Preparation of Pure Nb₂O₅ from Gabal El-Faliq Pegmatite, South Eastern Desert, Egypt

Abstract

The present work is concerned with selective dissolution of niobium from Gabal El Faliq concentrate sample by using fluoride-free process. The chemical analysis of concentrate sample revealed the presence of high amount of ZrO₂, Nb₂O₅, Re₂O₃, and HfO₂ assaying 33.6, 1.99, 4.8, and 1.2%w/w respectively. Several economic minerals including zircon, samarskite, fergusonite, and xenotime were recorded in the studied sample. About 98% of Nb content was selectively dissolved at the optimum conditions of 1:2 concentrate sample: KOH weight ratio, 2.5 h fusion time, and fusion temperature of 550 °C. Organic solvent extraction process using Aliquat 336S in xylene was effectively used for preparing pure Nb₂O₅ (98.95%).

1 Introduction

Gabal (G.) El-Faliq area, southeastern Desert of Egypt, is located between latitude 24° 35′ and 24° 42′ N and longitude 34° 28′ and 34° 35′ E. This area is covering about (250 km²) and it is composed mainly of ophiolitic mélange, gneisses, older, younger granitoids, post granitic dykes, and veins which include pegmatites [42]. The most important rock unite in the studied area is the post granitic dykes and veins. The younger granitoids and post granitic dykes and veins also attracted the interest of many authors because they are typically associated with anomalous concentrations of rare and some economically important elements such as Sn, W, Mo, U, Nb, Ta, REE, Zr, and Rb. It is worthy to mention herein that all the granitoid rocks of G. El Faliq area are cut and crossed by several pegmatite bodies. These bodies are trending (NNE-SSW) and ranging in length from 50 m to several meters. Also, they occur as pockets or lenses (10–20 m in length) at the margin and the core of the gneisses rocks as well as ophiolitic mélange. The present minerals of G. El Faliq area are classified into (a) secondary uranium minerals (uranophane and meta-autunite), (b) niobium–tantalum minerals (columbite, euxenite, and fergusonite), (c) sulphide minerals (pyrite and barite), and (d) accessory minerals (allanite and zircon) [35, 42, 43].

Niobium and tantalum are widely used in today’s technology due to their unique properties; they have been utilized widely in the steel, electronic, and other high-tech industries [7, 22]. Chemical separation or purification of these metals from their ores and concentrates has been studied using different methods. Among these methods, alkali fusion using sodium hydroxide and sodium carbonate (or the potassium salts) were used as leaching agents [6, 7, 12, 50, 53], and KHSO₄ roasting [3]. Some niobium–tantalum ore types can also be processed using chlorination, fusion with ammonium fluoride and bifluoride [20, 25, 37], direct acidic dissolution with either H₂SO₄ [15], or a combination of H₂SO₄ and HF [23, 27, 39]. Also, most niobium–tantalum ores were dissolved with concentrated HF [14, 20, 40, 41]. Due to high volatility of HF, about 6–7% of it is lost during the decomposition process which is hazardous [10, 11, 21, 22]. Furthermore, a large amount of wastewater containing fluoride is generated that needs to be treated. More importantly, this method is only appropriate for high-grade niobium–tantalum ores [32].

With respect to pure Nb and Ta recovery from their leach liquors, many researchers studied in details the separation and purification of Nb and Ta via liquid–liquid extraction processes. In this regard, several solvents were applied for this purpose. One of the most important and widely industrially applied is tri-n-butyl phosphate (TBP) [4, 5, 20, 36]. Also, many workers as Salles [44], El-Hussaini [14], Agulyansky et al. [2], Nikolaev and Maiorov [34], Yang et al. [52], Toure et al. [47], and Htwe and Lwin [23] used methyl isobutyl ketone (MIBK) for separating pure compounds of Nb and Ta, while Mayorov and Nikolaev [31], Agulyansky et al. [2], and El-Hazek et al. [13] used octanol. In this target, Tertiary amines, e.g., trioctyl amine, Alamine 336, and Aliquat 336S, were effectively applied by Sato et al. [45, 46], Deblonde et al. [8], Mishra et al. [33], and Zhu and Cheng [54]. The following equation illustrates the extraction process of Nb and Ta from oxalate media with tertiary amines [54].

$$ 3{{\left[{\left({\mathrm{C}}_8{\mathrm{H}}_{11}\right)}_3\mathrm{NH}\right]}^{+}}_{\left(\mathrm{Org}\right)}+{{\left[\mathrm{NbO}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3\right]}^{3-}}_{\left(\mathrm{Aq}\right)}\to {\left[{\left({\mathrm{C}}_8{\mathrm{H}}_{11}\right)}_3\mathrm{NH}\right]}_3{\left.\mathrm{NbO}\left({\mathrm{C}}_2{\mathrm{O}}_4\right)\right]}_{3\left(\mathrm{Org}\right)} $$

The main purpose of the present work is the selective alkaline dissolution of Nb from Gabal El Faliq concentrate sample depending essentially on the fluoride-free process with breakdown of the associated refractory minerals, e.g., zircon and xenotime. Finally, the preparation of pure Nb₂O₅ using Aliquat 336S in xylene was carried from the prepared leach solution.

2 Experimental

2.1 Materials

All the utilized chemicals in the experimental work were analytical grade including potassium hydroxide pellets (99%) [Prolabo], sulfuric acid (97%) [Adwic], oxalic acid (99%), ammonium hydroxide (30%) and xylene (99%) [Adwic], nitric acid (69%) [BDH], and Aliquat 336S (99%) [Sigma Aldrich]. On the other hand, the working concentrate sample of Gabal El Faliq area was subjected to both mineralogical and chemical specification and also to processing procedures.

2.2 Chemical and Mineralogical Specification

The sample under study was selected from Gabal El-Faliq, southwestern Desert, Egypt, and was subjected to physical upgrading process using shaking table. The concentrate sample was subjected to major and minor oxide chemical analysis together with analysis of Nb, REE, Zr, and Hf besides U and Th elements using Axios advanced WDXRF-PANalytical (Netherland). In this regard, the analysis of Nb and Zr during all experimental work was carried out by Microwave Plasma Atomic Emission Spectrometer (4200 MP-AES, Agilent Technologies). The REE were estimated using 0.05% Arsenazo III at λ₆₅₄ nm using Y as a reference ([29]), while U in all performed procedures was measured by oximetric titration against NH₄VO₃ in the presence of diphenyl amine-4-sulfonic acid sodium salt as an indicator [30]. The scanning electron microscopy (CAM SCAN series 4 ISIS 200 E/X system with pentajet detector) (SEM-EDX) was used for confirming the Nb cake and ICP (720 ICP-OES, Agilent Technologies) was used for identifying of the Nb pure product.

To study the mineralogical composition of the studied concentrate sample, the latter was subjected to X-ray diffraction analysis (PHILIPS PW 223/30) for identification of mineral content. Moreover, heavy mineral separation procedures were applied; for this purpose, the sample was ground to 60 mesh size to liberate the heavy minerals. The ground sample was then washed with distilled H₂O for removing slimes; then, it was sieved into the size interval of 0.4, 0.3, 0.2, and 0.1 mm. Heavy liquid separation procedures were applied using bromoform (sp.gr. 2.8) to bring out the heavy fraction. The mineral grains of the same color in the heavy fraction were separately picked up under the binocular microscope. The separated mineral grains were then investigated by X-ray diffraction analysis to identify the mineral crystals structure.

2.3 Optimization of the Fusion Process

Several experiments were performed by mixing constant weight (5 g) of the fine ground concentrate sample with a solid KOH (melting point 360 °C) as the fused reagent in a porcelain crucible. The mixture was then fused at different temperatures ranged from 300 to 650 °C for different periods of times (1–3 h). After cooling, the fused matrix was leached with distilled H₂O in S/L ratio of 1/4 for 30 min at room temperature. The spent residue was washed twice with distilled H₂O. After filtration using filter paper, Nb in the prepared alkaline solution was analyzed and its leaching efficiency (%) was calculated using the equation:

$$ \mathrm{Leaching}\ \mathrm{efficiency}\left(\%\right)=\frac{\mathrm{Metal}\ \mathrm{content}\ \mathrm{in}\ \mathrm{the}\ \mathrm{leach}\ \mathrm{liquor}\ }{\mathrm{Metal}\ \mathrm{content}\ \mathrm{in}\ \mathrm{the}\ \mathrm{ore}}\times 100 $$

2.4 Preparation of Impure Niobium Cake

Nb ion species in the prepared alkaline leach liquor was completely precipitated by neutralization process using diluted H₂SO₄ to pH 8.5. The precipitated Nb cake was then leached with oxalic acid solution for the dissolution of its Nb content with some associated impurities. Aliquat 336S diluted in xylene was used at ambient temperature 35 ± 5 °C for extraction of Nb from its oxalate leach liquor.

2.5 Optimization of Nb Extraction Process

Several relevant effective extraction parameters such as Aliquat 336S concentration, pH value, contact time, and phase volume ratios (O/A) were tested to optimize the loading process of Nb. The loaded Aliquat 336S was stripped with different mineral acids, e.g., H₂SO₄, HCl, and HNO₃ of 0.4 M concentration at A/O ratio of 1/1 and contact time of 5 min to optimize the Nb stripping process. All the experiments were carried out in shaking funnels. Finally, the concentrated Nb strip solution was treated with an ammonia solution to precipitate Nb as Nb-hydroxide cake. The latter was carefully washed with distilled water and then ignited at 650 °C to prepare a pure Nb₂O₅ product which identified with XRD, while its purity was determined using ICP.

3 Results and Discussion

3.1 Chemical and Mineralogical Characterization of the Sample

Complete chemical composition of Gabl El-Faliq concentrate sample is represented in Table 1. From this table, it is clearly evident that the concentrate is mainly composed of ZrO₂ together with SiO₂ as well as Fe₂O₃ and Al₂O₃ which assaying 33.6, 39, 10, and 5.9%w/w, respectively. In the meantime, there are considerable concentrations of valuable economic oxides, e.g., Re₂O₃ [sum of all rare earth elements oxides (La₂O₃, CeO₂, etc…)], Nb₂O₅, Ta₂O₃, and HfO₂ assaying 4.8, 1.99, 0.045, and 1.2%w/w, respectively. Moreover, the present concentrate sample contains remarkable concentrations of radioactive nuclear elements, e.g., U and Th assaying 800 and 340 ppm, respectively.

Table 1 XRF chemical analysis of Gabl El-Faliq concentrate sample

For mineral identification, the bulk concentrate sample was characterized by means of XRD analysis as shown in Fig. 1; it revealed the presence of zircon, silica, and hematite as main components, while the obtained data of XRD analysis of separated mineral grains revealed the presence of valuable minerals in close association with non-notable (gangue) minerals. Accordingly, the studied sample refers to the presence of the following mineral association (Figs. 2, 3, and 4).

Fig. 1

XRD pattern of bulk concentrate sample

Fig. 2

XRD pattern of the identified zircon mineral grains

Fig. 3

XRD pattern of the identified fergusonite- and smarskite-heated mineral grains

Fig. 4

XRD pattern of the identified zircon and xenotime mineral grains

3.1.1 Zircon Mineral

Zircon (ZrSiO₄) is a common accessory mineral in igneous, metamorphic, and sedimentary rocks. It may form large well-developed crystals in granites and pegmatites. This mineral is extremely resistant to chemical and physical weathering. Zircon was identified by XRD analysis (Fig. 2) in association with xenotime and separated in abundant amounts from pegmatite sample of G. El Faliq. Some crystals show corrosion, zonation, and overgrowth phenomena; they have varying shades of honey color due to the effect of radioactivity.

3.1.2 Nb, Ta, and REE Minerals

Samarskite is common and locally abundant enough that it has the potential to be a valuable source of REE. It is usually associated with fergusonite as Nb, Ta, and REE mineral. Samarskite has dark pitchy to velvety black to dark brown color, irregular massive grains with sub-conchoidal fracture. XRD data confirmed its presence in the separated grains from the pegmatite of G. El Faliq (Fig. 3).

3.1.3 Rare Earth Minerals

In the study area, fergusonite (Y, Er) (Nb, Ta, Ti) O₄ and xenotime were recorded as parent REE minerals in pegmatite of G. El Faliq. It is worthy to mention herein that fergusonite is mainly associated with samarskite. Fergusonite-Formanite series of chemical formula (Y, Er) (Nb, Ta) O₄-(Y, Er) (Nb, Ta) O₄ have brown color with no specific crystal shape but usually metamicted (Fig. 3). Fergusonite end member is the most common mineral. The chemical heterogeneity of fergusonite is represented by the variation in Re₂O₃ and Nb₂O₅ [49]. Fergusonite, ideally YNbO₄, occurs as an accessory mineral in granitic pegmatites, mostly in rare earth element (REE)-enriched pegmatites [17, 18, 38], and often in combination with one or more Y, Th, Nb, Ta, and Ti oxide accessory minerals [28]. The XRD data confirmed the detection of fergusonite mineral (Fig. 4).

On the other hand, xenotime is an (YPO₄) REE-phosphate mineral; Eu, Er, Tr, and Yb, as well as metal elements such as Th and U (all replacing Y), are the expressive secondary components of xenotime. Xenotime is used chiefly as a source of yttrium and HREE (Dy, Yb, Er, and Gd). It is found in pegmatites and other igneous rocks, as a minor accessory mineral. Associated minerals include biotite, quartz, and zircon; xenotime is typically translucent to opaque in shades of brown to brownish yellow and has a variable habit. It may form as minute grains or as extremely thin (less than 10 μ) coatings on detrital zircon grains (Fig. 4).

In this context, oxy–hydroxy minerals are either primary phases (e.g., magnetite) or oxidation products of the mineralized rocks occurring at different ratios in the studied area (e.g., hematite and goethite) in close association with quartz and kaolinite minerals and also with the valuable minerals which refer to hematization process.

3.2 Alkaline Fusion of the Concentrate Sample

The mentioned chemical and mineralogical composition of Gabl El-Faliq concentrate sample reflected the significant high-grade type of mineralization and reflected also the importance of the processing of such concentrate sample for recovering its valuable elements. According to the huge assemblage of mineral constituents in the working concentrate, it was found necessary to apply the alkaline breakdown using KOH due to:

• *Selective dissolution of Nb away from Zr and REE, where it forms water-soluble K-niobates and insoluble (Zr- REE) hydrous oxides.

• **Molten KOH forms with zircon mineral (ZrSiO₄), a water-soluble K-silicates and K-zirconate. It also forms with xenotime mineral (YPO₄), soluble K-phosphate, and acid soluble RE-hydroxide.

All of these can be summarized by the following equations which represented the chemical reactions between the molten KOH and the studied sample [1, 19, 50].

$$ {\displaystyle \begin{array}{l}\left(\mathrm{Fe};\mathrm{Mn}\right)\mathrm{O}{\left(\mathrm{Ta};\mathrm{Nb}\right)}_2{\mathrm{O}}_5+6\mathrm{KOH}\to 2{\mathrm{K}}_3\left(\mathrm{Ta};\mathrm{Nb}\right){\mathrm{O}}_4+\left(\mathrm{Fe};\mathrm{Mn}\right)\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}\\ {}6{\mathrm{K}}_3\left(\mathrm{Ta};\mathrm{Nb}\right){\mathrm{O}}_4+\left(5+\mathrm{n}\right){\mathrm{H}}_2\mathrm{O}\to {\mathrm{K}}_8{\left(\mathrm{Ta};\mathrm{Nb}\right)}_6{\mathrm{O}}_{19}\cdotp {\mathrm{nH}}_2\mathrm{O}+10\mathrm{KOH}4\mathrm{FeO}+{\mathrm{O}}_2\to 2{\mathrm{Fe}}_2{\mathrm{O}}_3\\ {}{\mathrm{ZrSiO}}_4+4\mathrm{KOH}\to {\mathrm{K}}_2{\mathrm{ZrO}}_3+{\mathrm{K}}_2{\mathrm{SiO}}_3\left(\mathrm{dipotassium}\ \mathrm{metasilicates}\right)+2{\mathrm{H}}_2\mathrm{O}\\ {}{\mathrm{ZrSiO}}_4+6\mathrm{KOH}\to {\mathrm{K}}_2{\mathrm{ZrO}}_3+{\mathrm{K}}_4{\mathrm{SiO}}_3\left(\mathrm{potassium}\ \mathrm{orthosilicates}\right)+3{\mathrm{H}}_2\mathrm{O}\\ {}\mathrm{Y}\left({\mathrm{PO}}_4\right)+3\mathrm{KOH}\to \mathrm{Y}{\left(\mathrm{OH}\right)}_3+{\mathrm{K}}_3{\mathrm{PO}}_4\end{array}} $$

In this context, several relevant factors such as sample/KOH mixed weight ratios, fusion time, and fusion temperature were properly studied in a manner to optimize the Nb dissolution process.

3.2.1 Effect of Concentrate Sample/KOH Weight Ratios

The effect of concentrate sample/KOH weight ratios upon the leaching efficiency of niobium was studied in the range between 1/1 to 1/2.5, while the other fusion conditions were fixed at 2 h as fusion time and fusion temperature of 550 °C. The obtained data (Fig. 5) showed that Nb dissolution efficiency improved obviously with decreasing the concentrate sample/KOH weight ratio, where it increased from 64.5 to 91.9% with decreasing the ratio from 1/1 to 1/2. However, further decreasing in the weight ratio more than 1/2 reflected very slight improvement in Nb dissolution efficiency. This may be attributed to the consumption of KOH in the dissolution of interfering metal ions especially Si⁴⁺ during the dissolution of niobium [50]. Thus, 1/2 weight ratio is preferred as the optimum ore concentrate/KOH weight ratio.

Fig. 5

Effect of concentrate sample/KOH weight ratio upon dissolution efficiencies of niobium (2 h fusion time and 550 °C fusion temperature)

3.2.2 Effect of Fusion Temperature

Data in Fig. 6 indicated the effect of changing fusion temperature from 400 to 650 °C upon Nb dissolution efficiency from the working ore material concentrate at ore concentrate/KOH weight ratio of 1/2 and fusion time of 2 h. It was observed that the best Nb dissolution efficiency (95.4%) was achieved at the suitable fusion temperature of 550 °C. Also further increase in fusion temperature up to 650 °C did not show any significant change in Nb dissolution efficiency, where the reaction between Nb₂O₅ and KOH occurred much more readily than other reactions and was complete at only 550 °C [50].

Fig. 6

Effect of fusion temperature upon dissolution efficiencies of niobium (2 h and 1/2 concentrate sample/KOH weight ratio)

3.2.3 Effect of Fusion Time

This effect was studied in the time periods ranged between 1 to 3 h, where the other parameters were fixed at fusion temperature of 550 °C and concentrate sample/KOH weight ratio of 1/2. Data represented in Fig. 7 emphasized that Nb dissolution efficiency reached its maximum value (97.9%) at the time period of 2.5 h.

Fig. 7

Effect of fusion time upon dissolution efficiencies of niobium (1/2 concentrate sample/KOH weight ratio and 550 °C fusion temperature

3.3 Preparation of the Alkaline Leach Liquor

In the light of the foregoing mentioned alkaline fusion process of Gabal El-Faliq concentrate sample, it could be concluded that the optimum fusion conditions required for dissolving 97.9% of niobium are as follows:

• Concentrate sample/KOH weight ratio: 1/2

• Roasting time: 2.5 h

• Roasting temperature: 550 °C

On the other hand, it is worthy to mention herein that the other associated metal values, e.g., REE, Zr, U, and Th, were left in the spent residue as insoluble hydrous cake. These determined optimum fusion conditions were used to prepare the pregnant leach liquor for subsequent recovery of niobium. Accordingly, 200 g of the concentrate sample was used to yield 2 L of alkaline leach liquor. The latter was found to assay 1.36 g/L of Nb and 0.03 g/L of Ta with chemical composition as shown in Table 2 and its pH value higher than 12. Yttrium showed small solubility with potassium hydroxide (11.3%) and this may be attributed to the technical viability of the alkaline leaching of yttrium [48, 51]. The prepared alkaline leach liquor was first subjected to silica gel precipitation. This was carried out by adjusting the pH value of the prepared leach liquor via H₂SO₄ (97%) to be 10. Almost Si⁴⁺ ions were precipitated as gelatinous Si(OH)₄ with precipitation efficiency of 93%, while small amounts of Nb⁵⁺ ions (not exceed 0.95% of total Nb content) were lost. After filtration, the precipitate of gelatinous Si(OH)₄ was then ignited at 850 °C for 1 h and identified using XRD (Fig. 8).

Table 2 Chemical composition of the working alkaline leach liquor

Fig. 8

XRD patterns for the confirmation of SiO₂

3.4 Extraction and Purification of Niobium

The filtrate of the working alkaline solution which contains almost niobium (1.34 g/L) and the remains of silicon (0.8 g/L) was subjected for further acidification to pH 8.5, where almost Nb⁵⁺ ions (99.8%) were already precipitated via hydrolysis process [50]. After filtration and washing, the prepared Nb cake was collected and a small amount of the cake was examined using SEM-EDAX, where it revealed that its purity attained only 55.3% with the interfering impurities of Si, Fe, K, and Y as shown in Fig. 9.

Fig. 9

SEM-EDAX chart for identification of Nb precipitated cake

Therefore, further purification using solvent extraction Aliquat 336S in xylene was needed. For this purpose, the previously prepared hydrous Nb cake was completely dissolved in 8% oxalic acid solution and stirring for 30 min at 80 °C to form the corresponding stable anion Nb-oxalate complexes [Nb(C₂O₄)₂]⁻ and/or [Nb(C₂O₄)₃]³⁻ at pH 0.3–4.3 [24]. Complete dissolving of Nb, Fe, and small portion of Si was attained, while Y was kept as insoluble Y-oxalate [16]. The prepared Nb-oxalate solution of pH 2.8 was found to assay 3.15 g/L of Nb and 0.39 g/L of Fe as well as 0.25 g/L of Si with leaching efficiencies of 98.9, 84.7, and 14.3% respectively.

3.4.1 Optimization of Loading Step

The maximum loading of [Nb(C₂O₄)₃]³⁻ anion complex from the aqueous phase of Nb-oxalate solution to the organic phase of Aliquat 336S required the investigation of several effective loading parameters as follows:

Effect of Aliquat 336S Concentration

This effect was already studied using the Aliquat 336S concentrations ranged between 2 and 5% in xylene from the prepared oxalate leach liquor of pH 3 at O/A volume ratio of 1/1 and contact time of 10 min at ambient temperature (35 ± 5). After separation, Nb and Fe were analyzed in the raffinate to calculate their extraction efficiencies. The obtained data conducted in triplicate with low standard deviation of 1 ± 0.4% (Fig. 10) revealed that Nb and the interfering Fe loading efficiencies were progressively improved from 69.2 and 48.4% to 92.9 and 91.8%, respectively, as the Aliquat 336S concentration increased from 2–5%, where further increase to 6% showed a limit increase in the Nb loading efficiency to 94.1%, while that of the interfering Fe increased to 96.5%. Indeed, Aliquat 336S concentration of 5% was very suitable to apply for Nb extraction process. Thus, the optimum conc. of Aliquat 336S was 5%.

Fig. 10

Effect of Aliquat 336S conc. upon Nb and Fe extraction efficiencies [pH 3, O/A volume ratio of 1/1 and contact time of 10 min at ambient temperature (35 ± 5)]

Effect of pH Value

Data given in Fig. 11 represented the effect of increasing pH values of the prepared oxalate solution from pH 3 to pH 5.5 upon Nb and Fe loading efficiencies using 5% (v/v) Aliquat 336S at O/A volume ratio of 1/1 and contact time of 10 min. It was noticed that Nb loading efficiency increased up to 95.3% at the pH 3.5 and decreased gradually up to 61.3% at pH 5.5. This may be explained that the present soluble anion Nb complex [Nb(C₂O₄)₃]³⁻ would be hydrolyzed to [Nb(C₂O₄)₂(OH)₂]³⁻ precipitated, at higher pH values more than pH of 4.5 [54]. In an opposite manner, the loading efficiency of Fe increased gradually with increasing the solution pH values. So, the pH value 3.5 was considered the optimum value due to the highest extraction efficiency of Nb.

Fig. 11

Effect of pH values upon Nb and Fe extraction efficiencies (5% Aliquat 336S, O/A volume ratio of 1/1 and contact time of 10 min at ambient temperature)

Effect of Contact Time

The effect of contact time (shaking time) upon the extraction efficiencies of Nb and Fe from the oxalate leach liquor adjusted to pH of 3.5 was investigated by contacting 5% (v/v) Aliquat 336S at different contact time ranging from 5 to 10 min at O/A ratio 1/1. The obtained results (Fig. 12) emphasized that the loading efficiency of Nb achieved its maximum value (97.9%) at the contact time of 10 min [26]. It was also noticed that further increase in the contact time to 12 min had an opposite effect. With respect to Fe loading, efficiency was found inversely proportional to the contact time.

Fig. 12

Effect of time values upon Nb and Fe extraction efficiencies (pH 3.5, 5% Aliquat 336S, and O/A ratio 1/1 at ambient temperature)

Effect of Organic/Aqueous Volume Ratio and Constriction of Macabe-Thiele Diagram

This effect was studied at different O/A ratios 3/1,…1/1,…. 1/3 using 5% Aliquat 336S at a contact time of 10 min and pH of 3.5. From the obtained data (Table 3 and Fig. 13), it was observed that the Nb loading efficiency at O/A ratios 2/1 (98.7%) and 1/3 (99.8%) was higher than O/A of 1/1 (97.9%). But these ratios were not applied because they yielded lower Nb concentrations in the organic phase. On the other hand, the Nb loading efficiencies at O/A ratios 1/2 and1/3 are 80.1 and 69.7%, respectively, which were lower than that of 1/1 (97.9%). The same manner was already found for the loading efficiency of Fe; it was 98.9% and 97.5% at O/A ratios of 2/1 and 3/1 and lower to be 65.8% and 48.3% at the ratios 1/2 and 1/3 (Table 4 and Fig. 14). Indeed, the suitable O/A ratio applied for the best Nb and Fe loading efficiencies was 1/1.

Table 3 Effect of O/A ratio upon the extraction efficiency of Nb

Fig. 13

Macabe-Thiele diagram for Nb extraction

Table 4 Effect of O/A ratio upon the extraction efficiency of Fe

Fig. 14

Macabe-Thiele diagram for Fe extraction

From the above study, the co-extraction of both of Nb and Fe was already attained with achieved loading efficiencies of 97.9 and 93.8%, respectively, from the prepared Nb-oxalate solution adjusted to pH 3.5 using 5% Aliquat 336S at O/A ratio of 1/1 and contact time of 10 min. The loaded organic solvent was then directed to the stripping process to bring out the loaded Nb and Fe to the aqueous solution until regeneration of the utilized organic solvent to be reused.

3.4.2 Optimization of Stripping Process

The stripping process was conducted to attain the maximum Nb stripping efficiency from one hand and for its separation from the co-extracted Fe from the other hand. For this purpose, the following stripping parameters would be investigated.

Effect of Stripping Reagent Types

After separation and determination of both Nb and Fe in the strip solution, the obtained data (Fig. 15) showed that 0.4 M H₂SO₄ acid gave very low Nb stripping efficiency (4.3%) with high Fe stripping efficiency (55.5%) from the loaded Aliquat 336S_. Therefore, H₂SO₄ acid would be firstly utilized and its effective stripping factors would be studied for selective stripping of Fe ([9]), while both HCl and HNO₃ acids yielded co-stripped Nb and Fe strip solution.

Fig. 15

Effect of reagents type upon Nb and Fe stripping efficiencies (A/O ratio 1/1 and contact time 5 min)

Effect of H₂SO₄ Acid Concentrations

This effect was conducted via contacting the loaded solvent with H₂SO₄ acid solution in different concentrations ranging from 0.2 to 1 M at A/O volume ratio of 1/1 and contact time of 5 min. A remarkable improvement of Fe stripping efficiency from 51.4 to 87.8% was attained with increasing H₂SO₄ concentration from 0.2 to 0.7 M with maximum Nb stripping efficiency not exceeding 8.2% as presented in Fig. 16, while the further increase in the strip solution concentration to 1 M showed improvement in both Fe stripping efficiency to 91.5% and Nb stripping efficiency to 20.3%. Indeed, 0.7 M H₂SO₄ acid was already chosen to remove almost of Fe from the loaded solvent.

Fig. 16

Effect of H₂SO₄ conc. upon Nb and Fe stripping efficiencies (A/O ratio 1/1 and contact time 5 min)

Effect of Contact Time

Figure 17 represents the influence of varying contact time periods from 3 to 9 min when 0.7 M H₂SO₄ solution was contacted with the loaded Aliquat 336S at A/O ratio of 1/1. It was actually noticed that not less than 90.5% of the co-extracted Fe was removed with the smallest Nb efficiency (8.2%) which was attained at a contact time of 5 min.

Fig. 17

Effect of contact time upon Nb and Fe stripping efficiencies (0.7 M H₂SO₄ and A/O ratio 1/1)

It was concluded from the above study that 0.7 M H₂SO₄ acid concentration should be firstly applied for removing 90.3% of Fe with a minimum amount of Nb (not exceeding 8.2%) at a contact time of 5 min and A/O ratio of 1/1. Another stripping process would be subsequently applied using HNO₃ solution for Nb stripping. Different Nb effective stripping factors should be studied as follows.

Effect of HNO₃ Acid Concentration

The effect of HNO₃ acid concentration upon Nb stripping efficiency was studied in the range from 0.5 to 1.2 M with shaking time for 5 min at A/O ratio 1/1. Data (Fig. 18) reflected the great improvement of Nb stripping efficiency from 87.2 to 95.4% when HNO₃ acid concentration increased from 0.5 to 0.8 M, while the further increase in strip solution concentration more than 0.8 M had very limited improvement in stripping efficiency which increased to 96.2%.

Fig. 18

Effect of HNO₃ conc. upon Nb stripping efficiency (5 min and A/O ratio 1/1)

Effect of Stripping Time

The effect of stripping time upon Nb stripping efficiency was studied in the range from 5 to 10 min using 0.8 M HNO₃ solution at A/O ratio of 1/1. The obtained results (Fig. 19) revealed that the stripping time 6 min is sufficient to regenerate not less than 99.1% of the loaded Nb.

Fig. 19

Effect of stripping time upon Nb stripping efficiency (0.8 M HNO₃ and A/O ratio 1/1)

Effect of Aqueous/Organic (A/O) Ratio and Constriction of Macabe-Thiele Diagram

The equilibrium state of Nb stripping process from the loaded Aliquat 336S was investigated using 0.8 M HNO₃ solution at stripping time of 6 min via applying different A/O ratios of 2/1, 1/1, 21/2, and 1/3. Table 5 and Fig. 20 clearly reflected that almost 100% of the loaded Nb was stripped at A/O ratio of 2/1 compared with 99.1%, 63.9%, and 52.6% at A/O ratios of 1/1, 1/2, and 1/3, respectively. Indeed, the ideal applied A/O volume ratio was 1/1 which is also beneficial for Nb stripping with efficiency of 99.1%.

Table 5 Effect of A/O ratio upon the stripping efficiency of Nb

Fig. 20

Macabe-Thiele diagram for Nb stripping

3.4.3 Preparation of Nb Product

From the previous data, it was found that the maximum value 99.1% of Nb stripping efficiency was achieved using 0.8 M HNO₃ solution at stripping time of 6 min and applied A/O ratio of 1/1. The strip nitrate solution rich in Nb was treated with 30% NH₄OH solution where Nb was completely precipitated at pH 7.5 [13]. After filtration and washing, the obtained Nb hydroxide cake was ignited at 650 °C for 1 h. A small portion of the product was confirmed by XRD analysis (Fig. 21) and chemically analyzed for its purity. The final Nb₂O₅ product was achieved with global recovery of 86% and purity of 98.95% with low concentration of associated impurities, e.g., 0.9% SiO₂, 0.05% Fe₂O₃, and 0.1% TiO₂.

Fig. 21

XRD pattern of final pure Nb product

4 Conclusion

An alkaline fusion process using KOH followed by water leaching was applied for a selective dissolving of Nb as water-soluble potassium niobates from Gabal El-Faliq ore material concentrate, where 97.9% of the total Nb content was already dissolved at 1/2 concentrate sample/KOH weight ratio and fusion temperature of 550 °C for a fusion time of 2.5 h. The prepared alkaline solution of pH higher than 12 was firstly neutralized at pH of 10 to remove almost of Si⁴⁺ as Si(OH)₄ gel, while complete precipitation of Nb and other associated impurities, e.g., Fe, Ti, and Y, were attained by further neutralization at pH 8.5.

The precipitated Nb-hydrous cake assaying 55.5% of Nb was dissolved in oxalic acid solution and subjected to Nb purification process using Aliquat 336S in xylene. The co-extracted Nb and Fe with loading efficiencies 97.9% and 93.8%, respectively, were attained using 5% Aliquat 336S concentration at pH 3.5 and shaking time of 10 min at O/A ratio of 1/1. Two subsequent stripping processes were performed using both 0.7 M H₂SO₄ for removing 90% of Fe away from the Aliquat 336S and 0.8 M HNO₃ for stripping 97.9% of Nb. Finally, this process was successfully applied to prepare Nb₂O₅ product with purity of 98.95% and global recovery of 86% as summarized in the proposal flowsheet (Fig. 22).

Fig. 22

Proposed technical flowsheet for the processing Gabal El-Faliq concentrate sample