Selective separation of thorium from rare earth elements liquor during the alkaline processing of Egyptian monazite concentrate

Abstract

Monazite ore processing is a complicated matrix which is mainly attributed to the presence of high phosphate (25% of P₂O₅) and sulfate concentration, hence, complicate the extraction and separation of REEs and other elements. Therefore, this work developed technical systematic studies for monazite processing using alkali solution, resulting in the removal and recovery of phosphate as a valuable product. Moreover, solvent extraction technique was used for selective separation of Th(IV) followed by the chemical precipitation of REEs. According to the obtained results, it could be stated that the early washing step is more efficient for removal of 92.8% of P₂O₅ hence; the separation of REEs from the matrix becomes more achievable. The subsequent extraction process of thorium (Th) revealed that secondary amine could be selectively and efficiently used for separation of Th and Fe with 100 and 98% efficiency respectively from U and REEs. In the residual aqueous solution, REEs were quantitatively precipitated as REEs-oxalate then separated from U.

Keywords

Monazite rare earths thorium precipitation solvent extraction

1 Introduction

With their unique applications in numerous fields, the demand for rare earth elements (REEs) has increased significantly [1], driving the development of new processes to recover rare earths from a variety of new resources [2, 3]. Among a large number of rare earth minerals, only three are mainly used for rare earth production, namely bastnasite with the composition of (RE)(CO₃)F, monazite of (RE)PO₄ and xenotime of YPO₄ [4 –6]. Monazite is a rare earth phosphate also containing thorium and uranium as associated metals [7, 8]. Its structural group consists of arsenate, phosphate and silicate[9, 10].

Several technological methods are used for industrial processing of monazite. Commercially, these methods include the acid leaching in sulfuric or alkaline leaching in sodium hydroxide. In acid digestion step, monazite is treated with 98% H₂SO₄ at 0–230°C. The sulfate (SO₄^2–) ion of H₂SO₄ acts as a ligand which reacts with REs as in Equation 1. $2 {REPO}_{4 (s)} + 3 H_{2} {SO}_{4} \to (RE)_{2} ({SO}_{4})_{3 (s)} ↓ + 6 H^{+} + 2 {PO}_{4}^{3 -}$

But the use of acids results in loss of phosphate as H₃PO₄. Thus, the decomposition of RE phosphates with hot, concentrated alkali was found to be suitable as tri-sodium phosphate was recovered as a by-product, which can be used as a fertilizer as illustrated in Equation 2 [11]. $2 {RE (PO}_{4}) + 6 NaOH \to 2 {RE (OH)}_{3} + 2 {Na}_{3} {PO}_{4}$

During the decomposition by strong alkali such as sodium hydroxide, the REEs in the forms of insoluble phosphates in monazite are transformed into the rare earth hydroxides. The rare earth hydroxides obtained from the alkaline processing can be easily dissolved in acid after removal from supernatant solution of alkaline phosphates [12].

Numerous techniques are available for the separation and recovery of REEs such as chemical precipitation, ion exchange, solvent extraction and adsorption [3 –15]. Among these, solvent extraction (SX) method has wide range of application in hydrometallurgical separation process owing to its great potential on high selectivity, effective separation and high metal enrichment [16]. In solvent extraction systems, phosphates [7 –19], amines [20] and carboxylic acids [21, 22] are usually studied.

Monazite contains 0.2–0.4% uranium as U₃O₈ and 4.5–9.5% thorium as ThO₂, depending on the region of origin/occurrence [23]. In Egypt, monazite is among the economic minerals found in the Egyptian black sands which are concentrated along the beach of the Nile Delta. 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) [24]. Monazite ore processing is a complicated matrix which is mainly attributed to the presence of high phosphate (25% of P₂O₅) and sulfate concentration, hence, complicate the extraction and separation of REEs and other elements. Therefore, this work developed technical systematic studies for monazite processing using alkali solution, resulting in the removal and recovery of phosphate as a valuable product. Moreover, solvent extraction technique was used for selective extraction and separation of Th(IV) followed by the chemical precipitation of REEs.

2 Experimental

2.1 Chemicals and reagents

The monazite used in this work was obtained from Nuclear Materials Authority, Egypt. All chemicals and reagents used were analytical reagent grade and their solutions were prepared with distilled water. All REEs standards were obtained from Sigma Aldrich (99.9%).

2.2 Instruments

Optical emission inductively coupled plasma (ICP-OES, Hudson, New Hampshire, 03051, U.S.A) was used for measurements and detection of various elements concentration. The chemical composition of the high grade concentrate (HGM 92%) was determined by X-ray fluorescence spectrometry using a PW-2400 spectrometer (Phillips, Netherlands).

2.3 Experimental procedure

Trials were performed for digestion of high grade monazite (HGM 92%) using alkaline technique. Representative HGM sample was subjected to XRF analysis to determine its original elemental compositions. The alkaline digestion of monazite was achieved by mixing 20 g of monazite (92%) slowly with 40% caustic soda solution then stirred for 3 h at 140°C. The mixture was then diluted with another 20% NaOH and stirred for 1 h at 105°C. Filtration was carried out (filtrate, sample no. P1), and the yellowish brown monazite cake (residue) was washed with 75 ml dist H₂O at 100°C for 10 min. (4 times). Each two washing steps were collected together in one sample (samples no. P2, P3). The residue was then dried (in the oven) and the weight of the dry residue was found to be 19 g. The dry residue was divided into 3 portions (6 g each). The resulted yellowish brown cake after monazite digestion was shown in Fig. 1.

Fig.1

Yellowish brown cake after monazite digestion by caustic soda.

The leaching step for the 3 portions was carried out using different acids (conc. HCl, conc. HNO₃, conc. H₂SO₄). The first portion was leached with 20 mL conc. HCl at 80°C for 1 h (3 times) giving 3 dissolved samples (C1, C2 and C3).

The second portion was leached with 20 mL conc. HNO₃ at 80°C for 1 h (3 times) giving 3 dissolved samples (N1, N2 and N3). The third portion was leached with 20 mL conc. H₂SO₄ at 80°C for 1 h (3 times). Unfortunately, in case of sulfuric acid leaching, the whole reaction mixture was converted to refractory solid material and hence it was excluded.

The leached samples by HCl were then collected together to give the HCl leached solution with total volume of 25 mL (sample no. F1). 8 mL of this sample was diluted into 100 mL and the pH was readjusted from 0.77 to 2.5 (sample no. F2). Different portions (10 mL each) from sample no. F2 were taken and shaken individually with 10 mL of 0.2 M of different organic extractants (Octylamine, N-Methylaniline, N,N-Dimethylaniline (Fig. 2) and Tri-N-Butyl Phosphate (TBP)) for 30 min., with the phase ratio 1 : 1. All the tested extractents were dissolved individually in petroleum ether at 25°C. The aqueous phase (sample no. F3) separated from the organic phase (N-Methylaniline) was divided to 3 portions and mixed individually with different concentration of oxalic acid (2, 3 and 4%). All the above presented samples of digestion, washing, leaching and extraction were subjected to ICP analysis.

Fig.2

The chemical structure of the investigated amines.

3 Results and discussion

3.1 Monazite digestion, washing and acids dissolution

The data for the elemental composition of the monazite sample (92% grade) are represented in Table 1.

Table 1
XRF analysis for monazite sample (92%)

Compound formula Concentration (%) Compound formula Concentration (%)

F 0.290 ZrO₂ 11.195

MgO 0.453 Rh 0.048

Al₂O₃ 0.980 PdO 0.039

SiO₂ 5.455 SnO₂ 0.187

P₂O₅ 10.051 I 0.053

SO₃ 0.119 La₂O₃ 8.745

Cl 0.078 CeO₂ 15.617

K₂O 0.049 Pr₂O₃ 1.403

CaO 1.572 Nd₂O₃ 6.358

TiO₂ 4.054 Sm₂O₃ 1.016

Cr₂O₃ 0.836 Gd₂O₃ 0.776

MnO 1.000 HfO₂ 0.425

Fe₂O₃ 12.134 WO₃ 0.042

ZnO 0.365 IrO₂ 0.036

As₂O₃ 0.006 PbO 0.142

SrO 0.006 ThO₂ 5.386

Y₂O₃ 1.132 U 0.289

Compound formula	Concentration (%)	Compound formula	Concentration (%)
F	0.290	ZrO₂	11.195
MgO	0.453	Rh	0.048
Al₂O₃	0.980	PdO	0.039
SiO₂	5.455	SnO₂	0.187
P₂O₅	10.051	I	0.053
SO₃	0.119	La₂O₃	8.745
Cl	0.078	CeO₂	15.617
K₂O	0.049	Pr₂O₃	1.403
CaO	1.572	Nd₂O₃	6.358
TiO₂	4.054	Sm₂O₃	1.016
Cr₂O₃	0.836	Gd₂O₃	0.776
MnO	1.000	HfO₂	0.425
Fe₂O₃	12.134	WO₃	0.042
ZnO	0.365	IrO₂	0.036
As₂O₃	0.006	PbO	0.142
SrO	0.006	ThO₂	5.386
Y₂O₃	1.132	U	0.289

In the metallurgy process, the reaction between the phosphorus and RE results in the formation of various phosphides causing pulverization of the RE-complexes [25, 26]. Therefore, it is very essential to eradicate phosphate present in the ore in order to enhance the dissolution of REEs.

During the monazite digestion, all the original phosphorus in the resultant hot slurry was converted into trisodium phosphate while rare earths and the other associated metals were converted into hydrous metal oxide cake (Yellowish brown monazite cake).

According to the digestion process, the alkaline digestion of monazite giving 2 products:

hot slurry, which contain mainly P₂O₅ (92.8%) and traces from Fe₂O₃, ThO₂, La₂O₃, CeO₂ and Nd₂O₃ (Table 4, washing step); hydrous metal oxide cake, which contain major Fe₂O₃, U, ThO₂, La₂O₃, CeO₂, Nd₂O₃ and Y₂O₃ (Table 4, HCl and HNO₃ Leaching).

The solution species resulting from alkaline digestion can actually be represented according to Equations 2, 3, 4. ${Th}_{3} {(PO}_{4})_{4} + 12 NaOH \to 3 {Th (OH)}_{4} + 4 {Na}_{3} {PO}_{4}$ ${UO}_{2} {HPO}_{4} + 2 NaOH \to {UO}_{2} (OH)_{2} + {Na}_{2} {HPO}_{4}$

The washing step was repeated different times until all the trisodium phosphate and free caustic soda had been removed from the cake.

The data present in Table 2, showed the percentage element concentrations (%) after digestion, washing and leaching of monazite. These results were recalculated relative to 100 g monazite taken in consideration the volume of the sample (after digestion, washing and leaching) and the weight of monazite sample.

Table 2

The percentage elements concentrations (%)

Elements	Fe	P₂O₅	Th	U	La	Ce	Nd	Y
Samples
P1	0.269	0.656	0.09	0	0.056	0.048	0.053	0
P2	0.23	2.296	0.315	0	0.196	0.168	0.184	0
P3	0	6.38	0.338	0	0.21	0.18	0.197	0
C1	1.02	0.34	2.1	0.15	1.52	2.83	1.05	0.18
C2	0.8	0.2	1	0.07	1.1	2.11	0.67	0.12
C3	0.53	0.11	0.8	0.041	0.48	1.26	0.48	0.06
N1	0.52	0.21	1.7	0.1	0.35	1.15	0.54	0.138
N2	0.35	0.13	0.9	0.07	0.58	1.05	0.6	0.08
N3	0.29	0.08	0.8	0.04	1.07	1.24	0.84	0.05

To evaluate the efficiency of each step, the data in Table 2, for the 3 different stages; washing (P1, P2, P3), leaching with HCl (C1, C2, C3) and leaching with HNO₃ (N1, N2, N3) were collected and presented in Table 3.

Table 3

Elements concentration by % (collected stages)

Elements	Fe	P₂O₅	Th	U	La	Ce	Nd	Y
Samples
Washing (P1+P2+P3)	0.499	9.33	0.743	0	0.462	0.4	0.43	0
HCl Leaching (C1+C2+C3)	2.35	0.65	3.9	0.261	3.1	6.2	2.2	0.36
HNO₃ Leaching (N1+N2+N3)	1.16	0.42	3.4	0.21	2	3.44	2	0.27

The efficiency % of the each metal relative to its original concentration (previously shown in Table 1) were calculated after conversion these metal ions into the corresponding metal oxide to be compatible with the compound formula in the XRF analysis. According to the results illustrated in Table 4, it could be stated that, washing step is more efficient for removal of 92.8% of P₂O₅. Therefore, it could be easy to separate the phosphate species from the complicated monazite matrix, hence; the separation of Th and REEs from the matrix becomes more achievable. Moreover HCl leaching step is more efficient rather than HNO₃. The solution species resulting from extraction with HCl can be represented according to Equations 5, 6, 7. $\begin{matrix} RE (OH)_{3} + 3 HCl \to {RECl}_{3} + 3 H_{2} O \\ {Th (OH)}_{4} + 4 HCl \to {ThCl}_{4} + 4 H_{2} O \\ {UO}_{2} (OH)_{2} + 6 HCl \to UCl 6 + 4 H_{2} O \end{matrix}$

Table 4

Removal efficiency (%) relative to XRF analysis

Elements	Fe₂O₃	P₂O₅	ThO₂	U	La₂O₃	CeO₂	Nd₂O₃	Y₂O₃
Samples
Washing (P1+P2+P3)	11.75	92.8	15.7	0	12.4	3.14	15.8	0
HCl Leaching (C1+C2+C3)	55.37	6.4	82.4	90.3	83.1	48.8	80.7	80.7
HNO₃ Leaching (N1+N2+N3)	27.33	4.2	71.8	72.6	53.6	27	73.4	60.6

Despite the high leaching efficiency of HCl, it could not separate REEs from the other co-existing cations such as U, Th and Fe. Therefore, the next trials were focused on separation of REEs from the hydrochloric acid leachate.

3.2 Organic solvent extraction of REEs from hydrochloric acid leachate

Series of experiments were attempted to separate REEs from HCl leachate with solvent extraction technique using different basic and acidic extractants followed by precipitation of REEs. The data presented in Table 5, showed the percentage elements concentration (%) after extraction.

Table 5
The percentage concentration (%) of the investigated elements [Extractant concentration is 0.2 M]

Elements Fe Th U La Ce Nd Y

Samples

F2 0.062 0.044 0.029 0.148 0.237 0.117 0.003

Octylamine 0.0003 0.005 Zero 0.025 0.02 0.011 Zero

N-Methylaniline 0.0013 Zero 0.028 0.15 0.23 0.114 0.003

N,N-Dimethylaniline 0.002 0.005 0.022 0.15 0.23 0.114 0.003

TBP 0.06 0.042 0.03 0.015 0.23 0.11 0.003

Elements	Fe	Th	U	La	Ce	Nd	Y
Samples
F2	0.062	0.044	0.029	0.148	0.237	0.117	0.003
Octylamine	0.0003	0.005	Zero	0.025	0.02	0.011	Zero
N-Methylaniline	0.0013	Zero	0.028	0.15	0.23	0.114	0.003
N,N-Dimethylaniline	0.002	0.005	0.022	0.15	0.23	0.114	0.003
TBP	0.06	0.042	0.03	0.015	0.23	0.11	0.003

These results were recalculated relative to 100 gm monazite to give the percentage elements concentration (%) as shown in Table 5. The percentage extraction efficiency for each organic extractants towards the investigated metal ions was calculated relative to its original sample no. 2 (F2) as shown in Table 6.

Table 6

Extraction efficiency % relative to F2 [Extractant concentration is 0.2 M]

Elements	Fe	Th	U	La	Ce	Nd	Y
Samples
Octylamine	99.5	88.6	100	83.1	91.6	90.6	100
N-Methylaniline	98	100	3.4	Zero	Zero	Zero	Zero
N,N-Dimethylaniline	97	88.6	24.1	Zero	Zero	Zero	Zero
TBP	Zero	Zero	Zero	Zero	Zero	Zero	Zero

According to Tables 5 and 6, it is clear that Octylamine as a primary amine is efficient to extract all the investigated elements, while N-Methylaniline is efficient for extraction of Fe and Th and less efficient for REEs and U. Also, N,N-Dimethylaniline is efficient for extraction of Fe, Th and U and less efficient for REEs. On the other hand, TBP is not effective for all of these investigated elements.

Based on the chemical structure of the investigated amines, Fig. 2, it could be observed that Octylamine is more basic than both of N-Methylaniline and N,N-Dimethylaniline [27], This is due to: the inductive effect (+I) of the long chain alkyl groups in Octylamine (C₈H₁₇NH₂) which increases the electron density on the N-atom.

Also, the lone pair electrons on the N-atom in both of N-Methylaniline (C₆H₅NHCH₃) and N,N-Dimethylaniline [C₆H₅N(CH₃)₂] are delocalized over the benzene ring by the ′R effect of ′C₆H₅ group. At the same time (C₆H₅N(CH₃)₂) is more basic than (C₆H₅NHCH₃) due to the presence of two electron-donating ′CH₃ groups in [C₆H₅N(CH₃)₂].

This explains why aliphatic amines are stronger bases than aromatic amines. Hence, the order of increasing the basicity of the given amines is given as follows:

C₈H₁₇NH₂ >C₆H₅N(CH₃)₂ >C₆H₅NHCH₃

Moreover, it is known that the higher the basic strength, the lower is the pK_b values, where the pK_b values increase in the order:

C₆H₅NHCH₃ (pK_b is 9.3)>C₆H₅N(CH₃)₂ (pK_b is 8.92)>C₈H₁₇NH₂ (pK_b is 3.35)

The obtained results are in a good agreement with many previously published results. According to Kim et al. [28], zero percent extraction was observed for all rare-earths at all pH values studied using 1 mol/L Alamine 336 [tertiary amine, N,N-Dioctyl-1-Octanamine (C₂₄H₅₁N)] or Aliquat 336 [quaternary ammonium salt, Tri-Octylmonomethylammonium chloride (C₂₅H₅₄ClN)]. Similarly, Amaral and Morais [29], proved that Primene JM-T [primary amine, Dimethylheptadecan-1-amine (C₁₉H₄₁N)] can extract thorium with concentration not exceed 0.15 mol/L, in which increasing concentration of Primene JM-T causes an increase in extraction of rare earth elements, but this fact was not observed for Alamine 336 (tertiary amine) in the concentration range investigated which confirm that REEs can be extracted by primary amines not by secondary or tertiary amines.

After the solvent extraction step, the aqueous phase (sample from the leachate by HCl then mixed with the organic phase N-Methylaniline and separated to give sample no. F3) that contains U and REEs was treated with different concentration of oxalic acid to precipitate and separate REEs from U according to Equation 8. The results are shown in Table 7. $2 {REE}^{+ 3} + 3 (C_{2} O_{4})^{- 2} \to {REE}_{2} (C_{2} O_{4})_{3 (s)} ↓$

Table 7

Aqueous phase concentration (mg/L) after oxalic acid precipitation

Elements	U	La	Ce	Nd	Y
Samples
F3	143	770	1210	593	17
2% Oxalic acid	143	47	84	53	9
3% Oxalic acid	143	21	51	19	0
4% Oxalic acid	143	9	18	6	0

In this respect, The solubility product value of Y₂(C₂O₄)₃ is 5.1×10^–30, La₂(C₂O₄)₃ is 6.0×10^–30, Ce₂(C₂O₄)₃ is 4.0×10^–31, Nd₂(C₂O₄)₃ is 1.3×10^–31, Yb₂(C₂O₄)₃ is 9.5×10^–31, thorium oxalate dihydrate (Th(C₂O₄)₂.2H₂O) is 5.01×10^–25.

According to Table 7, it can be noticed that, oxalic acid is efficient for precipitation of REEs (more than 95%) leaving U soluble as U-oxalate complex [25], [The tri hydrate UO₂C₂O₄.3H₂O is obtained when oxalic acid is added to a uranyl salt solution (Equation 9). ${UO}_{2_{(a q)}^{- 2}} + H_{2} C_{2} O_{4 (aq)} \to {UO}_{2} {(C_{2} O_{4})}_{(aq)} .3 H_{2} O$

Although uranyl oxalate is slightly soluble in water (0.45 g per 100 gm H₂O at 20°C), its solubility behavior completely changed in the presence of the mineral acids such as HCl, HNO₃ and H₂SO₄.

Finally, secondary amine (0.2 M N-Methylaniline at pH 2.5) was efficiently applied for separation of Fe and Th with 98 and 100% respectively from U and REEs, followed by precipitation of REEs as oxalate. The overall process of monazite digestion, leaching and extraction could be summarized and represented schematically as shown in Fig. 3.

Fig.3

Schematic flowchart for monazite digestion by caustic soda followed by acid leaching, organic extraction and precipitation.

4 Conclusion

Alkaline digestion of high grade monazite has been investigated. The results clearly demonstrate that washing step after digestion is efficient for removal of 92.8% of P₂O₅. HCl is preferred rather than HNO₃ for the leaching of all elements from the yellowish brown monazite cake. Fe(III) and Th(IV) were separated from U and REEs using 0.2 M N-Methylaniline at pH 2.5 as a secondary amine while REEs were selectively precipitated as oxalate then separated from the soluble U-oxalate complex.

Footnotes

Acknowledgments

Authors are thankful to the Science and Technology Development Fund (STDF), Egypt, for the financial support, Grant No 5021. We would also like to acknowledge the Nuclear Materials Authority, Egypt for providing the required high grade monazite.

References

Kim

and Osseo-Asare

, Aqueous stability of thorium and rare earth metals in monazite hydrometallurgy: Eh′pH diagrams for the systems Th–, Ce–, La–, Nd′(PO₄)′(SO₄)′H₂O at 25°C, Hydrometallurgy 113-114 (2010), 67–78.

Krebs

D.G.I.

and Furfaro

, The Kvanefjeld process, ALTA, Uranium-REE Conference, May 25′June 1, Perth, Western Australia, 2013.

Long

K.R.

, Gosen

B.S.V.

, Foley

N.K.

and Cordier

, The principal rare earth elements deposits of the United States – a summary of domestic deposits and a global perspective, Scientific Investigations Report. U.S. Geological Survey, 2010.

Jordens

, Cheng

Y.P.

and Waters

K.E.

, A review of the beneficiation of rare earth element bearing minerals, Miner Eng 41 (2013), 97–114.

Kanazawa

and Kamitani

, Rare earth minerals and resources in the world, J Alloy Compd 408-412 (2005), 1339–1343.

Xie

, Zhang

T.A.

, Dreisinger

and Doyle

, A critical review on solvent extraction of rare earths from aqueous solutions, Miner Eng 56 (2014), 10–28.

Loureiro

F.E.V.L.

, Terras raras no Brasil, depositos, recursos identificados, reservas, Rio de Janeiro, CETEM/CNPq/MCT, (Estudos e documentos) 21 (1994), 183.

Miyawaki

and Nakai

, Crystal chemical aspects of rare earth minerals, In: Jones

, Wall

, Williams

C.T.

(Eds.), Rare Earth Minerals: Chemistry, Origin and Ore Deposits. Mineralogical Society Series, 21, 1996, Chapman and Hall, London.

Fleiscer

, Rosenblum

and Woodruff

, The distribution of lanthanides and yttrium in the minerals of the monazite family, Geological Survey Professional Paper, File Report, 1990.

10.

Toledo

M.C.M.

and Pereira

V.P.

, Ocorrência e variabilidade de composiçao dos fosfatos do grupo da monazita em carbonatitos, Pesquisas em Geociências 30 (2003), 83–95.

11.

Panda

, Kumari

, Jha

M.K.

, Hait

, Kumar

J.R.

and Lee

J.Y.

, Leaching of rare earth metals (REMs) from Korean monazite concentrate, J Ind Eng Chem 20 (2014), 2035–2042.

12.

Abdel-Rehim

A.M.

, An innovative method for processing Egyptian monazite, Hydrometallurgy 67 (2002), 9–17.

13.

Atanassovaa

, Synergistic solvent extraction and separation of lanthanide (III) ions with 4-benzoyl-3-phenyl-5-isoxazolone and the quaternary ammonium salt, Solvent Extr Ion Exch 27 (2009), 159–171.

14.

Faust

S.D.

and Aly

O.M.

, Chemistry of Water Treatment, Butter Worth Publishers, 1976, NY, USA.

15.

Omar

H.A.

and Moloukhia

, Use of activated carbon in removal ofsome radioisotopes from their waste solutions, J HazardMater 157 (2008), 242–246.

16.

Radhika

, Kumar

B.N.

, Kantam

M.L.

and Reddy

B.R.

, Liquid- liquid extraction and separation possibilities of heavy and light rare-earths from phosphoric acid solutions with acidic organophosphorus reagents, Sep Purif Technol 75 (2010), 295–302.

17.

Asano

, Itabashi

and Kawamoto

, Separation of iron(III) by di(2-ethylhexyl) phosphate/4-methyl-2-pentanone extraction, Journal of the Iron and Steel Institute of Japan 87 (2001), 623–625.

18.

Mishra

R.K.

, Rout

P.C.

, Sarangi

and Nathsarma

K.C.

, A comparative study on extraction of Fe(III) from chloride leach liquor using TBP, Cyanex 921 and Cyanex 923, Hydrometallurgy 104 (2010), 298–303.

19.

Principe

and Demopoulos

G.P.

, Comparative study of iron(III) separation from zinc sulphate-sulphuric acid solutions using organophosphorus extractants, OPAP and D2EHPA: part II. Stripping, Hydrometallurgy 79 (2005), 97–109.

20.

Saji

and Reddy

M.L.P.

, Liquid′liquid extraction separation of iron(III) from titania wastes using TBP-MIBK mixed solvent system, Hydrometallurgy 61 (2001), 81–87.

21.

Pouillon

and Doyle

F.M.

, Solvent extraction of metals with carboxylic acids - theoretical analysis of extraction behavior, Hydrometallurgy 19 (1988), 269–288.

22.

Preston

J.S.

, Solvent extraction of metals by carboxylic acids, Hydrometallurgy 14 (1985), 171–188.

23.

Borai

E.H.

and El Afifi

E.M.

and Shahr El-Din

A.M.

, Selective elimination of natural radionuclides during the processing of high grade monazite concentrates by caustic conversion method, Korean, J Chem Eng 34(4) (2017), 1091–1099.

24.

Osman

, Solvent extraction study on uranium and thorium from sulphuric acid solution and its technological application, M.Sc. thesis, Zagazig University; 1998.

25.

Gupta

C.K.

and Krishnamurthy

, Extraction Metallurgy of Rare Earths, 2005, Boca Raton, FL, CRC.

26.

Qun

, Gan-feng

, Cun-zhi

and Shi-rong

, Micro-mechanism of disintegration of RE-silicide alloy containing phosphorus, J Rare Earth 19 (2001), 284–287.

27.

Borai

E.H.

and Shahr El-Din

A.M.

, EL-Sofany

E.A.

, Sakr

A.A.

and El-Sayed

G.O.

, Extraction and separation of some naturally occurring radionuclides from rare earth elements by different amines, Arab Journal of Nuclear Science and Applications 47 (2014), 48–60.

28.

Kim

J.S.

, Kumar

B.N.

, Radhika

, Kantam

M.L.

and Reddy

B.R.

, Studies on selection of solvent extractant system for the separation of trivalent Sm, Gd, Dy and Y from chloride solutions, Int J Miner Process 112-113 (2012), 37–42.

29.

Amaral

J.C.

and Morais

C.A.

, Thorium and uranium extraction from rare earth elements in monazite sulfuric acid liquor through solvent extraction, Miner Eng 23 (2010), 498–503.