Sulfuric acid leaching of Kab Amiri niobium–tantalum bearing minerals, Central Eastern Desert, Egypt

Sulfuric acid leaching of Kab Amiri niobium–tantalum

bearing minerals, Central Eastern Desert, Egypt

Omneya M. El-Hussaini*, Mohamed A. Mahdy

Nuclear Materials Authority, P.O. Box 530, El Maadi, Cairo, Egypt

Received 2 June 2001; received in revised form 15 April 2002; accepted 17 April 2002

Abstract

The ore under study was obtained from the Kab Amiri area located close to the northern boundary of the Central Eastern

Desert. The ore is constituted of different kinds of refractory minerals. These are mainly represented by the niobium–tantalum

rare earth-bearing minerals namely euxinite, samarsakite and fergusonite, beside the uranium refractory minerals davidite and

zircon. Increasing demand for niobium, tantalum, titanium, uranium, thorium and rare earth elements has stressed the

importance of developing methods for their recovery and processing into marketable form. In the present work, Kab Amiri ore

was subjected to direct agitation leaching with sulfuric acid. Variables such as acid concentration, temperature, time, ore to acid

ratio and oxidant effect were studied. When finely ground ore (� 74 Am) was reacted upon for 2 h with a mixture of sulfuric

acid (10.8 M) and nitric acid (5.3 M) in the ore to acid weight ratio of 1:3 at 200 jC, almost complete recovery of both niobium

and tantalum was achieved while the leaching extents of thorium and total rare earths were 86% and 70%, respectively. On the

other hand, the recovery of both uranium and titanium did not exceed 60% due to their presence in the refractory mineral

davidite. D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Leaching; Sulfuric acid; Refractory minerals; Niobium–tantalum ores

1. Introduction

Niobium and tantalum elements occur in a great

variety of ores. Their commercial value in the metal-

lurgical, nuclear and space industries, etc. make their

recovery desirable in relatively concentrated form.

The total production of tantalum from raw materials

as reported by the Tantalum–Niobium International

Study Center (T.I.C) (Linden, 1999) exceeded 5

million tonnes Ta2O5 during 1999, while the total

supply of niobium from raw materials is now running

at approximately 150–160 million tonnes Nb2O5. The

increased demand has firmed prices for tantalum raw

materials, which in turn has encouraged new produc-

tion from small operations in Africa, Brazil and

Thailand. On the other hand, prices have been

reported to be as low as $53 per kilogram of niobium

in ingot form to as high as $222 per kilogram of

niobium in special shapes.

The chemical study of niobium and tantalum

elements shows that there are large numbers of

procedures for their ore decomposition. All niobium

minerals can be readily decomposed with hydrofluoric

acid with almost complete dissolution. This method is

widely used for the dissolution of columbite–tantalite

minerals (Gupta and Suri, 1994a). Alkali fusion in

combination with acid leaching is one of the first

0304-386X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0304 -386X(02 )00045 -2

* Corresponding author. Fax: +20-2-758-5832.

www.elsevier.com/locate/hydromet

Hydrometallurgy 64 (2002) 219–229

methods that was adopted on an industrial scale to

achieve simultaneous breakdown of columbite–tanta-

lite and upgrading niobium and tantalum values (El-

Hussaini, 1996; Eckert, 1995). Some minerals have

been considered for the recovery of niobium, tantalum

and other associated metal values include euxinite,

samarsakite and fergusonite. These minerals can be

processed by the following: chlorination (Gupta and

Suri, 1994b), alkali fusion followed by acid leaching

(Foos, 1960), fusion with ammonium fluoride and

Fig. 1. Separation of niobium– tantalum minerals by physical methods.

O.M. El-Hussaini, M.A. Mahdy / Hydrometallurgy 64 (2002) 219–229220

bifluoride (Gupta and Suri, 1994c), direct acid dis-

solution with H2SO4 (Bock, 1979a; Vacariu et al.,

1999), or combination of H2SO4 and HF (Krismer and

Hoppe, 1984).

The niobium–tantalum rare earth-bearing minerals

which occur in the form of multiple oxides are

commonly associated with uranium minerals. These

minerals are opened by different methods including

acid leaching, atmospheric alkaline leaching, pressure

alkaline leaching, roasting and chloride roasting fol-

lowed by acid and alkaline leaching (El Shazly and El

Hazek, 1970).

Owing to the refractory nature of some uranium

minerals such as davidite, more aggressive conditions

are required for leaching than those used in conven-

tional uranium circuits (Ritcey et al., 1993).

The present work is concerned with the leaching

process performed upon Kab Amiri ore. This area is

located near the Qena–Safaga Road, i.e., close to the

northern boundary of the Central Eastern Desert. Two

radioactive anomalies were reported in the Kab Amiri

monsogranite: one in the pegmatites and the other in

the silica veinlets. The pegmatites host rocks are

composed of quartz cores surrounded by alkali feld-

spar outer zones. Both cores and outer zones are

separated by an incomplete zone of muscovite. The

muscovite zone shows high radioactivity, within

which visible black metallic minerals are detected.

The X-ray diffraction (XRD) results of the black

metallic minerals showed that they are composed of

zircon (Zr,U,Th,Hf)SiO4, euxinite (Y,Er,La,Ce,U,Th)

(Nb,Ta,Ti)(O,OH) and davidite (Fe,La,U,Ca)(Ti,Fe,V,

Cr)(OH,O) minerals. On the other hand, the petro-

graphic studies of silica veinlets indicate that their

radioactivity is mainly due to the presence of zircon

mineral (Ammar, 1993).

2. Methods and material studied

The niobium–tantalum raw material under study

was selected from Kab Amiri. The ore was ground to

� 500 Am and physically treated with heavy liquids,

as shown in Fig. 1. Grains of buff and black colors

were separated from the heavy fractions. They were

differentiated as the economic and the associated

minerals, respectively. The mineralogical composition

detected by X-ray diffraction (XRD, using Phillips

PW 223/30) and the chemical analysis of the selected

ore sample (� 74 Am) are given in Table 1.

In order to study the distribution of minerals in

different grain sizes, the ore was crushed to � 590 Amthen the fractions were separated to various sizes

from � 500 to � 74 mesh. The distributions of the

economic elements in each size were determined as

shown in Table 2. From the obtained data, it is clear

that the elements are variably distributed among the

separated fractions.

The agitation leaching process was performed

under different conditions summarized in Table 3, to

achieve the best leaching efficiency of different ele-

ments. Direct leaching has been applied to the ore of

� 74 Am size using sulfuric acid (97–99%) obtained

from ADWIC. Nitric acid (69–72%) obtained from

BDH was used as an oxidant. The leaching tests were

Table 1

Mineralogical and chemical analysis of Kab Amiri ore material

Gangue

mineralsaEconomic

mineralsbAssociated

mineralsbChemical

analysis

%

Quartz Euxinite Hematite Nb2O5 16.16

Alkali Feldspar Samarsakite Ta2O5 12.08

Muscovite Fergusonite TiO2 4.48

Phlogopite Daviditea FeOc 23.40

Fluorite Zircona REEd 0.09

ThO2 0.04

U3O8 0.23

ZrO2 0.19

LOIe 0.31

a From Ammar (1993).b The grains were heated at 1000 jC before XRD analysis.c Total iron as FeO.d Total rare earth elements oxides.e LOI: loss on Ignition at 900 jC.

Table 2

Distribution percentage of elements of Kab Amiri ore material in

different grain sizes

Grain size Weight Distribution (%)

(Am) (%)Nb2O5 Ta2O5 TiO2 REE2O3 ThO2 U3O8

� 500 8.2 10.6 7.9 17.5 14.2 10.0 17.5

� 420 6.6 8.4 6.4 12.6 10.7 7.8 12.5

� 297 6.1 8.9 6.2 13.8 9.1 6.9 9.0

� 250 10.6 8.7 13.2 13.5 14.6 10.6 13.3

� 125 17.2 15.2 19.0 11.0 15.8 17.2 17.5

� 74 51.3 48.2 47.4 31.7 35.6 47.5 30.1

O.M. El-Hussaini, M.A. Mahdy / Hydrometallurgy 64 (2002) 219–229 221

carried out in 100 ml stoppered Pyrex containers. A

thermometer was placed through a fitted opening, and

the reaction vessel was heated using a hot plate with

magnetic stirrer.

The determination of niobium, tantalum, total rare

earth elements, titanium, thorium and uranium in

solution were carried out using a UV–VIS spectro-

photometer (Shimadzu UV-160A). The values of total

iron and zirconium were obtained by Atomic Absorp-

tion Spectrophotometer (UNICAM 969).

3. Results and discussion

Relating to the process of the present work, sulfuric

acid (considered to be an inexpensive reagent) was

used for leaching the niobium–tantalum rare earth

minerals. The effects of different factors on the

leaching extents of the minerals’ constituents were

studied in detail as follows.

3.1. Effect of temperature

Increasing the temperature was found to be effec-

tive in increasing the leaching extents of the elements

composing the economic minerals. Above 300 jC,hydrolysis may occur thus the leaching extents de-

creased. The results are shown in Fig. 2a and b. This

set of experiments was conducted using concentrated

sulfuric acid (18 M), on the ground ore to � 74 Am at

an ore to acid ratio of 1.0:2.5 for 2 h.

Under these conditions, niobium reached its max-

imum leaching extent of 73.5% at 200 jC, while most

of tantalum may precipitate as tantalum hydroxide

(Koerner et al., 1963) leaving 30.3% leached species

in the liquor. On the other hand, overheating causes

reduction and dehydration of some rare earth elements

forming insoluble sulfate salts (Vickery, 1961).

The ability of both titanium and uranium to be

leached with concentrated sulfuric acid at various

temperatures is less than 45%, Fig. 2b. This may be

attributed to their presence in the refractory minerals

davidite and/or zircon.

3.2. Effect of sulfuric acid concentration

Two sets of experiments were performed at 65 and

200 jC. The agitation leaching process was carried

out by mixing sulfuric acid with the ground ore to

� 74 Am in ore to acid ratio of 1.0:2.5 for 2 h.

3.2.1. Low temperature

Fig. 3 reveals that at 65 jC the leaching extents of

the studied elements did not exceed 50% except for

thorium, which reached 63%. At ordinary temperature,

thoriummay form the readily soluble sulfate salt which

in turn forms the octahydrate [Th(SO4)2�8H2O] in

excess of sulfuric acid (Mathur and Tandon, 1986).

It was reported by Bielecki et al. (1991) that at

temperatures ranging from 40 to 100 jC, most of

thorium quantities were dissolved by leaching the ore’s

residue with sulfuric acid ranging from 0.5 to 5.0 M.

3.2.2. High temperature

As shown in Fig. 4a and b, the economic elements

were more effectively leached with the lower concen-

trations of sulfuric acid than the higher, except for

titanium, which reached its maximum leaching extents

Table 3

Studied factors affecting the agitation leaching of Kab Amiri ore material using sulfuric acid

Factors Factors’ values Fixed conditions

Temperature

(jC)Sulfuric acid

concentration (M)

Time

(h)

Ore to

acid ratio

Oxidant,

HNO3 (M)

Temperature (jC) 33, 56, 65, 94, 135, 174, 200, 315 – 18.0 2 1:2.5 0.0

Sulfuric acid (M) (i) 18.0, 14.4, 10.8 65 – 2 1:2.5 0.0

concentration (ii) 18.0, 15.8, 14.4, 12.2, 10.8, 9.4 200 – 2 1:2.5 0.0

Time (h) 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 7.5 200 10.8 – 1:2.5 0.0

Ore to acid ratio (wt/vol) 1:1.5, 1:2.0, 1:2.5, 1:3.0, 1:5.0 200 10.8 2 – 0.0

Oxidant concentration,

HNO3 (M)

0.0, 2.2, 5.3, 8.4, 9.4, 12.7 200 10.8 2 1:3.0 –


at 15.8 M sulfuric acid. Above 12.2 M, sulfuric acid

leaching of thorium and rare earth elements was found

to be independent of the acid concentration.

At 200 jC, it is preferable to apply the leaching

process using 10.8 M sulfuric acid. Higher concen-

tration should be avoided due to the fact that concen-

trated sulfuric acid is consumed in attacking the

gangue constituents of the ore.

Lerner (1961) found that sulfuric acid of concen-

tration ranging from 9.2 to 13.2 M is sufficient to

decompose the niobium mineralization in niobium-

containing ores. This leads to numerous processing

advantages, the foremost of which is the significant

reduction in the operating costs.

3.3. Effect of time

The reaction between Kab Amiri ore ground to

� 74 Am and sulfuric acid of concentration 10.8 M in

ore to acid ratio of 1.0:2.5 was studied at 200 jC,while varying the leaching time from 0.5 to 7.5 h. The

results in Fig. 5a and b show that most of the elements

composing the ore reached their highest leaching

extents within a few hours. The leaching extents of

Fig. 2. (a) Effect of temperature on leaching extent of Nb2O5, Ta2O5, REO and ThO2 upon leaching the ground ore to � 74 Am with 18 M

H2SO4 in ore to acid ratio of 1:2.5 for 2 h. (b) Effect of temperature on leaching extent of TiO2 and U3O8 upon leaching the ground ore to

� 74 Am with 18 M H2SO4 in ore to acid ratio of 1:2.5 for 2 h.


both titanium and uranium at any time did not exceed

50% for the former and 35% for the latter due to the

refractory nature of davidite.

3.4. Effect of ore to acid ratio

A set of experiments was carried out to examine

the effect of ore to acid ratio. The experimental

conditions were kept constant as mentioned before

while varying the ore to acid ratio from 1.0:1.5 to

1.0:5.0. The results are illustrated in Fig. 6a and b. It is

observed that the leaching extents are high with the

exception of uranium and titanium, which are below

50%, due to their incorporation in the primary ura-

nium mineral davidite. However, it is possible to say

that the best leaching extents of most of the elements

were obtained at ore to acid ratio of about 1:3.

3.5. Effect of oxidant concentration

These experiments were performed to find the

minimum concentration of nitric acid needed for

maximum leaching. The other leaching conditions

were kept constant, in which the ground ore � 74

Am was mixed with sulfuric acid of 10.8 M in the ore

to acid ratio of 1:3 and the mixture was heated for 2h

at 200 jC. The results are shown in Fig. 7.

It was found that the increase in nitric acid con-

centration above 5.0 M decreases the leaching extents

of both niobium and tantalum. This may be due to the

formation of protective coating of insoluble niobium

and tantalum oxides at higher nitric acid concentration

(Bock, 1979b). The maximum leaching extent of rare

earths and thorium was obtained by mixing concen-

trated nitric acid of 12.7 M with 10.8 M sulfuric acid,

the recovery was 70% and 84%, respectively (Fig. 7).

There is the possibility that most of the insoluble rare

earth elements and thorium are associated with the

refractory minerals, i.e., davidite.

Slight improvement in uranium leaching was

obtained by mixing 9.4 M nitric acid with 10.8 M

sulfuric acid (Fig. 7). This may be attributed to the

nature of davidite and zircon, which require severe

conditions for complete decomposition such as leach-

Fig. 3. Effect of sulfuric acid concentration on leaching extent of Nb2O5, Ta2O5, REO, ThO2, TiO2 and U3O8 upon leaching the ground ore to

� 74 Am with H2SO4 in ore to acid ratio of 1:2.5 at 65 jC for 2 h.


ing above atmospheric pressure, or fusion at high

temperature. However, the sulfuric acid leaching proc-

ess is commonly applied on the Australian davidite

concentrates of Radium Hill, at atmospheric pressure

(George, 1958a).

On the other hand, leaching euxinite mineral from

Norway (George, 1958b) under milder conditions

than that of Kab Amiri resulted in 12% and 48%

leaching extent of uranium at sulfuric acid concen-

trations of 1.88 and 4.68 M, respectively, even in the

presence of NaClO3 as oxidant.

Examining the results in Table 4, it is possible to

say that the decomposition of the refractory niobium–

tantalum minerals (euxinite, samarsakite and fergu-

sonite) were mainly affected by sulfuric acid concen-

tration as well as leaching temperature. Gupta and

Suri (1994c) reported that concentrated sulfuric acid

was used for direct leaching of Indian samarsakite

Fig. 4. (a) Effect of sulfuric acid concentration on leaching extent of Nb2O5, Ta2O5, REO and ThO2 upon leaching the ground ore to � 74 Amwith H2SO4 in ore to acid ratio of 1:2.5 at 200 jC for 2 h. (b) Effect of sulfuric acid concentration on leaching extent of U3O8 and TiO2 upon

leaching the ground ore to � 74 Am with H2SO4 in ore to acid ratio of 1:2.5 at 200 jC for 2 h.


under nearly similar conditions to those performed

upon Kab Amiri ore. It was found that both niobium

and tantalum remained as insoluble residue, while

thorium, rare earth elements and uranium were trans-

ferred to the leach liquor. It is worth mentioning that

lower concentrations of sulfuric acid are recommen-

ded to leach niobium–tantalum minerals (Lerner,

1961).

Increasing the concentration of sulfuric acid,

under atmospheric pressure at 200 jC, or mixing

with nitric acid has no great influence in improving

the leaching efficiency of both U3O8 and TiO2 (Table

4) due to the fact that both elements are incorporated

in the primary uranium mineral davidite (Ammar,

1993). It was reported that (Gshneidner, 1999) the

recovery of rare earth elements is applied on indus-

trial scale, at the expense of uranium and thorium.

Utilizing sulfuric acid on leaching the quartz albite

vein dikes and the pegmatite dikes of Bokan Moun-

tain, Alaska USA, causes the dissolution of more

than 74% of yttrium, cerium and lanthanum of the

ore.

In view of the foregoing results, the optimum

conditions for leaching Kab Amiri ore is to mix

the ground ore to � 74 Am with a mixture of 10.8

M sulfuric acid and 5.3 M nitric acid in a ratio of

1:3 for 2 h leaching time at 200 jC. The obtained

leach liquor contains: 99.50% of Nb2O5, 93.38% of

Fig. 5. (a) Effect of agitation time on leaching extent of Nb2O5, Ta2O5, REO and ThO2 upon leaching the ground ore to � 74 Am with 10.8 M

H2SO4 in ore to acid ratio of 1:2.5 at 200 jC. (b) Effect of agitation time on leaching extent of U3O8 and TiO2 upon leaching the ground ore to

� 74 Am with 10.8 M H2SO4 in ore to acid ratio of 1:2.5 at 200 jC.


Ta2O5, 70.00% of REE oxides, 76.32 % of ThO2,

48.44% of TiO2 and 44.98 % of U3O8 contents in

the ore.

4. Conclusion

This work is devoted to the study of the behav-

ior of not only niobium and tantalum using sulfuric

acid leaching, but also the associated elements such

as rare earth elements, thorium, uranium and tita-

nium in Kab Amiri ore. All these elements have

great commercial value in metallurgy and nuclear

industries. Thus, the recovery of each element indi-

vidually from their leach liquor is a most interesting

task.

From the present study, it became evident that Kab

Amiri ore of multi-oxides minerals (euxinite, samar-

Fig. 6. (a) Effect of ore to acid ratio on leaching extent of Nb2O5, Ta2O5, REO and ThO2 when the ground ore to � 74 Am was mixed with 10.8

M H2SO4 at 200 jC for 2 h. (b) Effect of ore to acid ratio on leaching extent of U3O8 and TiO2 upon mixing the ground ore to � 74 Am was

mixed with 10.8 M H2SO4 at 200 jC for 2 h.


sakite, fergusonite, davidite and zircon) decomposes

in sulfuric acid media. The results obtained from the

laboratory experiments can be summarized as follows:

(1) Sulfuric acid leaching of Kab Amiri ore

material was found effective for the dissolu-

tion of the economic elements in the leach

liquor—with the added advantages of mini-

mum acid consumption—and convenience for

further extraction procedures.

(2) The optimum leaching conditions were ob-

tained by mixing the ground ore to � 74 Am

with a mixture of sulfuric acid of concen-

tration 10.8 M and nitric acid of concentration

5.3 M in a ratio of 1:3, and heating for 2 h at

200 jC.(3) Under the mentioned conditions, almost

complete recovery of both niobium and

tantalum has been achieved while the leaching

extents of thorium and total rare earth

elements were 76% and 70%, respectively.

(4) The recovery of both uranium and titanium

did not exceed 60% during the leaching

process of niobium–tantalum minerals, even

Fig. 7. Effect of HNO3 concentration on leaching extent of Nb2O5, Ta2O5, REO, ThO2, TiO2 and U3O8 upon leaching the ground ore to

� 74 Am with 10.8 M H2SO4 in ore to acid ratio of 1:3 at 200 jC for 2 h.

Table 4

Effect of temperature and acids concentrations on leaching extent of the economic elements composing Kab Amiri ore

Temperature Acid concentration (M) Ratio Elements concentration (%)

(jC)H2SO4 HNO3

(wt/vol)Nb2O5 Ta2O5 REEa ThO2 TiO2 U3O8

63 10.8 0.0 1:2.5 28.34 21.19 21.11 31.58 3.57 9.61

14.4 0.0 1:2.5 30.07 18.71 23.33 63.16 2.32 6.11

18.0 0.0 1:2.5 29.21 20.20 48.89 63.16 4.84 6.55

200 10.8 0.0 1:2.5 96.47 86.34 66.67 68.42 45.47 25.76

14.4 0.0 1:2.5 90.28 45.45 61.11 63.16 55.92 13.93

18.0 0.0 1:2.5 73.45 30.30 61.11 63.16 44.73 7.86

200 10.8 0.0 1:3.0 97.77 90.00 72.11 72.11 44.64 17.90

10.8 5.3 1:3.0 99.50 93.38 76.32 76.32 48.44 44.98

10.8 6.4 1:3.0 85.83 84.52 84.21 84.21 44.64 32.31

a REE calculated as rare earth elements oxides.


in the presence of an oxidant. This is

attributed to the fact that they are mainly

incorporated in the refractory mineral davi-

dite.

(5) According to the grain size separation, it is

possible to conclude that the minerals of Kab

Amiri ore are variably distributed in all the

separated fractions.

Thus, the present study contributes to the evalua-

tion of Kab Amiri niobium–tantalum ore. It also

provides a case study for processing similar local

niobium–tantalum ores.

Acknowledgements

The authors wish to thank NMA for the permission

to publish this work. Thanks are extended to Dr. Saleh

E.S. Ammar for the provision of Kab Amiri’s ore

material, also to Dr. Tarek A. Amer for his help in the

physical separation work and Dr. Fathi Ammar for the

XRD analysis. Special thanks are due to Tantalum–

Niobium International study Center, Belgium, for

supplying some useful papers related to this work.

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Sulfuric acid leaching of Kab Amiri niobium–tantalum bearing minerals, Central Eastern Desert, Egypt

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