ORIGINAL PAPER

Process development for uranium and vanadium recoveryfrom Korean Okcheon black shale ore leach liquor

Joon-Soo Kim • Sung-Don Kim • Hoo-In Lee •

Jin-Young Lee • Jyothi Rajesh Kumar

Received: 23 April 2013 / Accepted: 12 July 2013 / Published online: 16 August 2013

� Springer-Verlag Wien 2013

Abstract The main goal of the proposed chemical meth-

odology is to extract uranium and vanadium without other

associated metals such as iron and aluminum. It also deals with

the development of recovery processes for uranium and

vanadium by preparation of yellow cake and vanadium pent-

oxide. Korean Okcheon black shale ore leach liquor contains

570 mg/dm3 uranium and 890 mg/dm3 vanadium with high

iron (12,560 mg/dm3) and aluminum (13,460 mg/dm3) metal

concentrations, at lower pH conditions were developed for

uranium recovery and then modify the extraction of raffinate

pH for vanadium recovery studies. By invoking a scrubbing

process, unwanted impurities were separated from loaded

organic phases, and the regenerated extractant was utilized for

further cycles. The metal contents in aqueous solution were

determined by using the advanced analytical technique

inductively coupled plasma optical emission spectrometry.

Finally in the present developed methodology, *95 % of

uranium was recovered in the form of yellow cake and*50 %

of vanadium was recovered as vanadium pentoxide.

Keywords Uranium recovery � Yellow cake preparation �Vanadium recovery � Korean ore

Introduction

Uranium is primarily utilized in the atomic power sector

and is considered to be one of the economic growth

parameters for countries. Uranium resources are limited

worldwide, so great attention is paid to uranium recovery

from naturally occurring ores and spent matrices. Analyt-

ical chemists are focusing their attention on new research

and development techniques for extraction and recovery of

uranium in a sustainable economic domain. Some countries

such as Korea have very limited uranium ore resources, and

the available ores have low-grade metal values such as

*300–600 mg/dm3 uranium and *900–2,000 mg/dm3

vanadium. Another important metal associated with ura-

nium is vanadium, often being used as a catalyst and

special alloy material in chemical industry applications.

The goal of the present scientific method is to extract and

recover both of these metals, viz. uranium and vanadium,

along with complete separation of other associated metals

such as iron and aluminum.

Electric power generation depends on many fuel sources

such as oil, gas, coal, nuclear, hydro, etc. Worldwide fuel

source ratios are presented in Fig. 1 [1]. The price of yel-

low cake steeply increased up to *65 US$/lb by the year

2008, recently coming down to *40 US$/lb in 2013. The

amount of uranium deposits in Korea is *115 million

tons, with average content of uranium component of about

300–600 mg/dm3, whereas the vanadium content is

900–2,000 mg/dm3. Okcheon black shale uranium ores

include brannerite [(U, Ca, Ce, Y)2(Ti, Fe)2O6] and

francevillite [(Ba, Pb)(UO2)2(VO4)2�5H2O].

Uranium can be recovered from crystal waste solutions

of zirconium oxychloride using N235 (Alamine 336, con-

taining trioctylamine/decylamine) with 10 % octanol;

0.5 g/dm3 uranium(VI) was selectively extracted from

thorium(IV), zirconium(IV), titanium(IV), yttrium(III),

scandium(III), and aluminum(III) under 5.0 mol/dm3

hydrochloric acid acidity conditions [2]. Dinonyl phenyl

phosphoric acid (DNPPA) dissolved in n-paraffin has been

J.-S. Kim � S.-D. Kim � H.-I. Lee � J.-Y. Lee �J. Rajesh Kumar (&)

Extractive Metallurgy Department, Mineral Resources Research

Division, Korea Institute of Geoscience and Mineral Resources

(KIGAM), Daejeon, Republic of Korea

e-mail: rajeshkumarphd@rediffmail.com;

rkumarphd@kigam.re.kr

123

Monatsh Chem (2013) 144:1589–1596

DOI 10.1007/s00706-013-1061-0

used as an extractant combined with neutral oxodonors as

synergists for studies of uranium recovery from nitric acid

solutions [3]. That study concluded that 1 mol/dm3 sodium

carbonate was more suitable for the uranium stripping

process [3]. Tri-n-butyl phosphate (TBP), tris(2-ethyl-

hexyl) phosphate (TEHP), and Cyanex 923 (a mixture of

four different trialkyl phosphine oxides) have been used as

a mixture for extraction, for which a synergetic coefficient

of 0.39–1.46 was calculated; Cyanex 923 was found to

form a better extractant mixture with DNPPA [3]. The

influence of sodium carbonate on uranium extraction was

established by varying the reagent concentration in the

range from 0.25 to 1.25 mol/dm3 for 0–48 h duration at

temperature of 25 �C and solid/liquid ratio of 0.11. Using

this method, the main achievement was extraction of

[80 % uranium (U3O8) with 1.00 mol/dm3 Na2CO3 at an

observed final pH value of 9.37 [4]. Study of the influence

of temperature led to the conclusion that complete uranium

recovery was possible at 65 �C within 1 h [4]. Uranium has

been extracted from uranium phosphate ore solutions using

25 % tributyl phosphate followed by a stripping process

using 0.5 mol/dm3 sodium carbonate, finally resulting in

yellow cake containing 93 % uranium [5]. Recovery of

uranium from Syrian phosphate deposits with 52–79 mg/

dm3 uranium content has been established [6]. Varying the

carbonate concentration for given bicarbonate concentra-

tions (0.05 and 0.1 mol/dm3) improved the uranium

recovery to about 55 %. Finally, this study indicated that

the presence of at least 0.05 mol/dm3 bicarbonate is

required for uranium recovery [6]. N-Phenylbenzo-18-

crown-6-hydroxamic acid (PBCHA) has been utilized for

uranium extraction between pH 3.0 and 8.0, showing that

pH 5.8–6.5 is quantitatively sufficient for uranium extrac-

tion [7]. Almost 100 % recovery of uranium has been

reported with a concentration factor of 110 [7]. Uranium

removal from metallic surfaces has been reported, with

surface analysis performed by X-ray photoelectron and

Rutherford backscattering spectroscopy methods [8]. Two

types of treatments, namely oxalic acid–hydrogen perox-

ide–citric acid (OPC) and citric acid–hydrogen peroxide–

citric acid (CPC), have also been applied in uranium

recovery studies. These treatments were applied to

29.8 ± 2.7 and 34.1 ± 3.4 lmol/dm3 uranium associated

with 2.26 ± 0.33 and 1.81 ± 0.23 mmol/dm3 iron, with

successful extraction of about 68 % and 94 % uranium for

the OPC and CPC combinations, respectively [8]. Uranium

was recovered from floated asphaltite ash with separation

of nickel, molybdenum, and vanadium. Using ammonium

carbonate [(NH4)2CO3], uranium and aluminum were

recovered [9]. (NH4)2CO3 (1.0–2.0 mol/dm3) quantita-

tively (100 %) extracted uranium and aluminum with zero

interference from other associated metals such as Fe, Ti, V,

and 1–5.3 % Mo, after which aluminum was separated

from uranium using ammonium chloride [9]. Modified fly

ash bed has been used for uranium recovery, with pH 5.3

being the optimum condition for complete uranium pre-

cipitation [10]. Extraction and recovery of uranium were

established by using Cyanex 923 as an extractant system

along with 5 volumes of 0.5 mol/dm3 sulfuric acid [11].

Based on previous studies, various other techniques such as

an electrochemical methodology [12], amidoximated

grafted polypropylene polymer matrix [13], nanoporous

silica adsorbent [14], tea waste [15], polyhydroxamic acid

sorbents [16], a molten salt technique [17], microemulsion

[18], supercritical CO2 [19], Ca-alginate-immobilized

Trichoderma harzianum [20], and hydrolytic wood lignin

[21] have also been adopted for uranium recovery.

The present scientific investigation focuses on extraction

and separation of uranium and vanadium from Korean

Okcheon black shale ore leach liquor. The amine-based

commercial extractant Alamine 336 (trioctyl/decylamine

with 95–100 % tertiary amine mix and B5 % secondary

amine content) dissolved in kerosene was used as a

potential extractant for uranium and vanadium extraction

studies, being mixed with isodecanal to prevent third-phase

formation in the loaded organic phase. Finally, successful

recovery studies were carried out to obtain yellow cake and

vanadium pentoxide materials from strip liquor. In the

present study, the following systematic scheme of experi-

ments was designed and carried out:

05

10152025303540

37.2%

24.2%19.7%

11.3%

4.4% 2.1% 1.1%

Fig. 1 Worldwide fuel sources for electric power generation

(adopted from Ref. [1])

1590 J.-S. Kim et al.

123

Step 1:

Step 2:

Results and discussion

The process development procedure for uranium and

vanadium recovery from Korean Okcheon black shale ore

is presented in Fig. 2. The first developed procedure was a

raw-ore leaching process using sulfuric acid, then the leach

liquor generated after residue separation was used for step-

by-step uranium and vanadium recovery processing. The

first extraction step was carried out with Alamine 336

dissolved in kerosene along with isodecanal as a phase

modifier. The uranium-loaded organic phase was scrubbed

to remove impurities such as Fe and Al. Then, a stripping

process was used to generate the strip liquor, which was

further used for yellow cake processing. The raffinate

solutions from the first extraction stage were used for

vanadium recovery. In this process, the first step was

conversion of the oxidation state of vanadium from three to

five using sodium chlorate as an oxidant, generating the

H3V2O7- anionic vanadium form. The extraction process

was applied with the Alamine 336 plus isodecanal system,

followed by the scrubbing and stripping processes for

vanadium. The generated ammonium vanadate strip liquor

with ammonium chloride was processed to obtain vana-

dium pentoxide.

Fig. 2 Proposed flowsheet for uranium and vanadium recovery from Korean Okcheon black shale ore

Extraction Washing and reuse of extractant

StrippingScrubbing Regeneration of extractant

Final productCalcination/dryingS/L filtrationPrecipitationStrip liquor

Process development for uranium and vanadium recovery 1591

123

Uranium extraction process

Effect of pH on uranium extraction

Korean Okcheon black shale ore leach liquor contains

570 mg/dm3 uranium, 890 mg/dm3 vanadium, 13,460 mg/

dm3 aluminum, 12,560 mg/dm3 iron, \150 mg/dm3 mag-

nesium and copper, and \40 mg/dm3 nickel and zinc. The

pH condition was varied from 0.2 to 0.8 (lower pH range)

to minimize iron extraction. The experimental conditions

were as follows: extractant system, 0.2 mol/dm3 Al-

amine 336 ? 5 % v/v isodecanal, phase ratio (O/A) 1:1,

temperature 25 �C. The extraction percentage increased

with the pH, and the present experiments demonstrated that

pH 0.6–0.8 is favorable for uranium extraction (Fig. 3)

with little other unwanted metals such as iron and alumi-

num. The percentages of metals extracted were 95–98 % of

uranium, 5–6 % of vanadium, 1–2 % of iron, and 2–3 % of

aluminum.

Scrubbing of uranium-rich loaded organic (ULO)

For removal of impurities such as iron, aluminum, and

vanadium from the loaded organic, a scrubbing process

was developed with dilute sulfuric acid solutions in the pH

range 0.5–2.0 (Fig. 4). The obtained results showed that

0.8–1.0 is the best pH condition for maximum removal of

impurities from uranium-rich loaded organic phase (ULO)

with two to three contacts.

Stripping of uranium from uranium-rich loaded organic

(ULO)

Back-extraction of the target metal from the loaded organic

phase is the most important step in the solvent extraction

process. A phase ratio of 1:1 at temperature of 25 �C with

0.25–1.0 mol/dm3 sodium chloride was applied for the

uranium back-extraction studies. The stripping percentage

increased with increasing sodium chloride concentration;

finally, with 1.0 mol/dm3 sodium chloride (pH adjusted to

1.0), *80 % stripping was achieved (Fig. 5). This indi-

cates that three to four stripping stages would be required

for complete uranium stripping in a continuous extraction

process.

Extractant regeneration and washing of Alamine 336

Treatment of the extractant for recycling and reuse in the

next cycle was studied. Na2CO3 (1.0 mol/dm3) was used as

an effective reagent for the Alamine 336 regeneration pro-

cess at recorded initial pH of 11.4 and equilibrium pH of

10.1. The other experimental parameters, namely phase ratio

(O/A) of 1, temperature of 25 �C, and duration of 20 min,

were fixed. Finally, the washing process was performed with

dilute H2SO4 at pH 2.0 and phase ratio (O/A) of 1.

Uranium recovery studies (yellow cake preparation)

The generated strip liquor was further processed for yellow

cake (U3O8) preparation. In the initial step, we added

Fig. 3 Effect of pH on the uranium solvent extraction process

Fig. 4 Effect of pH on scrubbing of uranium-rich loaded organic to

remove impurities

Fig. 5 Effect of NaCl concentration on uranium stripping

1592 J.-S. Kim et al.

123

proper equivalent moles of hydrogen peroxide to the strip

liquor to obtain *100 % precipitation, then adjusted the

pH for complete recovery by adding ammonium hydroxide

solution. Finally, the solid product (UO4�xH2O) was heated

to obtain a fine yellow cake. The total procedure is repre-

sented by the following equations:

UO2ðSO4Þ4� þ H2O2 þ xH2O! UO4 � 2H2O + 2Hþ þ 3SO2�4

3UO4 � 2H2O�!D U3O8 þ 2H2O ":

Effect of hydrogen peroxide on uranium precipitation

The influence of hydrogen peroxide on uranium precipi-

tation from the uranium strip liquor was studied. The

equivalent mole ratio was varied from 1:1 to 1:5; it was

found that, for values of 1:2 and above, more than *95 %

precipitation was achieved, and this ratio was fixed at 1:2

for total precipitation in subsequent experiments. The

experimental results are shown in Fig. 6.

Effect of pH on uranium precipitation

The precipitated uranium solid was further tested with pH

adjustment to obtain complete recovery. We slowly added

dilute ammonium hydroxide solution to the uranium pre-

cipitation for 3 h retention time. The pH was varied from

1.0 to 5.0, and it was found that pH 4.0–5.0 was the best

condition, offering *98 % recovery (Fig. 7). Finally, the

pH was adjusted to 4.0–5.0 for uranium precipitation, after

which the calcination process was carried out at 500 �C to

obtain yellow cake (U3O8).

Vanadium extraction process

The uranium extraction raffinate had the following com-

position of metals: 25 mg/dm3 uranium, 788 mg/dm3

vanadium, 8,913 mg/dm3 iron, and 9,738 mg/dm3 alumi-

num. Before the vanadium extraction process, the raffinate

solution was treated with sodium chlorate to change the

vanadium oxidation state from four (an unusual oxidation

state) to five. For each dm3 of raffinate solution, we used

5 g NaClO3, and 2 h time was applied for the vanadium

oxidation state change process, which can be represented

by the following equations:

VOþ2 þ NaClO3 þ Hþ ! VOþ2 þ NaCl + H2O

VOþ2 þ H2O! H3V2O�7 þ Hþ:

Effect of pH on vanadium extraction

After changing the vanadium oxidation state to five, the

raffinate solution pH was recorded as 0.8. The vanadium

extraction process was carried out with 0.2 mol/dm3 Al-

amine 336 diluted in kerosene, with 5 % isodecanal added

as a modifier. Other experimental parameters such as

extraction time of 20 min, phase ratio (O/A) of 1, and

temperature of 25 �C were invoked. The influence of pH on

the vanadium extraction was tested in the range of 0.2–1.0,

indicating that vanadium extraction increased with

increasing pH. Better extraction was observed for pH

Fig. 6 Effect of H2O2 addition on uranium precipitation from strip

liquor

Fig. 7 Effect of pH on uranium precipitation by adding NH4OH

Fig. 8 Effect of pH on the vanadium solvent extraction process

Process development for uranium and vanadium recovery 1593

123

0.8–1.0, resulting in *65 % extraction (Fig. 8). Iron and

aluminum were coextracted at approximately 10 and 9 %.

Increased pH would have given more vanadium, but at the

same time interference from other associated metals such

as iron and aluminum was found to increase drastically.

Complete vanadium extraction requires a greater number

of extraction stages in a continuous process.

Scrubbing of vanadium-rich loaded organic (VLO)

For removal of coextracted metals such as iron and alu-

minum from vanadium, a vanadium-rich loaded organic

phase (VLO) scrubbing process was established. Dilute

sulfuric acid solution at unity phase ratio with 20 min

scrubbing time at temperature of 25 �C was applied. The

influence of pH was studied in the range of 1.0–2.5, the

best results being observed at pH 1.0–1.5 for maximum

removal of iron and aluminum from VLO with two to three

contacts (Fig. 9). However, vanadium was also scrubbed at

up to 30 % along with the associated metal ions.

Stripping of vanadium from vanadium-rich loaded

organic (VLO)

After removal of impurities from VLO, the treated organic

phase was further used in a stripping process to back-extract

the vanadium. Sulfuric acid was used as an effective strip-

ping reagent in the vanadium back-extraction process, being

tested in the range from 0.2 to 0.8 mol/dm3 for 20 min

stripping time at temperature of 25 �C and unity phase ratio.

The obtained results indicate that the 0.6–0.8 mol/dm3

sulfuric acid range is good for vanadium stripping (Fig. 10).

Extractant regeneration and washing of Alamine 336

Alamine 336 recycling and reuse studies were performed

with 1 mol/dm3 sodium carbonate as the regeneration

reagent with unity phase ratio (O/A), temperature of 25 �C,

and duration of 20 min. After this process, the organic

phase was washed with dilute sulfuric acid solution (pH 1)

at phase ratio (O/A) of 2. Finally, the regenerated

extractant Alamine 336 was used for further cycles.

Vanadium recovery studies (vanadium pentoxide

preparation)

Vanadium stripped from VLO and the generated strip

liquor were used for further vanadium pentoxide prepara-

tion by adding ammonium chloride, followed by the

calcination process, which can be presented as follows:

VO2ðSO4Þ�ðVO�3 Þ þ NH4Cl! NH4VO3 þ SO2�4 þ Cl�

NH4VO3�!D

V2O5 þ NH3 " :

Effect of ammonium chloride on vanadium precipitation

The influence of ammonium chloride on vanadium pre-

cipitation from the vanadium strip liquor was studied in the

equimolar range from 2.0 to 8.0, revealing 6.0–8.0 equiv.

ammonium chloride as the best condition (Fig. 11) for

precipitation of vanadium as NH4VO3. Ammonium chlo-

ride was added to the strip liquor and slowly stirred with

precipitation duration of 2 h during the total process.

Effect of pH on vanadium precipitation

After vanadium was precipitated with ammonium chloride,

the influence of pH was tested in the range from 7 to 10; it

was observed that pH 9–10 is a good condition, offering

*99 % metal recovery (Fig. 12). The filtered precipitate,

i.e., NH4VO3, was dried and calcinated at 900 �C, finally

yielding vanadium pentoxide (V2O5) with 98 % purity.

Conclusions

We have determined optimum conditions for uranium and

vanadium recovery from Korean Okcheon black shale ore.Fig. 9 Effect of pH on scrubbing of vanadium-rich loaded organic

(VLO) for removal of impurities (iron and aluminum)

Fig. 10 Effect of H2SO4 concentration on vanadium stripping

1594 J.-S. Kim et al.

123

The first stage of the experiment concentrated on the ura-

nium extraction process. Based on the obtained results, it is

concluded that 0.6–0.8 is the optimum pH condition for

maximum uranium recovery (95 %), using 0.2 mol/dm3

Alamine 336 ? 5 % v/v isodecanal. Using dilute sulfuric

acid (pH 0.8–1.0), impurities such as iron, aluminum, and

vanadium were scrubbed within 30 min. At temperature of

25 �C, phase ratio of 1:1, and 1.0 mol/dm3 sodium chloride

(pH maintained 1.0), *80 % of loaded metal was back-

extracted (stripped) in the first stage. Equimolar hydrogen

peroxide precipitation applied to the strip liquor gave

yellow cake material at conditions of pH 4.0–5.0 with

calcination at 500 �C.

The second stage, for vanadium recovery, started with

raffinate solutions from the first extraction stage. Sodium

chlorate was used to change the vanadium oxidation state

from four to five, as shown in the following equations:

VO2? ? NaClO3 ? H? ? VO2? ? NaCl ? H2O,

VO2? ? H2O ? H3V2O7

- ? H?. Based on results for the

influence of pH, it is concluded that pH 0.8–1.0 enables

extraction of *65 % vanadium, along with 9–10 % of

other associated metals such as iron and aluminum. Using

dilute sulfuric acid solution, the impurities were removed,

but 30 % vanadium was also found to be scrubbed. The rest

of the vanadium from the loaded organic phase was strip-

ped with 0.6–0.8 mol/dm3 sulfuric acid, with final slow

addition of ammonium chloride to strip the liquor for

vanadium pentoxide preparation.

The presented scientific chemical methodology was

successfully applied for simultaneous recovery of uranium

(final yield 95 %) and vanadium (final yield 50 %) from

Korean Okcheon black shale ore leach liquor.

Experimental

Analysis of uranium, vanadium, and other metals was

carried out using an inductively coupled plasma optical

emission spectrometer (ICP-OES) manufactured by

Thermo Scientific, USA (model iCAP 6000, series ICP

spectrometer) with the following operating conditions:

radiofrequency (RF) power = 1,350 W, pump rate = 45,

aux. gas flow = 0.5 dm3/min, neb. gas flow = 0.6 dm3/

min, and purge gas flow = normal. The main metals were

determined at the following wavelengths: uranium at

409.014 nm, vanadium at 311.071 nm, iron at 238.204 nm,

and aluminum at 394.401 nm. A Thermo Scientific (USA)

pH meter was used for pH measurements, being stan-

dardized every day using the following standards: 1.68,

4.01, 7.00, and 10.01 supplied by the same company. A

shaking incubator (model SI-300/300R/600/600R) was

used for solvent extraction, scrubbing, stripping, regener-

ation, and washing of extractant for reuse experiments.

This shaker and the temperature-controlled oven for

material drying were supplied by Jeio Tech, Korea. AH

Jeon Industrial Company Limited supplied the furnace for

the calcination process.

The present study used kerosene as diluent (boiling

point 180–270 �C, density 0.80), sulfuric acid (98 % pur-

ity), ammonium chloride (NH4Cl), sodium chlorate

(NaClO3), and sodium carbonate (Na2CO3), supplied by

Junsei Chemicals Co. Ltd., Japan, and other chemicals such

as sodium hydroxide (NaOH) and hydrogen peroxide

(H2O2), supplied by Oriental Chemical Industries, Korea.

The modifier, isodecanal, was supplied by Dongnam

Chemical Company Limited, Korea, being used without

any further processing. All other reagents used were of

analytical-reagent grade. The commercial-grade extractant

Alamine 336 (trioctyl/decylamine) was supplied by Cognis

Corporation USA and used as supplied without further

purification. The general reactions of the amine-based

Fig. 11 Effect of NH4Cl addition on precipitation while stripping

vanadium

Fig. 12 Effect of pH on vanadium precipitation from stripping

solution

Process development for uranium and vanadium recovery 1595

123

extractant are shown below in two steps, i.e., protonation

and ion exchange (adopted from [22]):

Protonation : R3N½ �orgþ HA½ �aq$ R3NHþA�½ �org

Exchange : R3NHþA�½ �orgþ B�½ �aq

$ R3NHþB�½ �orgþ A�½ �aq

where [R3N] represents Alamine 336.

This amine-based extractant can strip when using basic

reagents such as sodium chloride, sodium carbonate,

ammonium sulfate, etc.; For example, the ammonium

carbonate stripping process for Alamine 336 can be pre-

sented as follows (adopted from [22]):

R3NHþB�½ �orgþ 2Naþ þ CO2�3

� �aq

$ 2 R3N½ �orgþ H2½ �aqþ CO2 þ 2 Naþ½ �aqþ2 B�½ �aq:

General extraction procedure

Equal volumes of aqueous phase containing desired con-

centrations of uranium, vanadium, and other elements and

organic phase containing the extractant (Alamine 336

diluted in kerosene with 5 % isodecanal) were equilibrated

for 20 min at 25 ± 0.5 �C in a glass-stoppered separating

funnel using a mechanical shaker. The solutions were then

allowed to settle, the phases were separated, and the metal

content in the aqueous phase was determined by ICP-OES.

The concentrations of metal ions in the organic phase were

then obtained by mass balance. The distribution ratio, D,

was defined as the ratio of the concentration of a metal ion

in the organic phase to that in the aqueous phase. The

general agreement between the distribution ratio values

obtained was within ±5 %. The uranium and vanadium

extraction processes can be represented by the following

equations:

UO2ðSO4Þ2�2 þ xððR3NHÞ2SO4Þ ! xðR3NÞ2 � UO2ðSO4Þ2�2þ xHþ

H3V2O�7 þ xððR3NHÞ2SO4Þ ! ½ðR3NHÞxþ ðHV2Ox�7 Þ�

þ xSO2�4 :

Acknowledgments This work was supported by the Energy Effi-

ciency and Resources (Physical and Chemical mineral dressing/

refining of low grade uranium ore) of the Korea Institute of Energy

Technology Evaluation and Planning (KETEP) grant funded by the

Korea Government Ministry of Knowledge Economy. Thanks are also

due to Cognis Company, USA for providing all amine-based ex-

tractants as gift samples.

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