Journal of Minerals & Materials Characterization & Engineering, Vol. 7, No.1, pp 83-95, 2007
jmmce.org Printed in the USA. All rights reserved
83
Investigation of Dissolution Kinetics of A Nigerian Tantalite Ore
in Nitric Acid
A. A. Baba*
1
, F. A. Adekola
1
, O. I. Dele-Ige
1
and R. B. Bale
2
1
Department of Chemistry, University of Ilorin,
P. M. B. 1515, Ilorin Nigeria.
*Corresponding author: (e-mail):[e-mail]
Tel: [phone])
2
Department of Geology, University of Ilorin,
P.M.B.1515 Ilorin-Nigeria
ABSTRACT
A study of the kinetics of the dissolution of a Nigerian tantalite ore in Nitric acid
has been examined. A detailed investigation on the quantitative leaching of the mineral was
also carried out. The effects of some parameters such as acid concentration, contact time,
temperature, particle size and the stirring rate on dissolution action have been
investigated. Experimental results indicate that the dissolution rate is diffusion controlled
via hydrogen ion [H
+
] action, with reaction order of 0.64. About 81.3% of tantalite ore
was dissolved within 120min., using 8.06M HNO
3
solution with <0.040mm particle size.
Activation energy, Ea, of 22.37kJ/mole was obtained for the process. However, elemental
analysis by inductively coupled-plasma mass spectrophotometry showed that the major
elements in the ore are Ta(39.9%), Fe(24.9%), Ti(4.03%), S(12.4%) and Mn(2.39%). The
x-ray diffractometry also confirmed the ore to exist in a single phase with tantalite
characteristics peaks at 3.31A
0
, 2.55A
0
and 1.74A
0
. The ore is not radioactive by its virtue
of its very low thorium and uranium contents.
1.0 INTRODUCTION
The principal source of tantalite or tantalum is an isomorphous series of minerals
that contain columbium (niobium), iron, manganese and tantalum oxides. Tantalum and
columbium have strong geochemical affinity and are found together in most rocks and
minerals in which they occur. Tantalite-columbite, which is the major source for tantalum,
occurs mainly as accessory minerals disseminated in granitic rocks or in pegmatite
associated with granites. The proper name for the mineral is tantalite when tantalum
predominates over columbium. When the reverse is true, the proper name is columbite [1].
Economic mineral concentrations occur where as in Nigeria or South-east Asia,
weathering has led to residual or placer deposits or where, as in Bernic Lake deposit in

84 A.A. Baba, F.A. Adekola, O.I. Dele-Ige and R.B. Bale Vol.7, No.1
Canada, the pegmatites contain a high concentration of these minerals. The minerals
consist essentially of complex oxides of calcium, columbium, sodium and tantalum in
combination with hydroxyl ions and fluoride(s). Microlite may contain as much as 70%
tantalum oxide and pyrochlore generally contains less than 10% tantalum oxide. Macrolite
occurs mainly in the albitized zones of granite pegmatites, often associated with tantalite or
columbite. Struverite, which is a titanium-bearing oxide, is a low-grade source of tantalum
that is recovered from tin-mining wastes in South-east Asia. Struverite typically contains
about 12% each of tantalum and columbium oxides [2].
The United States which has no tantalum mining industry, must import all its
tantalum source materials for processing. Tantalum mineral production comes mostly from
tantalite and columbite mining operations in Australia, Brazil and Canada and from smaller
mining operations in certain African countries including Nigeria [3].
Numerous occurrences of the speciality metals-columbite, tantalite, beryllium and
lithium have been reported associated mainly with the anorogenic. Younger Granites of the
Plateau and the pegmatites of the Older Granite suite especially in Central Africa. The
economic viability of the occurrences of the mineral in Nigeria is in general not known
because detailed exploration work has not been carried out. However, the occurrences at
Udegi (Opaga Oto) and Angwan Doka, in (middle belt) of Nigeria were investigated under
the Nigerian Mining Appraisal and Monetization Programme (NIMAMOP) [4].
Australia, which is the largest producer, accounts for about 25% of the World’s
annual tantalum requirements. Tantalum is also obtained from low-and high-grade
tantalum-bearing tin slags, which is a by-product from tin smelting, principality from
Australia, Brazil and Asia. Low-grade tin slags, however, might first be treated by a
pyrometallurgical technique to upgrade them to a synthetic concentrate before delivery [5].
Consequently, tantalite is the most important mineral form of tantalum, a speciality
widely used in the production of electronic components, mainly in tantalum capacitors,
super conductors, super alloys, aerospatial and nuclear among others [6, 7].
This work is prompted by the fact that this is very limited work in the aqueous
chemistry and physico-chemical properties of tantalite ore found in Nigeria. The only one
is our work on the hydrochloric acid leaching, Baba et. al [8]. The results obtained indicate
that the dissolution reaction is topochemical and is greatly influenced by hydrogen ion [H
+
]
concentration.
However, most of the reported works are in the areas of tantalite beneficiation,
geochemistry and phase characterizations [6,9-11]. Nevertheless, many researchers have
reported several findings in terms of chemical nature, aiming at extracting tantalum in
different media. These among others include: analysis of tantalum loaded tributyl
phosphate for the content of major, minor and trace elements using microwave
decomposition and ICP-OES [12].

Vol.7, No.1 Investigation of Dissolution Kinetics of A Nigerian Tantalite Ore 85
Various chemical treatments and techniques have been developed for niobium and
tantalum extraction from ores, which include reduction, chlorination, alkaline fusion and
acid dissolution [13 – 15].
The extraction of tantalum from tantalum source materials involves dissolution with
hydrofluoric acid followed by liquid-liquid extraction with methyl isobutylketone (MIBK).
This procedure efficiently recovers tantalum in a form that can be further processed into
tantalum oxide and potassium fluotantalate. Potassium fluotantalate is reduced with
metallic sodium to produce tantalum metal powder. The tantalum metal powder produced
by the sodium reaction process is treated to covert the metal to a form suitable for use as
capacitor-grade powder and as feed stock for tantalum wire and sheet. A solid state reaction
between tantalum oxide and carbon under vacuum conditions produces tantalum carbide [1,
16].
Consequently, this study gives a detailed data on the use of the nitric acid for
chemical leaching of a Nigerian tantalite ore. The dissolution kinetics of the ore is also
evaluated. No reported works vis-à-vis, leaching a Nigerian tantalite ore with nitric acid is
available. This work is inscribed within the programme being undertaken on the dissolution
kinetics of tantalum in different aqueous media [8].
2.0 EXPERIMENTAL
2.1 Material/Analysis
The tantalite mineral used for this investigation was sourced from Oke-Onigbin
field in Kwara State of Nigeria. The mineralogical purity of the ore was investigated using
Philips PW 1800 X-ray diffractometer with CuK α (1.54A
0
) radiation, generated at 40kV
and 55mA. The inductively coupled plasma-mass spectrophotometry (ICP-MS) was used
for the elemental analysis of the tantalite mineral.
Analar grade HNO
3
acid was used in this research work and doubly distilled water
was used in the preparation of all solutions.
2.2 Equipment and Methods
The solution was investigated in a 500ml glass reactor equipped with a mechanical
all-glass stirrer. The reactor was filled with 200ml of nitric acid solution which was heated
to the desired temperature [24].
The leaching of tantalite mineral with HNO
3
at different concentrations was done as
described previously [8]. The concentration of HNO
3
which gave the maximum dissolution
(i.e.8.06M) was subsequently used for the optimization of other leaching parameters
including temperature and particle size. Energy of activation and constants were evaluated
from the Arrhenius plots.

86 A.A. Baba, F.A. Adekola, O.I. Dele-Ige and R.B. Bale Vol.7, No.1
3.0 RESULTS AND DISCUSSIONS
3.1 Mineralogical Studies
3.1.1 Elemental analysis by ICP-MS
The ICP-MS results showed that Ta(39.9%) Fe(24.9%), Ti(4.03%), S(12.4%) and
Mn(2.39%) formed the major elements in the ore. The minor elements include Ca, P, Mg,
Al, Na, K, Nb, Zn, Pb, As, Sb, Ce, La and V. However, elements such as Mo, Cu, Ag, Co,
U, Au, Th, Sr, Cd, Zr, Li, R, W, Y, Rb and Hf are recoded at trace levels [8].
3.1.2 XRD analysis
The ore chemical composition dominated by Ta, Fe, Ti, S and Mn indicated ore
minerals constituent to be a mixture of Tantalite [(Ta, Nb)O
2
], ilmenite (FeTiO
3
) and pyrite
(Fe,Mn)S. The Nb and Mn, by their respective affinity for Ta and Fe are probably
incorporated into the tantalite and pyrite phases respectively. The ilmenite and pyrite
informs on the reducing conditions of formation of the ore while the lanthanum value
points to its lithosphere origin. The ore is not radioactive by virtue of its very low Thorium
content [17-19]. This reason also suggests why some Ta/Nb minerals cannot be shipped out
of the country, because of their levels of radioactivity. The limits of allowed uranium and
thorium contents are 0.1% U
3
O
8
and 0.1% ThO
2
[29]. Figure 1 presents the X – ray
diffraction spectrum with tantalite characteristics diffraction peaks at 3.31A
0
, 2.56A
0
and
1.74A
0.
Fig 1. XRD showing the characteristics peaks of Tantalite

Vol.7, No.1 Investigation of Dissolution Kinetics of A Nigerian Tantalite Ore 87
3.2 Dissolution Studies
3.2.1 Effect of HNO
3
concentration
The results obtained as seen in fig.2, showed that the rate of tantalite ore dissolution
is affected directly by the hydrogen ion [H+] concentration. This agrees with previous
grades [8, 20-22]. This is true up till 8.06M HNO
3
after which there was a decline in the
amount of the ore dissolved. Therefore, the optimum concentration of HNO
3
that favours
the dissolution is put at 8.06M, which was used for further studies.
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
020 40 60 80100120140
Contact Time (min)
Fraction of tantalite ore dissolved (X)
0.5M
1.0M
2.0M
4.0M
8.06M
12M
Fig. 2: Fraction of tantalite ore dissolved (X) vs time at different concentrations of HNO
3
.
Experimental conditions: Mass of tantalite ore used = 0.5g, [HNO
3
] = 8.06M, Temp. =
55
0
C, Stirring rate = 360rpm, particle size = < 0.04mm.
To determine the reaction order with respect to [H
+
] ion concentration, the
shrinking core model 1-(1 – X)
1/3
versus time at different HNO
3
concentrations was used in
the linearization (Fig. 3).

88 A.A. Baba, F.A. Adekola, O.I. Dele-Ige and R.B. Bale Vol.7, No.1
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
020406080100 120 140
Contact Time (min)
1-(1-X)
1/3
0.5M
1.0M
2.0M
4.0M
8.06M
Fig. 3: Plot of 1-(1 – X)
1/3
vs. time at different [HNO
3
].
From Fig. 3, the slope of each lines were calculated and recorded as apparent rate
constants, k
1
, from which the plot of lnk, versus ln[HNO
3
] was obtained (fig.4) for the
evaluation of reaction order for the process.
From Fig. 4, the reaction order with respect to [H
+
] ion concentration is calculated
to be 0.64, i.e. ([H
+
]
0.64
). This is in accordance with the earlier works published [20, 22,
23].
3.2.2 Effect of particle size
Four different particle sizes were examined in this work. The results obtained are
presented in Table 1.
Experimental conditions: Mass of tantalite ore used = 0.5g, [HNO
3
] = 8.06M, Temp
= 80
0
C, stirring rate = 360rpm. Contact time = 120min.
From table 1, it is evident that the rate of tantalite ore dissolution increases with
decreasing particle size [24]

Vol.7, No.1 Investigation of Dissolution Kinetics of A Nigerian Tantalite Ore 89
-8
-7.5
-7
-6.5
-6
-5.5
-5
-4.5
-4
-1.000-0.5000.000 0.500 1.000 1.500 2.000 2.500
ln [HNO
3
]
ln K
1
Fig. 4: lnk
r
versus ln[HNO
3
]
Table 1: Results of the effect of particle size
Particle size, x (mm) Fraction of tantalite ore dissolved (%)
<0.040
0.040<x<0.050
0.05<x<0.063
0.0634<0.080
81.30
75.43
71.06
67.14
3.2.3 Effect of stirring speed
As shown in Table 2, the amount of the tantalite ore dissolved increases up to
360rpm and assumes a near constant value afterwards.
Table 2: Results of the effect of stirring rate
Stirring rate (rpm) Fraction of tantalite ore dissolved (%)
0
90
180
270
360
450
47.81
54.95
62.11
73.49
81.30
76.52
Experimental conditions: Mass of tantalite used = 0.5g, [HNO
3
] = 8,06M, Temp. = 80
0
C,
particle size = < 0.40mm, contact time = 120min.

90 A.A. Baba, F.A. Adekola, O.I. Dele-Ige and R.B. Bale Vol.7, No.1
3.2.4 Effect of temperature
The effect of the temperature on the tantalite ore dissolution is shown in Fig. 5.
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
0.900
020 40 60 80100120140
Contact Time (min)
Fraction of tantalite ore dissolved (X)
80
70
60
40
28
0
C
0
C
0
C
0
C
0
C
Fig. 5: Fraction of tantalite ore dissolved vs. time at different temperatures.
Experimental conditions: Mass of tantalite used = 0.5g [HNO
3
] = 8,06M, Particle size =
<0.040mm, Stirring rate = 360rpm, Temp = 28 – 80
0
C, Contact time = 120min.
As seen in Fig. 5, tantalite ore dissolution in HNO
3
increases with leaching time and
with increasing temperature in the range of 28-80
0
C.
3.3 Kinetic Studies
The shrinking core model considers that the leaching process is controlled either by
the diffusion of reactant through the solution boundary layer, or through a solid product
layer, or by rate of the surface chemical reaction. The simplified equations of the shrinking
core model when either diffusion or the surface chemical reactions are the slowest step can
be expressed as follows, respectively [24, 25]:
( ) tkX1X
3
2
1
a
3
2
=−−− (1)
( ) tkX11
r
3
1
=−− (2),

Vol.7, No.1 Investigation of Dissolution Kinetics of A Nigerian Tantalite Ore 91
where X is the fraction of tantalite ore reacted, t is the reaction time and k
d
, k
r
are the rate
constants, respectively, which are calculated from equations (1) and (2) respectively.
Equation (1) reveals that if the diffusion through the product layer controls the
leaching rate, there must be a liner relation between the left side of equation and time. The
slope of the line is the rate constant, k
d
. It must be directly proportional to 2
ro
1(ro is the
initial radius of the solid particle). If the surface reaction controls the rate, the relation
between the left side of equation (2) must be linear. The slope of this line is called the
apparent rate constant, kr and must be directly proportional to
o
r
1
[24].
The rate constant values, kd and kr calculated from equations (1) and (2)
respectively and their correlation co-efficient for each temperature are given in Table 3.
Table 3: The kd, kr, and correlation coefficients values for different temperatures.
Temperature (0C)
Apparent rate constants
Correlation coefficient(R
2
)
k
d
(10
-
4
min
-
1
) k
r
(10
-
4
min
-
1
) k
d
k
r
28
40
60
70
80
1.43
3.33
7.24
8.26
10.90
1.14
1.80
2.77
3.03
3.54
0.993
0.981
0.998
0.993
0.991
0.996
0.997
0.997
0.996
0.995
In this study, these results indicate that the dissolution rate of tantalite ore in HNO3
solution is diffusion controlled reactions and not by chemical surface. This is contrary to
the mechanism of HCl dissolution of tantalite recently published [8]. This could be
explained by the oxidizing characteristics of HNO3 as different from HCl. The apparent
rate constants for tantalite dissolution increased with increase in temperature up to 800C.
The application of this kinetic model is presented in Fig. 6.

92 A.A. Baba, F.A. Adekola, O.I. Dele-Ige and R.B. Bale Vol.7, No.1
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
020406080100 120 140
Contact Time (min)
1-(1-X)
1/3
80
70
60
40
28
0
C
0
C
0
C
0
C
0
C
Fig. 6: Plot of
(
)
3
1
X11−− Experimental conditions vs time at different temperatures.
Experimental conditions: Same as in Fig.5
The Arrhenius plot considering the apparent rate constants, kr, was obtained by
applying equation (2) to leaching experimental data (Fig. 7).

Vol.7, No.1 Investigation of Dissolution Kinetics of A Nigerian Tantalite Ore 93
-7
-6.8
-6.6
-6.4
-6.2
-6
-5.8
-5.6
-5.4
-5.2
-5
2.800 2.900 3.000 3.100 3.200 3.300 3.400
1/T(K
-1
)x10
-3
ln K
r
Fig.7: lnkr vs 1/T (K-1).
From Fig 7, the calculated activation energy, Ea, for the process is 22.37kJ/mol.
This value clearly suggests that the reaction for this process is diffusion controlled as
proposed by several investigators, [20, 22, 23].
4.0 CONCLUSIONS
In this study, the dissolution kinetics of tantalite in nitric acid solution was
examined. The results showed that the reaction rate increases with hydrogen ion [H+]
concentration, reaction temperature, stirring speed, but decreases with particle size. The
dissolution kinetics was found to be governed by the shrinking core model as the diffusion-
controlled process. The reaction order with respect to [H+] ion concentration was found to
be 0.64 and a value of 22.37kJ/mol was obtained as energy of activation, Ea, for the
dissolution process.
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