Modified leaching and extraction of uranium from hydrous oxide cake of Egyptian monazite

Int. J. Miner. Process. 76 (2005) 101 – 110 www.elsevier.com/locate/ijminpro

Modified leaching and extraction of uranium from hydrous oxide cake of Egyptian monazite Y.A. El-Nadi, J.A. DaoudT, H.F. Aly Hot Laboratories Centre, Atomic Energy Authority, 13759 Cairo, Egypt Received 13 April 2004; received in revised form 25 October 2004; accepted 2 December 2004

Abstract A modified alkaline dissolution route of monazite was used to separate the hydrous oxide cake containing uranium, thorium, and rare earth elements from the phosphate matrix. It was found that uranium can be selectively leached by a mixture of sodium carbonate, sodium hydroxide, and hydrogen peroxide leaving thorium and rare earth elements as insoluble hydrous oxides. The factors affecting the extraction of uranium from the alkaline solution containing sodium carbonate and hydrogen peroxide were separately studied. Experimental results showed that tricarbonato uranium complexes are selectively extracted by Aliquat-336 diluted in kerosene from alkaline leach solution. Based on these results, uranium was purified from the leach solutions by extraction with Aliquat-336 diluted in kerosene with purity not less than 99%. D 2004 Elsevier B.V. All rights reserved. Keywords: uranium; monazite; alkaline leaching; extraction; Aliquat-336

1. Introduction The Egyptian beach sand deposits usually contain about 15% heavy economic minerals (ilmenite, rutile, magnetite, zircon, and monazite). The mean relative frequency of monazite in the beach deposits in some samples collected from Demietta and Rosetta (Egypt) attains 0.3 to 1.5%. El-Shazly (1965) has estimated the reserves of heavy economic minerals in million metric tons as about 30 in the top meter and about 615 in the top 20 m. Therefore, the Egyptian black sand T Corresponding author. Fax: +20 202 6352474. E-mail address: [email protected] (J.A. Daoud). 0301-7516/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2004.12.005

beach deposits are the chief thorium ore in Egypt due to the presence of monazite (Farrag, 1979). Gupta et al. (1979) studied the description of monazite breakdown viz. acid and alkali digestion and discussed the composition of the concentrates and their effect on solvent extraction processes. These authors reported that TBP-nitrate system is the most industrially attractive system. Eyal (1982) has studied the relative dissolution rates of the actinide isotopes 238U, 234U, 232Th, 230Th, and 228 Th upon leaching of monazite in a bicarbonate– carbonate solution and reported that the leachability of thorium was considerably reduced after heat treatment which causes a partial annealing of the alpha-recoil

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damage. Dong and Jinwen (1985) started with a monazite–U–Th–alkaline cake–HCl solution as the feed and used dimethylheptyl–methylphosphonate (DMHMP) and TBP as extractants in kerosene in order to separate the above elements. Tetracycline in benzyl alcohol was used (Petrauskas and Saiki, 1985) as an extracting agent to separate uranium from interfering elements in the determination of uranium and isotopic ratio 235U/238U by neutron activation analysis. This separation method gives a recovery of 97% for uranium and eliminates the interferences from matrices of pitchblende and monazite. Hammad et al. (1986) suggested an economic technique for separation and purification of thorium and uranium from their hydrous oxide cake concentrate, which could be obtained by alkali treatment of Egyptian monazite sands. The mineral monazite is primarily composed of rare earth phosphates especially those of low atomic numbers (cerium group) along with numerous other minor constituents among which thorium and uranium are of major interest (Katzin et al., 1986). The recovery of uranium and thorium from leach solutions, resulted from opening monazite, can be carried out by a variety of methods including ion exchange, solvent extraction, and chemical precipitation. Each of these various procedures can be applied to either acid or alkaline leach liquors, although in general they will not be equally applicable. Ion exchange, solvent extraction, or both are nowadays used to purify and concentrate uranium and thorium prior to a final product precipitation (Katzin et al., 1986). Monazite is very stable chemically and can be attacked by strong acid (usually sulphuric acid), which essentially transforms the metal phosphate ion to di-hydrogen phosphate salt and phosphoric acid and leaving the metal ions as water soluble salts, or by strong alkali (sodium hydroxide), which transforms the insoluble phosphates to insoluble metal hydroxides that can easily be dissolved in the acid after removal from the supernatant solution of alkali phosphates (Katzin et al., 1986). Olander and Eyal (1990) have used the interaction of three natural monazite specimens with a bicarbonate–carbonate solution for up to 6.8 years. Dissolution was observed to be incongruent with respect to uranium and thorium as well as their radiogenic daughters. Leaching was divided into a very rapid initial stage lasting a few hours and a slower process active for the

remaining time. Preannealing of a specimen at 800 8C depresses the elemental thorium leach rate but enhances the amount of 228Th/232Th fractionation. Kraikaew et al. (1996) used continuous liquid–liquid extraction technique in laboratory scale to purify uranium, resulted from monazite, by scrubbing thorium using various scrub solutions and flow ratios. The results indicated that the uranium extraction efficiency at flow ratio (solvent:feed) 3:2 and 6 stage numbers is more than 99.8%. The thorium scrub efficiency is more than 80% when using uranyl nitrate as scrub solution at flow ratio (feed:scrub) 7:1 and 4 stage numbers. Egyptian monazite (purity 97%) has been analyzed and was found to assay 5.9% ThO2, 0.44% U3O8, 26.55% Ce2O3, and 34.35% other rare earths (RE2O3) (Osman, 1998). Its hardness ranges from 5.0 to 5.5 (Moh’s Scale) and its specific gravity from 4.9 to 5.3 g/ cm3. At present, the alkaline digestion is preferred due to economic reasons. The temperature of reaction is lower than in acidic method, the resulted sodium phosphate may be used in the industry of fertilizers, and the alkaline method could be less complicated. Within this context, the present contribution is directed to separate pure uranium from Egyptian monazite by a modified alkaline leaching followed by purification of uranium by Aliquat-336 extraction.

2. Experimental 2.1. Chemicals and reagents The chemicals and reagents in the present work were used as received. Uranyl nitrate and thorium nitrate of analytical reagent grade (AR) were Fluka products, while sodium carbonate and hydrogen peroxide were obtained from Winlab. Methyl trioctyl ammonium chloride (Aliquat-336) and sodium hydroxide were Merck products. Kerosene (nonaromatic) was supplied by Misr Petrol Ltd., Egypt, while 1-octanol was obtained from BDH. The monazite was supplied by the Nuclear Materials Authority, Egypt, as 88% concentrate. 2.2. Procedure The particle size was measured using Laser light scattering instrument (Mastersizer X, Malvern Instru-

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ments). This instrument covers the size range from 0.1 Am to 2000 Am using different lenses, with resolution size discrimination of 100 size bands on any lens range. Uranium and thorium concentrations were spectrophotometrically determined by Arsenazo-I method (Marczenko, 1976) at 596 and 565 nm, respectively, using Shimadzu UV–Visible spectrophotometer model 160A, while radiation measurements were carried out using multichannel analyzer equipped with high purity germanium (HPGe) g-ray detector model 601 (USA). The detector is cylindrical crystal with 7.6-cm diameter and 11.3-cm length with 512-cm3 active volume. An energy dispersive XRF spectrometer containing a Mo X-ray tube working at 50 kV and 40 mA with a Mo secondary target was used. X-ray spectra collected by a Si(Li) detector (resolution of 170 eV for 5.9 KeV) and a S100 Canberra MCA card were analyzed by Axil software (Van Espen et al., 1983). Extraction of uranium from alkaline carbonate medium has been carried out at 25 8CF1 8C (except when studying the effect of temperature). In this concern, equal volumes (5 ml) of the aqueous solution containing 1 mg/ml of uranium in the used dissolving alkaline medium under investigation and an organic solution containing a known concentration of Aliquat336 dissolved in kerosene or other diluents used were equilibrated by shaking for 1 h. To determine the uranium concentration, known aliquot volumes were taken from the aqueous phase before and after extraction for spectrophotometric assay. The distribution ratio, D, was calculated from the relation: D¼

Co  C V 4 C V

ð1Þ

where, C o is the original metal mass (mg) in the aqueous phase before extraction, C is the metal mass (mg) in the aqueous phase after extraction, and V* and V are the volumes of the solution in the organic and aqueous phases, respectively. 2.3. Determination of uranium and thorium in monazite Assuming that, 238U and 232Th are in equilibrium with the daughter 234Th and 228Ac, the specific activities of the radionuclides should equal to the

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specific activity of the parent isotopes. Thus, the glines at 63.3 and 92.6 keV of 234Th was taken as a measure for 238U and the g-lines at 338.91 and 969.9 keV of 228Ac was taken as a measure for 232Th. Therefore, an accurate weight of 100 g of monazite sand was packed into a polyethylene container (250 cm3), closed tightly, sealed well by molten wax, and stored, to ensure the secular equilibrium between the main naturally occurring radionuclides (238U, and 232 Th) and their respective decay daughters. After secular equilibrium, the stored sample was counted on the HPGe-detector of the g-spectroscopic system, for sufficient time (2.5 h). The results of measured uranium and thorium concentrations in monazite were found to be 0.540 and 5.265 wt.%, respectively.

3. Results and discussion 3.1. Alkaline carbonate leaching Prior to this study, two monazite samples were ground, sieved, and the particle size of both was measured. The particle size of the ground sample was found to be of 16.81F0.28 Am whereas the nonground is 106.44F0.61 Am. In order to follow the chemical treatment of the samples, the monazite samples (10 g each) were treated with sodium hydroxide (50 wt.%) solution and boiled for 4 h at 140 8C. The solution was diluted up to 20 wt.% NaOH concentration and boiled again for another hour then filtered at 80 8C. Fig. 1 represents the XRF spectrum of the filtrate, which consists mainly of sodium phosphate and excess sodium hydroxide. The absence of peaks in this spectrum may indicate that there is negligible decrease of elements concentrations during the digestion method of monazite. The solid residue was washed several times with water, dried, and treated with alkaline carbonate solution composed of alkaline carbonate at 60 8C in order to separate the maximum content of uranium selectively by forming tricarbonato complex. It was found that 40 vol.% 1 M Na2CO3, 50 vol.% 1 M NaOH, and 10 vol.% (30%) H2O2 mixture is quite sufficient for the dissolution of 20 g/l U3O8 (El-Nadi, 1996). Since our solid residue was expected to contain uranium hydrous oxide rather than U3O8 making it is easier to be dissolved in the above mixture. Fig. 2 represents the

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Fig. 1. XRF spectrum of NaOH digestion solution of 10 g monazite.

XRF spectrum of the solution resulted from the first leaching step. It is clear that uranium peak is only found indicating the efficiency of the method of alkaline carbonate leaching for the removal of uranium. The leached carbonate solutions (50 ml for the first five steps followed by 25 ml for the last two) which treated the monazite samples (10 g) were diluted, and uranium was measured spectrophotometrically. The data given in Table 1 show that uranium could be separated easily from the monazite sample with smaller particle size (ground sample). Therefore, carbonate leaching mixtures contain mainly uranium, which justify the proposed method of alkaline carbonate treatment. By taking the summation of the average

values of uranium concentration for all methods used, it is concluded that 0.044 g of uranium was separated from 10 g of ground monazite sample indicating a recovery percent of 83% while 0.004 g of uranium was only separated from the non-ground sample representing 8% recovery. Thus, the non-dissolved uranium amounts are 17% and 82% for ground and non-ground samples respectively, which indicates the importance of grinding the monazite sample before treatment. After carbonate leaching, the precipitate was carefully washed with water and finally treated with nitric acid three times (100 ml each) with heating to dissolve the hydroxides (hydrous oxides) of the elements included. Fig. 3 represents the nitrate solution spectrum

Fig. 2. XRF spectrum of the first washing of the hydrous oxide cake by alkaline carbonate–hydrogen peroxide solution.

Y.A. El-Nadi et al. / Int. J. Miner. Process. 76 (2005) 101–110 Table 1 Spectrophotometric determination of uranium by the carbonate leaching of monazite samples (10 g) Carbonate leaching steps

Ground sample (16.8 Am)

Non-ground sample (106.4 Am)

Uranium concentration (ppm)a 1st 2nd 3rd 4th 5th 6th 7th Uranium recovery

38.4 31.7 24.2 20.5 10.1 7.8 6.2 83%

8.5 2.6 UDLb UDL UDL UDL UDL 6%

a ppm Concentration of uranium in the carbonate leaching solutions (50 ml for the first five steps followed by 25 ml for the remaining two steps). b UDL—under detection limit.

of these elements after dissolution in HNO3 using EDXRF method. As it is clear from this spectrum, the peaks of uranium are almost absent while no significant changes are noticed for the peaks of the other elements. Based on these findings, extraction of uranium from alkaline carbonate solution containing hydrogen peroxide was investigated. 3.2. Extraction of uranium from leach solution As found previously, more than 80% of uranium was leached with alkaline carbonate solution containing H2O2. Therefore, to obtain high-purity uranium, the extraction of U(VI) from the leach solution by Aliquat336 was investigated. The use of Aliquat-336 as extractant is based on the fact that U(VI) forms soluble

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anionic complexes with carbonate. Further, Aliquat336 behaves as liquid anion exchanger. Therefore, the extraction parameters investigated include effect of Aliquat-336, sodium carbonate, sodium hydroxide, hydrogen peroxide as well as temperature. 3.2.1. Effect of Aliquat-336 concentration In this concern, variable concentrations of Aliquat336 in different diluents (containing 5 vol.% octanol as a modifier) were utilized to extract a constant weight of 1 mg/ml of uranium using alkaline medium which consists of 20 vol.% 1 M Na2CO3, 25 vol.% 1 M NaOH, and 10 vol.% (30%) H2O2. Preliminary experiments indicated that 15 min were sufficient for reaching to equilibrium. The extractant concentration used covered the range, 0.5–15 vol.% in different diluents. The extraction percent (%E) of uranium plotted against the respective Aliquat-336 concentrations in Fig. 4 indicates that the extraction increases with Aliquat-336 concentration up to 5 vol.% then remains constant with further increase with relatively higher values for aromatic diluents compared with aliphatic ones. Due to practical and economic reasons, kerosene was used throughout this work. The slope of the log–log relation between Aliquat-336 concentrations and the corresponding distribution ratios, Fig. 5, was found to equal two which suggests that the extracted species contains two Aliquat-336 molecules. 3.2.2. Effect of Na2CO3 concentration Investigations of the carbonate concentration effect on the extraction of 1 and 5 mg/ml uranium (equivalent to 0.004 and 0.02 M, respectively) by 5 vol.% Aliquat-

Fig. 3. XRF spectrum of the nitrate medium after the dissolution of the hydrous oxide cake in 1 M nitric acid.

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Y.A. El-Nadi et al. / Int. J. Miner. Process. 76 (2005) 101–110 50 Phase Ratio aq.:org. = 1:1

40

Extraction percent (%E)

increased to 0.02 M, the D values obtained were more or less constant when the [CO32]/[U(VI)] was in the range 1–2. When the ratio increased to more than 2, the extraction of uranium starts to decrease parallel to the case of low uranium concentration. This indicates that the maximum extraction of uranium is obtained when the molar ratio between U(VI) and CO32 is two. This suggests that the main extracted species is of the type [UO2(CO3)2]2. Formation of higher uranium carbonate complexes, such as [UO2(CO3)3]4 will then decrease the extraction as experimentally found and given in Fig. 6. Moreover, the slope value of 1 for both cases may suggest the release of one carbonate species during the extraction.

Kerosene MIBK Benzene Xylene

30

20

10

0 0

2

4

6

8

10

12

14

16

18

Aliquat-336 % Fig. 4. Extraction percent of uranium (1 mg/ml) solution from (20 vol.% 1M Na2CO3, 25 vol.% 1M NaOH, and 10 vol.% (30%) H2O2) solution by Aliquat-336 in different diluents.

336 in kerosene were made using different concentrations of sodium carbonate together with a constant concentration of 50 vol.% 1 M NaOH and 10 vol.% (30%) H2O2. At low uranium concentration (0.004 M), the distribution ratio was found to decrease with increasing Na2CO3 concentration when the initial CO32 molarity to the molarity of U(VI) is more than 2.5. When the molar concentration of U(VI) was

3.2.3. Effect of NaOH concentration To investigate the effect of NaOH concentration on the extraction process, samples containing 1 mg/ml of uranium were prepared in different concentrations of sodium hydroxide and constant concentration of 40 vol.% Na2CO3 and 10 vol.% H2O2. The extraction was studied by a constant 5 vol.% Aliquat-336 in kerosene. The log–log relation between the molar concentration of NaOH versus the distribution ratio, D, shows that D increases linearly as the NaOH concentration decreases (slope 2), Fig. 7. These 5 U (0.004 M) U (0.02 M)

1

Distribution Ratio (D)

Distribution Ratio (D)

Phase Ratio aq.:org. = 1:1

0.1

Phase Ratio aq.:org. = 1:1

1

0.1

0.01

Slope = -1

Slope = 2

0.01 0.01

0.001 0.01

0.1

[Aliquat-336], M Fig. 5. Effect of Aliquat-336 concentration on the extraction of uranium (1 mg/ml) from (20 vol.% 1M Na2CO3, 25 vol.% 1M NaOH, and 10 vol.% (30%) H2O2) solution.

0.1

1

[Na2CO3], M Fig. 6. Effect of Na2CO3 concentration on the extraction of uranium (1 mg/ml=0.004 M) and (5 mg/ml=0.02 M) by 5 vol.% Aliquat-336 in kerosene from (50 vol.% 1M NaOH and 10 vol.% (30%) H2O2) solution.

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(Ciavatta et al., 1983; Kramer-Schnabel et al., 1992) with a high formation constant (b) according to the following equation:

Phase Ratio aq.:org. = 1:1

Distribution Ratio (D)

107

10 2 2 UO2þ 2 þ 2CO3 W½UO2 ðCO3 Þ2 

b2 ¼ 1  1017 ð2Þ

1 4 2 UO2þ 2 þ 3CO3 W½UO2 ðCO3 Þ3 

Slope = -2

b3 ¼ 1  1024 ð3Þ

0.1

0.01

0.01

0.1

1

[NaOH], M Fig. 7. Effect of NaOH concentration on the extraction of uranium (1 mg/ml) by 5 vol.% Aliquat-336 in kerosene from (40 vol.% 1M Na2CO3 and 10 vol.% (30%) H2O2) solution.

results show that the decrease of NaOH concentration enhances the extraction considerably. In this case, the pH decreases with decreasing the NaOH concentration until reaching 9.9 at 2 vol.% NaOH. This indicates that the changes in the pH will have a marked effect on the extraction process. 3.2.4. Effect of H2O2 concentration The mixture of Na2CO3 and NaOH containing 1 mg/ml of uranium was used to investigate the effect of the peroxide concentration on the extraction of 1 mg/ ml uranium by 5 vol.% Aliquat-336 in kerosene. For this experiment, variable amounts of 1 vol.% to 25 vol.% of H2O2 were used at constant 40 vol.% Na2CO3 and 50 vol.% NaOH concentrations. The distribution ratios were found to equal 0.1, 0.09, 0.1, 0.11, 0.11, 0.12, and 0.10 when the percent of H2O2 concentrations were 1, 5, 7.5, 10, 15, 20, and 25, respectively. These D values indicate that the final peroxide concentration has virtually no effect on the extraction of uranium from the alkaline solution used. Therefore, there is no need for strict control of hydrogen peroxide concentration (Korkisch and Hecht, 1972; Maya, 1982). 3.2.5. Extraction equilibrium In alkaline medium containing carbonate, uranium tends to form di- and tri-carbonato complexes

Based on the results obtained on the effect of Na2CO3 on the extraction of 0.02 M uranium, Fig. 6 and its maximum extraction at carbonate/U(VI) molar ratio of 2.0, the main uranium species extracted by the amine can be represented by: P

½UO2 ðCO3 Þ2 2 þ 2ðR3 RVNþ ÞOH P h i WðR3 RVNþ Þ2 UO2 ðCO3 Þ2 þ 2OH 2

ð4Þ

where R stands for Aliquat-336 and bars indicate the species in the organic phase. At high carbonate concentration, a large amount of carbonate is bound to the uranyl ion and the major complex is [UO2(CO3)3]4 with a high formation constant of (log b 3=23.92). Based on this information and the results obtained from the factors affecting the extraction of uranium in the present system, the following equation could be proposed to explain the reaction equilibrium: P

½UO2 ðCO3 Þ3 4 þ 2ðR3 RVNþ ÞOH P h i WðR3 RVNþ Þ2 UO2 ðCO3 Þ2 þ 2OH þ CO2 3 2 ð5Þ The extraction constant of this equation is given by: h i P   ðR3 RVNþ Þ2 ½UO2 ðCO3 Þ2 ½OH 2 CO2 3 2 Kex ¼  4 P UO2 ðCO3 Þ3 ½ð R3 RVNþ ÞOH  2 ð6Þ Or,   D½OH 2 CO2 3 Kex ¼ P ½ðR3 RVNþ ÞOH  2

ð7Þ

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Taking log of the two sides of Eq. (7) and rearrangement, the following relation is obtained: log D ¼ log Kex þ 2 log½ðR3 RVNþ ÞOH   2 log½OH  log½CO2 3 

ð8Þ

From this equation, the extraction constant is calculated under the different investigated extraction parameters. The total average value obtained for K ex is 0.317F0.013 which indicates that the proposed extraction equilibrium is valid with acceptable accuracy, under the investigated extraction conditions. 3.2.6. Effect of temperature The effect of temperature on the extraction of uranium was studied by varying the extraction temperature in the range 10–60 8C. In this concern, the aqueous phase employed is composed of 20 vol.% Na2CO3, 25 vol.% NaOH, and 10 vol.% H2O2, and the extraction was carried out by 5 vol.% Aliquat-336 in kerosene. The obtained results show that the extraction percent, %E, slightly increases by increasing the temperature. The extraction constants, K ex, of the extracted species are calculated by applying Eq. (6) and plotted as log K ex versus the reciprocal of the respective absolute temperatures, 1/T, Fig. 8. A

-0.35 Phase Ratio aq.:org. = 1:1

-0.40 -0.45

log Kex

-0.50 -0.55 -0.60 -0.65 -0.70 -0.75 2.8

3.0

3.2

3.4

3.6

3.8

straight linear relation with a negative slope is obtained. The temperature effect on the complex extraction could be evaluated in terms of their thermodynamic values calculated from the following relations: ln Kex ¼ 

DH þ C; C: constant RT

ð9Þ

dðlog Kex Þ d ð1=T Þ

ð10Þ

DH ¼  2:303R

DG ¼  RT ln Kex DS ¼

DH  DG T

ð11Þ ð12Þ

where DG is the free energy change, DH is the enthalpy change and DS is the entropy change. R is the universal gas constant (8.314 J mol1 K1 ) and T is the absolute temperature (K). From Eq. (10) and the slopes obtained from Fig. 8, the respective enthalpy variation (DH) are elucidated. The free energy (DG) are obtained by applying Eq. (11) at standard state, 298 K. Also, the entropy variations (DS) were given by applying Eq. (12). The calculated values of DH, DG, and DS were found 897.8 J/mol, 3.253 kJ/mol, and 7.9 J/mol K, respectively. The positive DH value obtained for the complex formation indicate the endothermic character of the extraction process, while the negative DS values indicate that the extraction is less random in nature. 3.2.7. Stripping investigations Among the different stripping agents, high concentration of sodium carbonate was chosen for stripping of U(VI) from the loaded organic phase because of its ability to form a highly strong complex with uranium. In this respect, different concentrations of sodium carbonate in the range 0.1–2.0 M were used and it was found that a concentration of 1 M is quite efficient for the stripping of uranium.

1/T x 103, K-1 Fig. 8. Effect of temperature on the extraction constant (K ex) of uranium (1 mg/ml) by 5% Aliquat-336 in kerosene from (20 vol.% 1M Na2CO3, 25 vol.% 1M NaOH, and 10 vol.% (30%) H2O2) solution.

3.3. Developed process The process of monazite opening and processing could be represented as shown schematically in Fig. 9

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109

Monazite Sand Grinding

Ground Ore Mesh Size: 200-270, 130 g

NaOH 50% ∆ 140ºC 4 hs Filtrate:

Filtration Na3PO4 and excess NaOH Precipitate: Hydrous Oxides of U, Th and REE

Several Washings

Na2CO3, NaOH, H2O2, 60ºC

Precipitate: Filtration Th + REEs Filtrate,

Aliquat-336 5 vol % Precipitation Ammonia Uranium ppt, 0.643 g

Uranium Tricabonate

Dilution 1+3 (pH~9), One Stage Extraction Na2CO3, 1M

Acidification HNO3

Stripping

Fig. 9. Modified flowchart of the alkaline digestion, leaching, and extraction of uranium from Egyptian monazite.

and explained as follows; monazite ore (130 g) was ground to mesh size 200–270, then digested by boiling with sodium hydroxide (50 wt.%) for 4 h at 140 8C. The solution was diluted to NaOH concen-

tration of 20 wt.% and boiled again at 105 8C for another hour, then filtered at 80 8C. The filtrate can be recycled for recovery of sodium phosphate and excess sodium hydroxide whereas the precipitate is washed

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with water and dried. The dried precipitate, which consists mainly of thorium and rare earths as well as uranium hydrous oxides, is leached ten times with alkaline solution (total 2.5 l) composed of 40 vol.% 1 M sodium carbonate, 50 vol.% 1M sodium hydroxide, and 10 vol.% (30%) hydrogen peroxide to separate uranium. The solution is filtered and the filtrate containing uranium is diluted (1+3 with water), adjusted to pH 9 and shaken with 5 vol.% Aliquat336 in kerosene in one stage solvent extraction (1:1 phase ratio) to recover uranium. The resulted uranium which is mainly as tricarbonato complex is transferred to uranyl nitrate form by addition of nitric acid and may be precipitated as hydroxide by addition of ammonia giving 0.643 g. A sample from the resulted uranium precipitate was dried, dissolved in nitric acid, and analyzed spectrophotometrically. The results showed that the precipitate is composed of 99.85% uranium and less than 0.01% of each of thorium and rare earths indicating the efficiency and purity of the suggested method of monazite treatment for separation of uranium. Therefore, this method selectively leads to get uranium product in almost a pure form.

4. Conclusion Based on the above experimental results, we can conclude the work in the following: â Alkaline carbonate solution dissolves uranium selectively. â Aliquat-336 in kerosene extracts uranium from alkaline carbonate medium. â Decreasing alkalinity (pH) increases the extraction. â It was found that the main extracted species is [R 2UO2(CO3)2], where R refers to Aliquat-336 molecule. â Increasing temperature enhances the extraction of uranium. â Sodium carbonate strip uranium efficiently from alkaline medium. â Aliquat-336 poorly extracts rare earth elements from nitric acid medium.

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