value [18, 191. The separation based on hollow fibre type units, which provide large surface area, and can hence be economical for use with expensive agents, can be expected to play an important role in future [20].



P r o c e s s Improvements in t h e TBP Refining P r o c e s s

The crude concentrates produced from the ores, by-products of phosphate industry and from the monazite processing are refined by separation of nuclear impurities using TBP. Fundamental studies [21] of the extraction of U and HNO, by TBP have been carried out. The extraction is found to be stepwise in nature and initially involves the extraction of HNO, alone. Subsequently, U enters the organic phase by displacing extracted HNO, molecules until the equilibrium is reached. Similarly the fundamentals of hydrodynamics in a mixer- settler have been examined [22]. The Sauter mean diameters and droplet distributions were measured using photographs of the dispersions. Overall extraction efficiencies were measured. Settler behavior was determined by measurement of dispersion band heights. It was characterized by a normalized band height corresponding to a specific throughput of 0.144 cm3/cm2/s. Normalized band heights depend on solution composition and stirrer speed. Crud formation was studied, but solid characteristics were such that dispersion band heights were reduced, i.e., coalescence in the settler was enhanced.

In the refining of uranium by TBP, ammonium uranyl tricarbonate (AUC), (NH,),UO,(CO,), can be produced directly by contacting the loaded organic with strong solution of ammonium carbonate [23]. The solubility limit of AUC, 200g/l, can be exceeded by optimising the process parameters. Reactions involved are:

4 NH: + [UO, (CO3),I4- = (NH,), UO, (CO,), (solid) (16)

AUC is a crystalline product and has excellent physical properties for subsequent processing to the nuclear fuel grade oxide.

TBP has also been used for purification of recycled material, particularly those containing fluoride [24-271. In the Canadian plant, digested slurry containing 18% solids by weight, with 150 g/l uranium and 20 g/l fluoride at 2N free acidity was processed in 15 nos. of MlXCO columns at 50°C by 22% TBP in Isopar M with solvent/feed


316 H . S l N G H A N D C . K . GUPTA

ratio of 3:l. Raffinate analysed <0.5 g/l uranium. Stripping of the loaded organic in 2 columns with water at 70°C in 1:l ratio gave a pregnant liquor containing 35 g/l uranium and 0.1 g/l fluoride. In the Indian plant, high fluoride in the plant feed was found to lead to stress-corrosion cracking a t weld joints of steel impeller to the shaft. A separate circuit was built to handle fluoride solutions in a polypropylene mixer settler. Fluoride solutions result from leaching of uranium slag in nitric acid. Typical leach solution analyses (g/l): U = 10-15, F = 1-3; Mg = 7 and Fe = 0.5-1. The free acidity is 2-2.5 N. A five stage mixer-settler battery operating a t an overall phase ratio of O/A = 114 yields an extract containing 40-60gU/l. The unit has an internal recycle from settler to same mixer stage to maintain a ratio of O/A= 1.5/1. Loading of extract is below saturation to ensure raf- finates are below 0.5 gu l l . This does not affect the purity since the feed solution is already pure with regard to nuclear poisons and the only separation required is for F. The extract contains only 0.05-0.1 gF/I and free adidity is 0.1 -0.2 N.

Other advances in the field of refining in India [25,28] include: (a) improvement in settling rate by use of 0.05 N acid for stripping instead of dimineralised water. Tests also showed coalescence rate doub- led by an increase of temperature from 30°C to 50°C, (b) Entrainment loss in alkaline processing of solvent was 9-15 ml/l, into the raffinate was 0.7-2 ml/l and the pure strip solution was 0.5-3 ml/l. A dual knit mesh packing of steel/polypropylene installed in the settler reduced the losses by a factor of five and reduced the dispersion band width by a factor of 2, (c) solvent degradation over a period of two years reduced the settling rate 15 times. Regular processing was found necessary. Used solvent could be subjected to vacuum distillation in the range 40°C-130°C and a residue separated for discarding as waste, (d) slurry extraction as described subsequently in this paper.

Uranium from Phosphates

In India by-product uranium from indigenous phosphorites is es- timated at 1695 tonnes [29]. As India imports bulk of its 2.5 million tonnes of P,O, requirements of phosphates from Morocco, Florida Jordan, Senegal etc. - all of which are uraniferous, the potential for recovery is much greater. Research has been carried out in India on



various aspects of uranium recovery from phosphoric acid [30-411. Initial studies were carried out on the pyrophosphates as extractants. However since the pyrophosphates were found to be unstable, further studies were carried out on phenyl phosphoric acids. These had been earlier considered as inefficient extractants by the researchers at O R N L in USA. Based on systematic work with octyl phenyl acid phosphates (OPAP), a process was finalised in India, which was later developed in ORNL and used in Canada. A highly sensitive dye method was developed for solvent loss determination. In 1987, an on-site pilot plant was operated to demonstrate the process feasibility and study the requirements for feed purification. Subsequently, a large scale pilot plant was operated on-site for technology demonstration. Currently processes have been developed for simultaneous separation of rare earth elements and cadmium. Industrial operation a t a large fertiliser plant is under consideration. The environmental aspects of radioactivity have been also investigated [42].

The pre-treatment to remove 'humic' matter has been studied. It has been shown by IR spectroscopy that the humic matter present in the acid can be separated by (a) clariflocculation with appropriate flocculant, (b) contacting the acid with a viscous D2EHPA solution to precipitate the impurities as 'gunk', which can be subsequently separated in a centrifuge or a filter, and (c) adsorption on activated carbon. The efficiency of separation as measured by the optical density (OD) a t 408 nm, was found to be good. In a typical plant trial a t 1000 I/h, the feed acid had a O D 1 1 . 5 , the O D of flocculated and aged acid was 0.4, and after gunk generation it was < 0.35. No carbon treatment was necessary. About 440 gram of gunk was generated from a cubic metre of acid. The uranium concentration in the gunk was low, -0.0045%. Solvent loss with gunk was about 2/3rd weight of the gunk. The acid oxidation study in 10 m3 reaction vessel showed that the hot acid could be oxidised to an e m f . of above 450 mV by sparging of air. Oxygen sparging was needed for oxidation to > 550 mV. In either case, heating of the acid was essential for effective oxidation. The extraction tests showed that as the e.m.f. increased from 453 mV to 571 mV, the distribution ratio increased from 9 to 29. The entrainment loss was found to be uniformly low, < 30ppm. The operation of equipment such as clarifier, oxidation reactor, carbon column, phase separation settler, froth flotation was found to be highly sensitive to


318 H. SlNGH AND C. K . GUPTA

the specific characteristics of the acid, attributed to the rock source e.g., Florida, Morocco, Jordan or Senegal. In all cases however, the plant could be tuned for optimal performance.

SELECTION OF DILUENT

Selection of a diluent for solvent extraction is important in the process technology. Diluent selection depends on physical properties such as flash point, viscosity, density, boiling range, evaporation rate and solubility; on chemical properties such as influence on the distribution ratio, selectivity, reaction with the extractant; hydrodynamic factors such as third phase formation, crud formation, entrainment loss; and safety factors such as the flammability hazard, toxicity rating and effluent control limits; besides the economic factors. Diluent selected to dissolve the extractant(s) may be aromatic, aliphatic or a mixture of the two types of hydrocarbons. The nature of diluent has a significant effect on the kinetics of process, including extraction and phase separation. A diluent which strongly solvates the extractant (for example a diluent high.'in aromatics) will not allow it to be present at interface for equilibration, while a diluent with low solvation (such as a mixture of aliphatic hydrocarbons) may .not dissolve adequate concentration of extractant for efficient extraction [43]. Hence it is necessary to have a compromise. Often a mixture of the two types of hydrocarbons is used, or a third component such as a phase modifier is added to prevent formation of a third insoluble phase.

The effect of raw and sulfonated kerosene-type diluent has been investigated in detail with reference to solvent extraction of uranium and co-extractable impurities from both nitric acid solutions and wet process phosphoric acid [44,45]. Kerosene as obtained from the petroleum distillation contains a mixture of paraffinic aromatic and napthenic hydrocarbons. Stability of the diluent to chemical attack depends on the presence of double bonds which are the sites for oxidation reaction and irreversible extraction of metals. In the study reported, the effect of chemical treatment involving several washings with 98% H,SO,, neutralisation with 5% Na,CO, and final washing with water was studied by IR spectroscopy and experimental measurement of extraction performance. A comparison of treated kerosene



(TK) and untreated kerosene (NK) showed that the treatment procedure decreased the unsaturated compounds, especially the aromatics, from 16% to 6%, reduced the density and viscosity, reduced the phase disengagement times by a bctor of two, and increased the distribution ratio. In the case of TBP extraction from nitrate media, the saturation loading increased by 13%, and for a six-stage extraction followed by eight stage stripping it was found that there was reduction in raffinate level from 670 mg/l to 90 mg/l and improvement in pregnant concentration from 64 g/l to 97.4 g/l. The barren organic concentration also decreased from 2.75 g/l to 0.03 g/l. There was greater inhibition of extraction of impurities. Similarly the results on extraction from wet process phosphoric acid showed that the treatment procedure improved the recovery of uranium by 30% while reducing the co-extraction of Fe and Al by about 25% and 15% respectively. Co-extraction of thorium and rare earths was also reported.

In a similar study [46,47] on the inter-phase transfer of U-VI from phosphoric acid to kerosene solutions of D2EHPA-TOPO, it was found that a sample with aromatic hydrocarbon content of 0.5% (Ker.-A) gave higher distribution ratio and faster transfer rate than sample with aromatic content of 18.3% (Ker.-B), particularly at lower temperatures. The activation energy was also higher, at around 12.23 kJ/mol for transfer from aqueous to organic for Ker.A as against 9.04 kJ/mol for Ker-B, indicating the role of diffusional processes. The activation energy for transfer in reverse direction was higher at 46.1 kJ/mol, indicating that the rate was chemically controlled. The effect on the distribution ratio was correlated by log D = - AHl(2.303RT). The value of AH was found to be -42.49 kJ/mol for Ker.-A and -39.86 kJ/mol for Ker.-B. The behaviour of distribution ratios is different for the two kerosenes. A plot of A log D vs T shows a negative trend. The values of D rapidly decrease with acidity, non- aromatic kerosene always giving higher values. The results are consistent with the fact that the dominant species is UO;' at low acidity and UO,(H,PO:-")n a t higher acidity [48]. Performance of diluents for uranium extraction from an Australian ore leach solution has been reported [49]. It was found that for uranium extraction with Adogen 364, an aromatic diluent, led to an interfacial crud formation, while a purely aliphatic diluent gave a lower distribution ratio. Optimum



results were obtained with a solvent having 16% aromatics, density of 0.808 g/l, viscosity of 1.97 cst @ 2 5 T , boiling range of 200-240°C and a flash point of 80°C. An excellent study on the selection of diluent for practical application in treatment of wet process phosphoric acid is presented with regard to cadmium extraction [50]. Wet process phosphoric acid (WPA) is a complex solution containing a large number of inorganic impurities present both in solution and in suspension. The large number of ions present lead to numerous chemical equilibria. The formation of complexes between anions (F, C1, SO,) and cations (Fe, Al, My) results in a complex system. The source of phosphate rock (Morocco or Florida) leads to additional complications. WPA acid made from Morocco rock has Cd level of 16 ppm, while that from Florida it is 7 ppm. The purification level of 1 ppm is desired. In-depth investigations showed that only an aromatic diluent Shellsol-A (flash point 5WC, i.e., 15°C above operating temperature) could be used. Its main component is I,2,4-tri-methylbenzene and water solubi-lity is 59ppm [51] as against the solubility of 0.85 ppm for n-octane [52] . The Florida acid gives a C1- level of 0.022 M but has high organic content while that from Morocco gives 0.035 M CI but organic content is much lower. Both CI- and organic level influence the extraction. The diluent with high aromatic content has higher toxic rating.

In India over the years kerosene, purified kerosene, normal heavy paraffin (NHP)-also called as Petrofin and n-dodecane have been used as diluent for TBP in liquid-liquid extraction of uranium. N H P of required quality is now produced indigenously by a process developed jointly by the Indian industry and BARC. The properties of locally produced N H P for re-processing are: n-paraffin content > 99%; aromatic and water < 100ppm; specific gravity: 0.75 a t 2WC; flash point > 70°C. Use of N H P has the advantage of greater stability due to presence of > 99% normal chain compounds, and avoidance of viscous red oil formation. While the products of TBP degradation are removed readily by carbonate treatment, degradation products of the diluent are not easily removed and get accumulated in the solvent rendering it ultimately unusable, as also observed during refining. Refined kerosene has the advantages of high flash point and higher fire safety, lower evaporation loss, low vapour concentration in air, permitting safer working environment without undue ventilation costs and improved phase separation.


URANIUM AND THORIUM SOLVENT EXTRACTlON 32 1

The specifications of N H P have been modified for economical use in uranlum recovery from phosphoric acid, Table V. Based on the experience gained, N H P is now in use for uranium separation from ore leach liquors, separation of rare earths and for uranium refining. It may be noted that as per the hazardous substance notification of Government of India [53], three classes are defined: 1) Flammable gases with boiling point below 20°C, 2) Highly flammable liquids with flash point below 21°C and boiling point above 2WC, 3) Flammable liquids with flash point below 55°C. The paraffin does not fall in above classes. Similarly as per IATA regulations only liquids with flash point upto 60.5"C classify as flammable.

HEAT EFFECTS IN SOLVENT EXTRACTION

Enthalpy changes accompanying solvent extraction have been a subject of interest from fundamental as well as applied considerations. Thus, for example, in the extraction of uranium by tri-n-butyl phosphate diluted with an aliphatic diluent, it has been found that the transfer of solute (both uranium and nitric acid) between the two liquid phases is accompanied by significant change in temperature. Sev- eral published data for heat of reaction involving TBP under standard

TABLE V Characteristics of the Indian heavy normal paraffin for extraction

ITEM TESTMETHOD TYPICAL

SPECIFIC GRAVITY (15/4"C) ASTM D-1298 0.74-0.76 COLOUR (SAYBOLT) ASTM D-156 430 VISCOSITY (cst) ASTM D-445 1.62 1 FLASH POINT ("C) ASTM D-93 > 65 REFRACTIVE INDEX ASTM D-I216 1.4358 DISTILLATION ("C, at 760 mm Hg) ASTM D-1160

IBP > 175 BP < 245

TOTAL AROMATICS (%) UOP-495 < I TOTAL NON-NORMAL (%) Capillary GC > 98

C-I0 18-30 C-l l 20-35 C-12 20-35 C-13 10-25 C-14 < I 5



conditions are available [54-581. The enthalpy change for uranium extraction by TBP, which involves a solvation mechanism, is -54.5 kJ/mol. The heat effect for acidic extractant D2EHPA is lower, - - 30 kJ/mol. This may be because in the reaction with acidic extractant no major amount of electrostatic energy is gained when U O i C exchanges for 2 H + , in contrast to the solvation mechanism with TBP which involves association or UO;' with 2NO;. Addi- tional differences arise due to bonding with four P - 0 groups in the case of DZEHPA, compared to the two P - 0 groups and two nitrate groups for TBP. Differences of the heat of hydration of UO;+, 2H' and 2NO; are also significant. The data obtained under standard conditions can be extended to cover the variations observed in practice [59,60].

The temperature profile across a cascade can be correlated with concentration profile by mathematical modeling of the heat and mass balance processes. Since the temperature profile is easier to measure on-stream in practice, i t provides an inexpensive technique for automatic process control [54-571. A computer program based on these principles [60] has shown that, in the case of a reprocessing flowsheet, temperatures can vary by over 10°C at various stages and these correlate with concentration successfully. The transient processes can be successfully simulated. A sensitivity analysis showed that rt: 1% error of temperature measurement at each mixer settler stage (i.e., k0.3"C for 30°C) does not lead to any serious errors in the estimation of concentration profile. The process control strategy for changes in solvent flow rate, scrub acid concentrations, T B P concentration, feed uranium concentration, feed flow rate can be successfully implemented.

Heat effects are equally important when the ambient temperatures difler significantly from the optimum process conditions. Thus for example in the case of uranium recovery from Domiasiat ore in India, the ambient temperatures can go down as low as 3"C, and insulation of mixer-settlers as well as heaters Tor the start up are essential in the extraction circuit. In the case of separation from phosphoric acid, the stripping of uranium from the extract requires temperatures above 50°C. In addition the strip acid is very viscous and forms an emulsion if temperatures fall below 40°C. Hence it was found necessary to install graphite heaters in the mixer itself.



PROCESS INSTRUMENTATION AND CONTROL

The efficient operation of solvent extraction plants is very strongly dependent on the level of process instrumentation and control. While much of the items are standard in the chemical industry, specific items of interest to the uranium extraction are discussed.

An important requirement for automatic process control of solvent extraction is the development of techniques for sensing the concentration levels of the extracted metal in the organic phase. Several methods, depending on the metal of interest and the accuracy required, have been reported. An interesting method involves laser-in- duced thermal lensing effect [61] as demonstrated for uranyl nitrate in 30% TBP solution [62]. A pulsed dye laser excites uranyl ions so as to create a local temperature gradient which changes the refractive index. A probe beam from He-Ne laser passing through the sample diverges. A fast Fourier transform spectral analysis helps determine the absorbance of the sample solution, and detect concentration difference of 0.0001 M of uranyl ion. Normal absorption spectroscopy is unable to resolve such low differences.

On-line analysis by X-ray fluorescence is becoming important for automatic control and process optimisation. In India installations have already been made in the ore concentrators of Hindustan Zinc Limited, Hindustan Copper Limited and Indian Rare Earth Limited. Similar application for solvent extraction are under consideration. The system uses an energy dispersive solid state detector and radio-isotope energy source for the X-ray analysis. Each element in the solution stream emits fluorescent X-rays of an energy that is characteristic of the element. The signals are processed for resolution into concentration measurements. Elements in the range of atomic numbers 16 to 92 (sulphur to uranium) can be measured besides measuring pH and temperature. Typical accuracy is 2-8% in the concentration range 0.001-1 g/l, and 1-2% for > 10 g/l.

ENTRAINMENT

Successful industrial operation of solvent extraction processes requires that the loss of solvent into the aqueous raffinate should be minimised.



The solvent loss is due to soluble loss as well as entrainment loss. The soluble loss, which is generally low, depends on the physico-chemical characteristics of the system selected for operation and is beyond the control of an operator. The entrainment loss is due to mechanical factors and can be significantly high. Experimental investigations have been conducted world-wide for understanding the mechanism of entrainment loss and development of equipment for its minimisation [63-711. In a commercial plant operating with large flows, even a ppm level loss can be unacceptable either in terms of the economics o r toxicity load on the environment if released as effluents o r lead to downstream technical problems.

Primary reason for entrainment from mixer-settlers, which are the preferred equipment for large scale operation of solvent extraction, is secondary liquid-liquid phase dispersion. The secondary dispersion consists of extremely fine droplets which do not coalesce in the conventional gravity settlers [64]. Secondary droplets are formed which are stable and their distribution depends on several factors such as: droplet-film particle size, curvature of the droplet film interface, density differences between the phases, viscosity ratio of the phases, im- pedance ratio of the phases, interfacial tension, interfacial potential, temperature effects, mechanical vibration effects, presence of third phase, mutual solubility, external electric field, external magnetic field, internal electrical field.

The aqueous raffinate needs to be passed through equipment such as post-settlers, flotation cells, absorption columns, centrifuges and electrical settlers for overcoming the limitations of ordinary gravity settlers in coalescence of secondary droplets. We have carried out investigations in a 1000 litreslhr pilot plant having a lamellar parallel plate coalescer as well as a flotation cell. The applications include (a) uraniferous phosphoric acid contacted with an organophosphorous synergistic mixture in aliphatic diluent, and (b) sulphate leach solution containing uranium contacted with 0.5 M tertiary amine in aliphatic kerosene and an alcohol as phase modifier. The details of the lamellar settler are shown in Figure 2. The entrainment levels were measured by a dye technique [33] and also confirmed by infra-red spectrometry. The efficiency of the lamellar settler was found to be > 90% and that of the flotation cells to be 80%. The secondary dispersion resulting from the last settler can be measured by laser size analyser. Droplets


URANIUM AND THORIUM SOLVENT EXTRACTION

CLEAN ACID OUTLET

SLUI PLATE PA

FIGURE 2 Parallel Plate Separator for recovery of entrained solvent.

of the dispersed phase diffract the incident laser beam due to differences in the refractive index of the organic phase and the aqueous phase. The emergent beam is detected by a multi-channel detector and the angle of dfiaction is correlated to the droplet size. The data is analysed on-line to obtain the size distribution. Typical results are shown in Figure 3. The fine droplets in the settler approach the corrugated parallel plate, get adsorbed on the hydrophobic surface, the intervening film between two droplets ruptures and coalescence takes place, as shown in Figure 4. Large coalesced drop rises to the surface where it is skimmed off.

The data obtained on droplet size distribution of the entrained phase can be used to model the performance of the lamellar settler by a population balance approach. The desired inclination of the plate separator has been reported to be 15". At this slope the fouling capacity of the unit is reduced and the load of dispersed phase is accept- able [71]. Solutions with higher solids and scaling tendency need higher slope, -45".

The performance of the primary settler in the mixer-settler battery has been the subject to considerable research over the years [65,72-751


H. SlNGH AND C. K. GUPTA

18.00 -

-

12.00 -

-

8.00 - VOLUME (%) -

4.00 -

0.00 5.00 10.00 15.00 20.00 25.00 DIAMETER (ym)

FIGURE 3 Laser size analysis ofentrained solvent drops.

L i & o & Heavy phase 1 droplet Heavy phase

continumn / Inclined

Coalescence

d a c e

FIGURE 4 Coalescence of secondary droplets in a Parallel Plate Separator.

as it is the source of the formation of entrained droplets. It has been shown that the data on settling can be correlated by a power law: H = K (Q, /A)q , where H =thickness of the dispersion band, Q, =flow- rate of the dispersed phase, A =settling area, K and q =empirical



constants. The scale-up based on this approach suffers from the limitations that the K and q values are highly variable, depending upon mixing intensity, phase continuity, temperature, phase ratio, impurities present in the aqueous and organic phase etc.

Hydrodynamic studies have shown that the droplet size is independent of the dispersed phase flow rate, the thickness of the dispersion band is a function of the specific flow rate of the dispersed phase but independent of the total flow of the dispersion and there are both vertical and horizontal zones of coalescence. In the horizontal zone the dispersion is uniformly spread with little variation in the drop size. In the vertical direction, the droplet size changes very significantly.

Dramatic reductions of the aqueous entrainment in the organic phase have been reached by using an externally applied pulsed D C electrical field across coated electrodes [67,68,75,76]. Field tests in the US have shown for amine extraction of uranium, the conventional loading rate can be increased six times by using an electrostatic coalescer.

Fibre bed coalescers provide another technique to reduce entrainment [76,77]. Modelling studies show how the inertial interception leads to coalescence as a function of internal pore properties, shape, area and hydraulic diameter. Fibre coalescer media have high surface area and rather large pore cross section which help enhance efficiency without increasing the pressure drop. Polypropylene fibres are superior media for promoting coalescence.

Structural factors influencing phase separation rates, and con- sequently the entrainment level, have been systematically investigated for systems employing tertiary amine extractants [78]. The extraction of uranium has also been evaluated. It was found that in the case of organic continuous (OC) mode, increasing the number of carbon atoms per chain (n) reduced the phase separation rate, whether the chain was linear or branched. For a constant value of n, the branched chain amines were found to separate faster than linear chain in the O C mode. A key structural factor for the O C separation was found to be the backbone chain length i . e., the longest chain in each alkyl group. Aqueous continuous (AC) dispersion separated fast, but was very sensitive to presence of colloidal silica as well as the use of actual leach solution instead of the synthetic one. Stabilisation of AC dispersion by silica and crud formation was explained by a model. Larger molecular weight amines with n z 10 were found to give faster settling


328 H. SINGH AND C. K. GUPTA

rate, less emulsion stabilisation and crud formation as well as higher uranium extraction and lower tendency for formation of a third phase. Solubility losses can also be expected to be lower. Results showed that uranium ore processing plants based on the amine process could improve performances by using branched chain extractants of higher molecular weight.

PURIFICATION O F AQUEOUS AND ORGANIC PHASES

Aqueous solution containing the metal value of interest must meet stringent requirements with regard to impurit& before it is considered suitable for industrial operation of solvent extraction. Presence of colloidal silica can lead to deleterious process performance, as discussed earlier with regard to the entrainment levels. Impurities in the commercial extractants, solvent degradation products, presence of solids and humic acids in the ore and their build-up over time can have a marked effect on the process performance [79,80]. Actual process testing is essential for each specific system. Mere measurement of physical parameters can lead to misleading conclusions. Higher interfacial tension can thus be expected to increase the phase separation rate since it can be expected to increase the thermodynamic instability which favours coalescence. But the opposite trends have been reported from the plant experience [79] and research studies [80]. The effect of impurities in the commercial extractants is pronoun- ced on phase separation in the case of aqueous continuous dispersions when ore leach solutions containing silica are used. Purification can lead to dramatic improvements. Similarly the effect of recycling water from a tailings pond back into the solvent extraction circuit in a mill is known to have caused emulsion problems [78].

Crud formation involving build-up of a film like solid phase at the interface of the aqueous and organic phases is a serious problem in the industry [81,82]. The interfacial layer of crud stabilises organic droplets and prevents their coalescence. A large amount of solvent gets trapped, leading to high losses and in extreme cases it can cause expensive shut-down. Crud formation is enhanced not only by Si, but also by Fe and to lesser extent by alkali metals. lncorporation of a scrub.stage before the stripping, apart from any benefits in terms of



purity, helps to improve the system hydrodynamics by isolating the crud to extraction stages only. Addition of fluoride is one of the methods of dealing with a siliceous crud problem [83]. Crud formation is also affected by the nature of diluent, aliphatic diluents like Exsol D 100 (aromatics < 0.9%, flash point 102"C, distillation range 235-262"C, non-flammable) producing less crud than aromatic diluents when contacted, for example, with wet process phosphoric acid [84].

Purification of a solvent by contacting with another extractant has been reported in a novel application [85]. Solvent containing 25%TBP in lsopar M was contaminated with tributuxyethyl phosphate (TBEP) leading to reduced extraction of uranium and higher phosphorous levels in the uranium product. Conventional sodium carbonate wash gave an organic containing 100 ppm uranium, higher than the required limit of 5 ppm. The strong complexing power of IONQUEST 201 (I-hydroxyethyl-1, 1-diphosphonic acid) and its abi- lity to form metal complexes which are soluble in water was utilised for purification of the contaminated solvent. The IONQUEST 201 product stream was decomposed by hydrogen peroxide into phosphoric acid and carbon dioxide for subsequent treatment.

Electrodialysis techniques have been investigated [86] for purification of uranium leach solutions to remove anionic impurities such as molybdates.

In our pilot plant test work, purification of feed solution has been found essential when organic impurities are present in the form of dissolved 'humic' matter and insoluble oils, greases, etc. due to leakages into the process fluid from gear-boxes etc. As solvent is repeatedly recycled, these impurities can accumulate. Once poisoned, there is little possibility of solvent purification. The humic matter resulting from dis- solution of organic impurities under strong lixiviation conditions has been recognized as a major problem in many plants. Harmful affects are (a) poor phase separation in settlers, lowering capacity (b) stable emulsions leading to solvent loss and plant shut-downs (c) aggravated problems in stripping, especially with 10% (NH&SO,. A detailed Chinese study on sandstone-type ores (using IR Spectroscopy, electron microscopy) used in acid leach and amine extraction has shown [87] following: (a) Organic continuous dispersion inseparable at humate level about 39 mg/l. Upto 30 mg/l the separation time increases linearly, from 30 sec to 300 sec., (b) Nature of crud varied from 100 nm spherical


330 H. SlNGH A N D C. K. GUPTA

particles to fibrous forms. (c) Decrease in uranium loading was to two-thirds of normal value. (d) Pre-treatment with activated carbon adsorption o r coagulation with higher molecular weight flocculant, removed organics. (e) The accumulated humus in organic phase could be removed by sodium carbonate scrubbing of solvent.

SOLVENT-IN-PULP PROCESSING

In the front end of the nuclear fuel cycle, extraction of uranium from ore leach solutions commonly uses solvent extraction. Mixer-settlers are the preferred choice of equipment. The mixer-settlers are however prone to failure if the feed solutions are not adequately clarified. The clarificati'on of the feed slurry is an expensive operation, accounting for nearly a third of the capital costs and 10-20% of the operating costs. Solvent in pulp (SIP) technology is under development for di- rect extraction from slurries without prior filteration [88-911. It also increases the overall recovery by reducing the soluble uranium losses. The equipment to be used for contacting needs to have the capacity to handle solids. An 'internal mixer-settler' has been tested for extraction by the DZEHPA solvent from sulphate leach solutions [88]. In this apparatus mixer and settler are combined in a single tank. Settling takes place in the bottom and top sections. Internal recycle of organic phase provides an O/A ratio of over 10 which helps in treating dense slurries, having solids in the range of 40-55% w/w. From the mixer tank, the aqueous is pumped from the bottom while the organic over- flows by gravity. The losses of organic could be reduced by varying parameters such as mixer speed, dilution of the effluent with water or addition of surface active agents such as organic sulfonates to levels of 0.03-0.06 m3/tonne of solids. High levels of entrainment were observed to be due to free standing droplets ranging in size from 15-150 micron. Slurry extraction with amine solvents was considered less feas- ible since the amines are preferentially adsorbed on the solid surface.

Alternatively pulsed columns have been tested [91,92]. When treating solids, the flow rate, amplitude and frequency need to be decreased by a factor of two compared to operations with clarified solutions. Under these conditions, uranium extraction from 20% solids by Alamine 336 could be achieved at a solvent loss of < 50 ppm. However



the throughput was low and the inventory of solvent high [92]. The ralfinate from uranium was processed for recovery of copper and nickel and by slurry extraction using LIX63 and D2EHPA. Feasibility of slurry extraction was demonstrated in a 250mm column at a throughput of 13.5 T P D solids in a slurry containing 35% solids with amine loss limited to 35 gmltonne [90]. The loaded organic was filtered through a plate and frame filter and the solids were repulped for solvent recovery and recycle. Acidification of the solvent after stripping was carried out to pH of 0.5 to prevent crud formation, which occurred at higher pH. The amine loss was of the order of 15 gmltonne of solvent for each settler [93]. A more extensive investiga- tion [94] showed that aqueous continuous phase gave much lower solvent losses, even though the operation in organic continuous mode was easier. Effect of the plate material was also reported. In a recent study [91] it was found that the presence of solids reduces the dispersed phase hold-up, increases drop sizes and leads to loss of the organic phase by formation of stable emulsions or adsorption of the solvent on the solid surface. The erosion of the pulsing stator can be expected to be high. The occluded solvent needs to be recovered by an additional step. A recent study on hydrodynamics of Rotating Disc Contactor has shown [95] that it can handle solids of the order of 20% by weight.

Solvent in pulp is a potential technology for economical uranium extraction and is a logical extension to proven technologies of resin- in-pulp and carbon-in-pulp [96].

In India, air-mixed slurry extraction units have been developed [97] and have been in operation in three plants in the country. It has helped to eliminate three stages of filtration and cake-repulping when conventional units based on clarified feed were in use. The unit is schematically shown in Figure 5. It has no moving parts and is without control valves. Air lift pump is used for inter-stage transfer of the aqueous phase and for mixing the two phases. Organic flows by gravity from stage to stage, with an overall level difference of 300 mm for seven stages..A large recycle of the organic, controlled by flow regula- tion of air used for air lift, is maintained to ensure operation in the aqueous dispersed mode and minimise the loss of solvent. Slurries containing 15% solids could be handled. The air is separated from the liquid-solid phases at the end of air lift in a disengagement chamber. It passes through mist eliminators and is exhausted through a header. At


332 H . SlNGH A N D C. K. GUPTA

FIGURE 5 Solvent in Pulp(S1P) for uranium refining.

the raffinate end, the aqueous phase is drawn by air metering. Mix- ing efficiency of the order of 85-90% has been achieved during the contacting in the air lift zone with a residence time of the order of seconds. The organic flow to subsequent stages is through the settler downcomer, which is so sized that there is upward flow of organic and downward flow of the aqueous. The recycle rate of organic at each stage is very important and an automatic control system has been developed.

EXTRACTION OF URANIUM AND THORIUM

Solvent extraction plays a vital role in the recovery and separation of uranium and thorium from monazite mineral concentrate of monazite



produced from beach sand minerals. It is also important when mined ores containing both uranium and thorium minerals are processed [98,99].

Extraction by TBP

Selective extraction of uranium, in the hexavalent form, from the tetravalent thorium from nitrate leach solutions by neutral organophosphorus compounds such as TBP has been practiced on commercial scale world-wide [loo-1021. The separation by TBP generally uses a low concentration of TBP, say 5-lo%, for selective extraction of uranium over thorium. From the raffinate, free of uranium, thorium extraction requires a higher TBP concentration, about 40%. Here care has to be taken to avoid formation of a third phase due to lower solubility of the extracted Th(NO,),. TBP in the non-polar diluents used. The selectivity of TBP is rather low. It can be improved by use of other phosphates and phosphonates, particularly by incorporating branched alkyl groups [102-1031. Fundamental studies on the TBP- nitrate system have been carried out for better understanding of the process [104-1061. The extraction rates of uranium and thorium increase with increasing nitric acid concentration and with saturation appearing a t nitric acid concentration higher than about 2.0 M. The rates increase linearly with NO; ion concentration, but decrease with H' ion concentration. Apparent activation energy of 16 kJ/mol for uranium and 43 kJ/mol for thorium was obtained from the temperature data. It suggests that the extraction rate of uranium and thorium is controlled by chemical reaction at lower temperature and by diffusion process at higher temperature. The thorium nitrate extraction by tri- alkyl phosphates is by solvation with 3 number of molecules as well as by 2 numbers. Uranium forms only di-solvates with TBP.

Separation by Carboxylic Acid Amides

Extractants of the carboxylic acid amides, N-alkyl type (R.CO.NHR'), developed as alternatives to organophosphorus extractants in the back-end of the nuclear fuel cycle, are of interest in the front end as well [107]. The effect of extractant structure on the selectivity of separation and stoichiometry of the extracted species has been studied



by varying the alkyl groups R and R'. Also studied are the N,N- dialkyl amides (R.CO.NR,') and non-substituted amides (R.CO.NH,). Studies were carried out by solutions of the amides in toluene.

Increasing steric bulk of the alkyl groups R and R', for N-alkyl amides, was found to cause a marked decrease in the extraction of thorium with the effect on uranium being lower. This significantly increases the selectivity between the two metals. The effect of group R' becomes strongly evident for tert-alkyl groups, or if both R and R' are branched. The alkyl group R has greater steric effect than R'. The structure of U-VI complex [lo81 can explain these differences as the amide ligands are coordinated to the metal ion by the carbonyl oxygen atoms. The significantly greater steric effect observed in the extraction of Th-IV are apparently due to the difficulty of assembling four nitrate ions and the required number of amide ligands around the central cation. Comparison with the separation factor for neutral organophosphorous compounds shows amides are capable of superior separation to both 1M TBP, having a factor of 1.89, and 0.15M di-n-butyl n-butylphosphonate, having a factor of 1.51.

The separation factors for the extraction from 2M sodium nitrate by O.SM N,N-Dialkyl Amides R.CO.NR, have been evaluated and results show that the separation factor for branched chain compounds are much higher. Thorium is appreciably extracted only by a compound containing R=CH,.

Extraction by amides is generally by solvation type mechanism [lO8-1 lo].

Where M n f denotes Th4+ or U O i f . The relation for the distribution ratio D follows:

log D = log Kex + n log[NO;] + plog[Amide] (18)

Experimental data [lo71 showed that values of p were in the range i.98-2.09 for uranium and to 2.75-3.37 for thorium, with the n values being 2 and 4 respectively. At lower amide concentrations, the value of p for thorium has been reported to be two [108].



Separation by Tri-sec-Butyl Phosphate

The steric effect on the differential extraction by phosphate or phos- phonate is the basis for using extractants with the introduction of branching at the first carbon atom of the alkyl group attached to the oxygen atom. The extraction of uranium is not affected but that of thorium is reduced due to the higher number of extractant molecules (3-4) in the thorium solvate.

Experimental studies have been reported on extraction by tri-sec- butyl phosphate (TsBP) [ l l 1-1 121 and tri-iso-butyl phosphate (TiBP) [113]. Results on uranium show that for low acidity, say upto 3 M, the differences in the three cases of TBP, TsBP, and TiBP are not very significant. The extraction of thorium decreases very significantly at high loading and high acidity when TBP is replaced by TsBP. Thus at 5M HNO,, from a feed containing 1 g/l U-VI and 50g/l Th-IV, the separation factor using 1.1 M for TBP is 6.8 while that for TsBP is six times better at 41.2. The selectivity can be further improved by lower loading and by using lower extractant concentration. An additional advantage of TsBP is the significantly higher, by a factor of 1.3 to 1.5, the limiting organic concentration for third phase formation in comparison with TBP. The higher solubility of the thorium solvate with TsBP also makes it a better choice for extraction of macro levels of thorium in the second cycle.

Separation by DZEHPA in a Carboxylic Acid Diluent

DZEHPA is generally used in a diluent in which the extractant is generally aggregated as a dimer. Extraction of uranium and thorium from an aqueous mineral acid using D2EHPA in a dimerising diluent yields very high distribution ratios [ I 14-1151. An alcohol can be used as a monomerising diluent [I161 but it yields very low distribution ratios. An alternative is to use carboxylic acid as a monomerising diluent [117]. Extraction behaviour for U-VI and Th-IV using 2-ethyl hexanoic acid as a diluent for DZEHPA has been reported for an aqueous chloride phase [118]. It is found that the extraction reactions correspond to


H. SINGH A N D C. K . GUPTA

where HY represents the monomer of D2EHPA. With H Z represen- ting the monomer of 2-ethylhexanoic acid, the extracted species are postulated to be Th(HY),(HYZ), and UO,(HY,)(HYZ). The distributions ratio K in terms of formality F (defined as the number of for- mula weights of solute per litre of solution), acidity [HC], and a constant K , is K = K,, F3/ [HfI4 for thorium and K = K,, FZ/ [H+Iz for uranium.

The value for K , are 70000 for thorium and 200 for uranium. Selec- tivity with respect to trivalent rare earths which have K, values 0.01-7, is excellent. The extracted metals can be stripped into an aqueous phase by 6 F HCI. I n fact the extractant scheme has been proposed for extraction of thorium from low-grade resources while purifying from rare earth contaminants. During the extraction by D2EHPA, regeneration of organic solvent can be achieved completely by contacting with aqueous solution of oxalic acid [I 191.

Similarly extraction with bis para-octylphenyl phosphoric acid, de- noted as HDO+P, has been investigated from chloride medium [I201 and i t is found that the K, ratio T h 4 + / U O i + is nearly 2.0 x lo5, and the selectivity w.r.t. Eu is over a million. O n the other hand if the extractant is bis-diisobutyl methyl phosphoric acid (HD(DIBM)P), then the K, ratio is reversed to - 2 x lo- ' and uranium extraction is favoured. The latter extractant has adequate selectivity in the nitrate medium also to be useful [121]. Thus the liquid-liquid extraction system can be selected to extract the minor component selectively, whether uranium or thorium.

Selectivity of uranium extraction over thorium from hydrochloric acid solutions using diphenyl sulphoxide (DPSO) has been investigated with respect to parameters such as concentration, temperature and acidity [122]. Sulphoxides containing the S - 0 group, in contrast to the P - 0 group of organophosphorous compounds, are simple compounds having a pyramidal molecular structure coordinat- ing by donating of the electron pair on the oxygen. Their stability is good and viscosity much lower than TBP. Uranium can be preferentially extracted from 5 M HCI with a neutral salting out agent at a low temperature using a higher concentration of thorium than uranium. The separation factor is 3690 and virtually all the uranium is extracted,



leaving thorium in the aqueous phase. Benzene is used as diluent and the method is appropriate for laboratory work.

Comparative extraction by di(2-ethylhexyl)sulphoxide and TBP from nitrate media has shown DEHSO is superior 11231. Use of binary mixtures for simultaneous recovery of Th and rare earths in nitrate medium has been recently patented [124]. The process comprises contacting a nitrate solution with an organic extractant compound containing two extractants, one from the group OP(OR1)(OR2)(OR3), OP(OR1)(OR2)R3, O P ( O R ' ) R 2 ~ 3 , and OPR1R2R3 [R1-R3 =

hydrocarbon radical.]; and second from the group P(OR4)=(OR5) 0 , H [R4,RS = hydrocarbon radical]. The solvent mixture has a high separation coefficient.

Use of dialkyl dithiophosphoric acids for thorium extraction has been reported [125]. Studies on use of chelating extractants for uranium and thorium have been reported [126-130). LIX84 (2-hydroxy- 5-nonylacetophenone oxime) and its mixtures with other chelating extractants or neutral donors showed very little extraction for thorium, although uranium was extracted. Extraction of uranium V1 from nitrate solutions by 10% (v/v) LIX 622 in a benzene diluent was found to increase with increasing equilibrium pH in the range 3-6. TBP acts as a synergistic modifier upto 2%(v/v), beyond which antagonistic effects are observed. Thorium-1V extraction by LIX 622 and its mixtures with Alamine 336 and Aliquat 336 was found to be poor in the pH range 1-4. Separation of Mo-VI, which is undesirable in uranium circuits, can be achieved at high acidity. The extraction corresponds to:

The dependence of distribution ratio D on the acidity is given as

log D = log K + log[NO;] + log[HL]

where K is the equilibrium constant. In the process it was found that pH adjustment by addition of am-

monia or caustic solution led to formation of turbidity and third phase, and had to be carried out instead by the addition of pyridine. Extrac- tion decreased with increasing concentration of CI and SO, ions.


338 H. S l N G H AND C. K. GUPTA

Supercritical fluid extraction of uranium and thorium from nitric acid solutions with organophosphorus reagents is an exciting new area which can lead to newer separation processes [131]. An organophosphorus reagent is dissolved in supercritical CO, by passing the fluid through a reagent vessel placed upstream of the extractor. Using TBPO or T O P 0 in supercritical CO,, effective extraction of uranyl and thorium ions can be achieved even from dilute HNO, solutions.

Plant Flowsheets for Uranium and Thorium Separation

Uranium ores of the Elliot Lake, Canada contain thorium and rare earths, and process flowsheets have been practiced for their recovery [132]. Ore containing'0.l WO uranium, 0.028% thorium and 0.057% rare earth oxides were processed by sulphuric leaching, clarification and uranium extraction by ion-exchange. Barrens containing (g/l) 0.13 thorium, 0.10 rare earths, 0.005 uranium, 0.04 titanium, 18.9 sulphate ion are processed in 3180 litres/min. solvent extraction plant. Or- ganophosphorus extractant is used at aqueous/organic ratio of 9-11:l in a mixer with contact time of 1-2 min and settler with a surface area of 149 m2. Phase separation is rapid and is complete in 30 sec. Solvent is stripped with ION H,SO, at organic/aqueous phase ratio of 4:l Strip settler has a capacity of 6813 litres. Mixed solvent loss is 333 ppm of barren aqueous. Solvent regeneration is carried out partially using HF. Thorium is recovered as a sulphate. Rare earths are co-precipitated.

In India monazite is the main source of rare earths and thorium, besides producing by-product uranium. A new process for separation of these elements by a solvent extraction route is shown in Figure 6. The composition of various streams is shown in Table VI.

BINARY EXTRACTANTS

Extraction of uranium by a binary mixture of Aliquat 336 and PC88A from aqueous phosphoric acid has been studied recently [133]. In the extraction studies using 5%(v/v) Aliquat 336 (trade name for tricapryl methyl ammonium chloride:R,R'NH,PO,), 5% (v/v) PC88A (trade name for 2-ethyl-hexyl phosphonic acid mono-2-ethyl hexyl ester: (HA),) and their 1 : 1 binary mixtures in xylene, it was found that there is a synergistic effect over the entire range of acid concentrations,



103'0 ALAMINE-336 INKEROSENE I . I

1 1 M P C S A INKEROSENE

Loaded Organic

EXTRACTION I STRIPPING

5%

, .

h J- Scmb Ti Strip

aEinate I1 RE. Th. Fc. Ti RE, Th. Fe. Ti Th, Ti. F

U &Fe STRIPPING

FIGURE 6 Flowsheet for separation of U, Th and rare earths.

Ra&nate I

bJcous O.lNHQ Y

Feed ,

TABLE VI Typical composition of streams for separation of uranium, thorium and rare earths by PC8BA/Alamine 336

Stream, gll- > CeO, R E 0 Nd,O, T h o , U ,O, Fe,O, T i O ,

Aqueous Feed Raffinate I U-Strip Raflinate I1 Scrub Th-Strip Alamine 336 Extract PC 88A Extract Stripped Alamine 336 Stripped PC 88A Ti-Strip

Flouride concentration in strippcd PC88A can be in the range of 1-100 ppm and in the Ti strip solution 5%. Streams correspond to the Figure 6 streams.

which was attributed to the formation of mixed adduct species, UO,A,(R,R'N,)(H,PO,), in the extracted organic phase. The distribution ratio was found to decrease in the acidity range 0.4-1.4 M


340 H . SINGH AND C. K. GUPTA

H,PO,. Effect of diluent showed that there was a decrease in extraction by a factor of 1.15 when diluent was changed from xylene to kerosene.

In a similar study, involving uranium extraction from phosphoric acid solutions [134], it was found that there was antagonism in using mixtures involving tri-isodecylamine (Alamine 310), TBP, di-n-pentyl sulfoxide (DPSO). Maximum extraction was found in the higher acidity range. Extraction was less by bis(2,4,4-trimethyl penty1)phosphininc acid (Cyanex 301) and its mixtures in the range 0.2-1.0 M H,PO,.

Uranyl extraction by mixtures of Cyanex301/Alamine 308 (triisooctylamine) with TBP/DDSO(didecyl sulfoxide) from chloride solutions has been reported [135]. Mixtures of Cyanex301 (RH) with TBP, DDSO and Alamine308 show significant synergism by the solvation mechanism. Extracted species is of the type UO,R,.L where RH is Cyanex 301 and L is for the synergistic neutral ligand TBP, DDSO or Alamine 308.

UO:&, + 2RH(org)=UO,R,(orgj + 2H + (aqj (23)

It is to be noted that Alamine 308 behaves like a neutral donor in the range 0.2 to 1.0 M HCI mixtures of Alamine 308 with TBP or DDSO result in a synergism where a species of the type (R,'NH),UO,Cl,L is extracted, with R,'N being the amine and L = TBP or DDSO.

Mixtures of Alamine 308 and Cyanex 301 used for extraction from aqueous phase at acidities above 2 M HCI were found to show strong antagonism.

Quarternary ammonium compounds (such as Aliquat 336) have better extractive power than the tertiary amines [136-1381 and the extraction of uranium with mixtures of Aliquat 336 and PC88A, in xylene diluent, from nitric-sulphuric acid medium has been investigated [139]. Aliquat 336 is a quarternary ammonium compound of the type tricapyryl methyl ammonium chloride. Extraction from 0.5-6 M HNO, solution showed maximum synergism at 3 M. The result is interesting since extraction with PC88A alone showed a minima in extraction a t 3 M. The synergistic coefficient SC defined as


URANIUM AND THORIUM SOLVENT EXTRACTION 34 1

log ((D,,)/(D, + D,)), reached a value of 0.251 at 3 M. Extraction from H,SO, medium is of interest from the point of view of application in the uranium ore processing, and both antagonistic and synergistic effects are important.

The effect at lower acid concentrations is attributed to the interac- tion between the extractants:

At high concentrations of the acid, the sulphate compounds of the quarternary ammonium salt are formed which decrease the antagonistic effect [140].

Extraction using a binary mixture of benzoic acid and cinnamic acid has been reported [141].

Synergism was reported in the solvent extraction of uranyl nitrate by tetrabutylalkyldiphosphonates (TBADP) and tetrabutylalkylene- diphosphonate-D2EHPA mixtures [143]. The TBADP tested included both the methylene and ethylene varieties:TBMDP and TBEDP with the structure as ((RO) 2(0)P - (CH)n - P(O)(OR)2) with n = 1 for TBMDP. A synergistic effect was observed when mixtures of TBADP- D2EHPA were used as follows:synergism is explained by the neutral donor ligand property [144], which also explains the superiority of TBEDP and TBMDP.

Extraction of thorium with binary mixture of PC88A and T O P 0 from an aqueous HCIO, media with various diluents has been investigated with regard to the synergistic effect [142]. Modelling of uranium and thorium extraction data has been carried out [145-1471.

LIQUID-LIQUID EXTRACTION AND URANIUM ENRICHMENT

Chemical exchange reactions leading to isotopic enrichment in two immiscible phases have been used in the nuclear industry. Solvent extraction systems have been the focus of intense research, especially in France and Japan [148-1521. The uranium ions exist in aqueous solution in forms of U-111, U-IV, U-V and U-VI. The precipitation as hydroxyl complexes is prevented by controlling the proton concentrations. The stable oxidation states of uranium in aqueous solution are


342 H. S l N G H AND C. K. G U P T A

U-IV and U-VI as UO:+ due to oxidation-reduction potential and disproportionation reaction of the five valency form of ion U 0 2 + . The standard reduction potentials in HCI media are as given in Table VII.

Since the potential between U-111 and U-IV is lower than that between H2 and H', U-I11 is oxidised to U-IV with the evolution of hydrogen gas. The U-V and U-VI strongly bond with oxygen to form UO:' and U 0 2 + . The U disproportionation of U 0 2 + produces the tetravalent and hexavalent ions. ths isotopic equilibrium between the Uranus and uranyl ion is the basis for chemical exchange:

The equilibrium constant K = 1 + 2e, where e the enrichment factor, is reported to be 1.0013. Early studies [I531 showed that either the separation factor was too small or kinetics were very slow. Require- ments of an economic chemical exchange process for isotopic enrichment of uranium were specified as (a) a single stage separation factor preferably greater than 1.005 but certainly greater than 1.001, and an exchange rate which is high enough to give an equilibrium in less than 60 sec per stage, (b) an efficient method of product and waste reflux, and (c) suitable means for continuously separating the two chemical forms of the element. Process energy of the advanced chemical enrichment process, based on dynamic enrichment factor, has been found to be much smaller than the value in the original process, both in the laboratory and applied to pilot plant process [150]. The French results on the solvent extraction process for enrichment were announced in 1977 [I521 Technical details were subsequently published [149,154,155]. The originality of the French process lies in the choice

TABLE VII Redox reactions lor enrichment by solvent extraction

Oxidation-reduction equlihrin Potenrial, mV



of the interacting species as trivalent and tetravalent uranium which has an enrichment factor of 2e = 0.025 at 40°C, about double of the conventional U-IV/U-VI or U-V/U-VI exchanges. The exchange coefficient further increases to 0.027 at lower temperature of 20°C. In the solvent extraction process, aqueous hydrochloric acid phase (4.6-5.2 M) retains 1.5-1.8 mol/l of uranium consisting 94% of the trivalent uranium while the organophosphorus phase (40% TBP in alkyl ben- zenes-Solvesso 150), retains the UCI,, 0.5-0.6 mol/l, enriched in the U-235 isotope. The U-III/U-IV kinetics are very fast and the interfacial exchange kinetics control the overall kinetics. The isotopic exchange is carried out in pulsed columns operating in the continuous organic phase, designed upto 150 mm in diameter and 50 m in height. Twin square cascades, each containing 20 large columns in series, can yield 500000 SWU/yr for a tails of 0.18% and 90 T/yr product of 3.5%. The main advantages include (a) low specific energy consump- tion, (b) low inventory (c) handling of uranium in aqueous forms which is safer, (d) modular system design (e) low sensitivity to fluctu- ations in the operating conditions and ease of control (f) small number of moving parts. Mixer-settler extraction system based on solvent consisting of tri-n-octylamine in benzene and extraction from HCI aqueous phase has been studied by British investigators [151,156]. The concentration level of the uranium in the organic phase containing upto 40% amine, was high, > 100 g/l. In 4 M HCI U-VI had good solubility in the organic phase but the U-IV was almost insoluble. Equilibrium data for U-VI correlates with a Langmuir isotherm: y = 16.91 x / ( l + 0.161 x), where x denotes aqueous concentration and y the organic concentration. Cascade design of mixer settlers showed that high aqueous recycle (of the order of several hundreds) and large number of stages (few thousands) will be needed for enrichment to 2-3% levels. The solvent inventory is high. Diluents other than benzene gave third phase formation o r a cloudy organic phase containing precipitates at high loading.

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