Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.10, pp.787-801, 2009 Printed in the USA. All rights reserved
Dissolution Kinetics and Leaching of Rutile Ore in
Hydrochloric Acid
Alafara A. Baba1*, Folahan A. Adekola1, Emmanuela E. Toye1 and
Rafiu B. Bale2
1Department of Chemistry, University of Ilorin, P.M.B 1515, Ilorin-Nigeria.
2Department of Geology and Mineral Sciences, University of Ilorin, P.M.B 1515, Ilorin-
*Corresponding Author:,
Experiments on the dissolution kinetics and leaching of rutile ore by hydrochloric acid have been
carried out. The influence of acid concentration, temperature, stirring speed and particle
diameter on the leaching of the ore were examined. The dissolution rates were greatly influenced
by the hydrogen ion concentration, temperature, stirring speed and particle diameter. Kinetic
data analysis showed that the dissolution mechanism followed a diffusion controled shrinking
core model with the surface chemical reaction as the rate controlling step. The study showed that
with 4M HCl solution, about 82.3 % of 10g rutile ore per litre of leachant at 80oC was dissolved
within 120min., using 0.045-0.075mm particle diameter at a stirring speed of 360rpm. The
reaction order with respect to hydrogen ion concentration was found to be 1.0, while
42.28kJ/mol was calculated for the activation energy of the dissolution process. Finally, the X-
ray diffraction spectrum showed that the residual solid which amounted to 18% of the initial
solid material contained silica (-SiO2) and are formed around the shrinking core of the
unreacted material.
Rutile is a mineral composed of titanium dioxide, TiO2 and one of three distinct titanium dioxide
polymorphs: ruble, anatase and brookite. Natural rutile may contain up to 10% iron and
significant amount of niobium and tantalum [1]. It has among the highest refractive indices of
any known mineral and also exhibits high dispersion. These properties have led to several
industrial applications, especially in the manufacture of refractory ceramics, pigment and
titanium metal [1].
788  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
Nigeria is an immensely mineral rich country with diverse metal ores, many of which are only
cuurently being evaluated by the Nigeria Geological Survey Agency. Available among the metal
ores is rutile, which occurrence has reported in Kwara, Niger, Plateau, Taraba, Osun and Kogi
States of Nigeria [2]. It has been observed that smelting plants in Nigeria export crude ores and
other concentrates to Europe where there are available facilities for the smooth extraction of
some precious metals including titanium. These metals are therefore innocent fallouts of the
country’s bid to earn revenues from its ores. Therefore, with a properly articulated policy on
solid minerals, the country stands to benefit technologically and economically from the huge
mineral deposits in the land [3].
Even though, the United States mines and processes titanium and titanium dioxide, it still
imports significant amounts of metallic titanium from Russia (36%), Japan (36%), Kazakhstan
(25 %) and other nations (3%). TiO2 pigment for paint is imported from Canada (33%), German
(12%), France (8%), Spain(6%) and other nations including African countries (36%) [4]. For
instance, in 2005, the Republic of Sierra Leone in West Africa had a production capacity of 23
percent of the world’s annual rutile supply, which rose to approximatelly 30 percent in 2008. The
reserves, lasting for about 19 years, are estimated at 259,000,000 metric tons (285,000,000 short
tons) [5].
A variety of problem such as high energy cost, shortage of high grade ores, processing of low
and complex ores and exploitation of smaller deposits have prompted the development of low
temperature hydrometallurgical processes for the extraction of base metals from their ores and
concentrates. The conventional hydrometallurgical processes for the extraction of a base metal
from a named ore or concentrate consist of a catalytic sulphating, roasting, leaching of the
metallic values, solvent extraction and selective stripping [6].
Chloride system in hydrometallurgy has been used for the treatment and recovery of precious
metals for a number of years [7]. The leaching of minerals including rutile is a subject of
considerable interest. The growing inability of the worlds’s natural rutile resources, now
principally derived from Australia to meet the raw material needs of the ‘chloride’ pigment
manufacturers, is one of the reasons for the present study [8].
Since there is virtually no reported work on the leaching or dissolution kinetics of any Nigeria
rutile ore, this work is therefore expected to provide useful data on the kinetic parameters on the
leaching of the ore for its subsequent beneficiation. The only existing document with respect to
Nigerian rutile is on elemental analysis by X-ray technique [9].
2.1 Material/Analysis
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The rutile sample was obtained from an ore deposit at Oke-Ode, Ifelodun Local Government
Area of Kwara State, Nigeria. The ore was crushed, ground and sieved with ASTM Standard
sieves into three size fraction: 0.045-0.075, 0.075-0.106 and 0.106-0.212mm. All experiments
were performed with particle size: 0.045-0.075mm, unless otherwise stated.
The elemental analysis of the ore was carried out by Inductively Coupled Plasma-Mass
Spectrometry (ICP-MS), Yogogawa Model HP-4500, equipped with auto sampler, peristatic
pump and Babington nebulizer under the following conditions: plasma, auxilliarry and carrier
gas flow rates of 15, 1.2 and 1.04L/min, respectively. The mineralogical purity of the ore was
examined using PHILIPS PW 1800 X-ray diffractometer (XRD) with CuK1(1.54Å) radiation,
generated at 40kV and 55mA. The cabinet houses a high speed, high precision Goniometer, high
efficiency generator (X-ray) and an automatic sample loading capacity. Doubly distilled water
and BDH grade HCl acid were used to prepare all solutions.
2.2 Equipment and Methods
Experimentals were carried out by agitation leaching using a covered 500ml Pyrex flask (glass
reactor) and mechanically stirred with a magnetic stirring bar at 0-540rpm. Typically and for
each run, 100ml of HCl solution of predetermined molarity was charged into the reactor and
heated to the required temperature (550C). Thereafter, 1.0g of rutile was added to the reactor and
the contents were well agitated [7, 8]. The concentration of HCl which gave the maximum
dissolution (4M) was subsequently used for the optimization of other leaching parameters
including temperature, stirring rate and particle size. Energy of activation, Ea, and constants
were determined from the Arrhenius plots. In all experiments, the fraction of the ore dissolved,
X, were calculated from the initial difference in weight of the amount dissolved or undissolved at
various time intervals up to 120min, after oven dried at about 600C [10]. The post-leaching
residual product at 800C in 4M HCl was then analyzed by XRD.
3.1 Mineralogical Studies
3.1.1 Elemental analysis b y ICP-MS
The result of the elemental analysis of the rutile ore by ICP-MS technique is summarised in
Table 1.
790  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
Table 1. Elemental analysis of the rutile ore by ICP-MS (expressed in percentage).
Element Fe Ti Zn Cu S Cd Nb Cr W Ag Ca Si Pb Sb
Conc. 1.34 40.94 0.79 0.47 0.78 0.93 0.018 0.032 0.005 0.014 2.41 18.46 0.06 0.003
O (oxygen) = 31.92 %, obtained by difference.
From Table 1, it is evident that the major elements detected by ICP-MS are Ti (40.94 percent), Si
(18.46%), Ca (2.41%) and Fe (2.34%), while Zn, Cu, S, Cd, Nb Cr and Ag are minor elements in
the ore. Other metals detected in the ore at trace levels were W, Sb, Th, V, Te and Mn. This
rutile, sourced from the North-central part of Nigeria with Ti content of about 41% is comparable
to 49% Titanium earlier reported for the rutile originating from the South-western part of Nigeria
3.1.2 Ore phase studies by XRD
Figure 1 shows the identified phases and their respective lattice plane with JCPDS file number in
the rutile ore by X-ray diffraction.
Fig. 1. X-ray spectra of rutile ore with the most probable compounds identified. Joint committee
on powder diffraction standard, File No. are put in brackets: (1,2): TiO2 [1 2 0] (29-1360); (3):
Fe2O3 [0 2 2] (16-0653); (4): SiO2 [1 0 1] (46-1045); (5): Fe3Ti3O10 [1 1 3] (47-0421); (6):
Ti3O5 [2 0 6] (23-0606).
The X-ray spectrum data in Fig.1 apparently complement the results of chemical analysis by
ICP-MS. It shows that titanium is present mainly as TiO2. With the result of ICP-MS, the TiO2
Vol.8, No.10 Dissolution Kinetics and Leaching of Rutile Ore 791
content can be estimated to be in the range of 68.3%. In addition, other phases identified include
-quartz (-SiO2) and Fe2O3, while Fe3Ti3O10 and Ti3O5 can be said to be present in traces.
3.2 Leaching Studies
3.2.1 Effect of stirring rate
The effect of stirring rate on the dissolution of 1.0g rutile ore was investigated in 4M HCl
solution with the 0.045-0.075mm size fraction of the rutile ore at 800C using stirring speed of 0-
540rpm for 120min. Table 2 summarizes the results of the effect of stirring speed on the rutile
ore dissolution.
Table 2. Effect of stirring speed on rutile ore dissolution in 4M HCl solution.
Stirring speed, min-1 Percent of rutile ore dissolved
0 39.83
90 50.27
180 63.75
270 74.08
360 82.23
450 82.11
540 82.11
The results from Table 2 showed that the rate of rutile ore dissolution was found to be dependent
on the stirring speed over range 0-540rpm. Above 360rpm, the stirring rate no longer influences
solid dissoluution. Therefore, steady rate was attained at 360rpm and was used for subsequent
3.2.2 Effect of HCl concentration
The effect of HCl concentration (0.5-8.06M) on the dissolution of 1.0g/L ore was investigated at
550C using 0.045-0.075mm size fraction of the ore. The fraction of the ore dissolved as a
function of leaching time for the different HCl concentrations is presented in Fig. 2
792  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
020 40 60 80100120140
Leaching time (min)
Fraction of rutil e ore di ss o lved
0.5M HC l
1.0M HC l
2.0M HC l
4.0M HC l
8.0M HC l
Fig. 2. Effect of HCl concentration on the dissolution of rutile ore at 550C.
From Fig. 2, it is evident that the leachant has a significant effect on the leaching of the rutile
ore. However, the fraction of the ore dissolved was moderate. The maximum percentage
dissolved did not vary appreciably when HCl concentration was doubled from 4M to 8M. The
respective values obtained were 56.7 and 51.3 %. Hence, 4M HCl was therefore retained for
subsequent studies.
3.2.3 Effect of temperature
The effect of temperature on the dissolution of 10g/L rutile ore in 4M HCl solution using 0.045-
0.075mm size fraction in the 28-80oC temperature range at a stirring rate of 360rpm was
investigated. From the results shown in Fig.3, it can be observed that increasing the temperature
is accompanied with increase in the dissolution rates. At 80oC about 82.37% of the rutile ore was
dissolved within 120min. However, tests at higher temperatures would be less suitable due to
increase loss of HCl vapour [8].
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020 40 60 80100120140
Leach i ng t ime (mi n)
Fraction of rutil e ore di ss o lved
Fig. 3. Effect of temperature on the dissolution of rutile ore in 4M HCl solution.
3.2.4 Effect of particle diameter.
The effect of particle diameter on the rate of rutile ore dissolution was examined in 4M HCl
solution at 80oC, using the three particle size fractions: 0.045-0.075, 0.075-0.106 and 0.106-
0.212mm. As expected, the results shown in Fig. 4 affirm that the rates of rutile ore dissolution
are inversely proportional to the average initial diameter of the particles.
794  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
020 40 60 80100120140
Leach ing time (mi n )
Fraction of rutil e ore di ss o lved
Fig. 4. Effect of particle diameter on the rutile ore dissolution in 4M HCl at 80oC
3.3 Discussion
3.3.1 Dissolution kinetic models
Understanding the mechanism of a leaching system is the main objective of this study. Leaching
of mineral particle may be described by a number of reaction models already proposed in the
literature [7]. Consequently, the dissolution rates of the rutile ore were analyzed with the
shrinking core models, under the assumption that the ore is a homogenous spherical solid phase
The leaching of rutile ore by hydrochloric acid may be written as:
TiO2 + 4HCl TiCl4 + 2H2O (1)
For this study and for better understanding of the leaching mechanism, two established Kinetic
models were used, as expressed by the following equations:
1 – (1- X) 1/3 = MkcCAt = k1t (2)
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1+2 (1- X) – 3 (1- X) 2/3 = 6uMDCAt = k2t (3)
Where kc is the first-order rate constant (mmin-1), M is the molecular weight of the solid reactant
(kgmol-1), CA is the acid concentration (molm-3), D is the diffusion coefficient (m2min-1), d is the
density of the particle (kgm-3), r is the initial radius of the particle (m), X is the fraction of rutile
ore dissolved at time t (min), k1(mmi n-1) and k2(m2min-1) are the overall rate constants and u is
the stoichiometric coefficient. Equation (2) is applicable to chemically controlled processes and
equation (3) referred to the diffusion controlled processes through the porous product layer [8,
Of the two shrinking core models tested, only equation (3) has been found to give a perfect
straight line, with a good correlation of 0.96. All the data shown in Fig. 2 were also found to fit
the model equation (3) and this is presented in Fig. 5
= 0.8763
= 0. 9639
= 0.9882
= 0.9979
020 40 60 80100120140
Leaching Time (min)
0.5M HCl
1.0M HCl
2.0M HCl
4.0M HCl
Fig. 5. Plot of 1+2(1-X)-3(1-X)2/3 versus leaching time at different HCl concentrations.
The experimental rate constants, k1, were evaluated from the slopes in Fig. 5 and the plot of lnk1
vs ln[HCl] were made as illustrated in Fig. 6.
796  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
Fig. 6. Plot of lnk1 versus ln[HCl]
The slope of the resulting plot (Fig. 6) gave the dissolution reaction order of 1.0, with respect to
hydrogen ion concentration, for HCl concentrations 4M. This showed that the dissolution
reaction follows a first order mechanism. Furthermore, the linearization of the data in Fig. 3 was
done using equation 3. This relation is presented in Fig. 7.
= 0.99 61
-1-0.50 0.5 1 1.5 2
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= 0. 9844
= 0.9986
= 0.9996
= 0.9998
= 0.9994
020 40 60 80100120140
Contact time (min)
Fig. 7. Plot of data extracted in Fig. 3.
The apparent rate constants, k1, k2 for the two shrinking core models examined at different
temperatures were calculated form the slopes of the straight lines obtained. These values and
their corresponding correlation coefficient rate are summarized in Table 3.
Table 3. The rate constants k1, k2 values with their correlation coefficient for rutile ore
dissolution in 4M HCl solution at different temperatures.
Apparent rate constants Correlation coefficient
k1 (10-4 min-1) k2 (10-4min-1) k1 k
28 0.93 3.17 0.8731 0.9844
40 1.61 8.67 0.9344 0.9986
60 2.25 15.50 0.9215 0.9994
70 3.02 25.63 0.8958 0.9996
80 3.63 34.51 0.9016 0.9998
798  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
The apparent rate constant, k2 derived from the slopes of the line in Fig. 7 were used to obtain
the Arrhenius relation in Fig. 8, from which the energy of activation, Ea, of 42.28kJ/mol was
calculated for the dissolution process.
= 0.9998
2.8 2.933.1 3.2 3.3 3.4
X 10
Fig. 8. Arrhenius relation of reaction rate against the reciprocal of temperature, for the data
extracted from Fig. 7.
The calculated activation energy from Fig. 8 seems to suggest a chemical control. Recent studies
showed that some diffusion controlled reactions have unusually high activation energy [8, 13,
and 16] For example, the reported energy of activation for the diffusion controlled dissolution of
titanium and iron from ilmenite by hydrochloric acid solution was 48.9 and 53.7kJ/mol,
respectively [14]. On closer examination, it appears that the rate controlling mechanism of
heterogeneous dissolution reactions is sometimes better predicted from plots of the kinetic
equations rather than from the activation energy value. In some instances, the same mechanistic
information is derivable from both variables [8].
The linearization of the kinetic curves in Fig. 4 was carried out by means of equation (3). The
values of the rate constants were plotted versus the reciprocal of the particle radii (1/ro), yielding
a linear relationship with a correlation coefficient of 0.9996 (Fig. 9). On the contrary, the plot of
Vol.8, No.10 Dissolution Kinetics and Leaching of Rutile Ore 799
the rate constants as a function of the square of particle radii (1/r02) did not give a linear
= 0. 9996
00.01 0.02 0.03 0.04 0.05 0.06
k X 10
Fig. 9. Dependence of rate constant (k) on 1/ro.
Hence, the linear dependence of the rate constant on the inverse of particle radius suggests that
the surface chemical reaction is the rate controlling step for the dissolution process [15].
3.3.2 Composition of the residual products
The X-ray diffraction analysis of the solid residual products resulting from the leaching at
optimal conditions are presented in Fig. 10.
The leaching residue amounted to 18% of the initial solid material. Its XRD data shown in Fig.
10 revealed the presence of silica (-quartz) as the only product identified. It is very important to
note the near absence of Ti and Fe in the residual product.
800  A.A. Baba, F.A. Adekola, E.E. Toye and R.B. Bale Vol.8, No.10
Fig. 10. The X-ray diffraction spectrum of the 10g/L solid residue at 80oC in 4M HCl, using
0.045-0.075mm particle diameter, showing - SiO2 as the major peaks identified at various d-
spacing. (1): -SiO2 (46-1045).
Based on the results of the mineralogical and leaching investigations, the following results can
be drawn from the study.
(i) The Inductively Coupled Plasma-Mass Spectrometry technique showed that the rutile
mineral used in this study exists mainly as TiO2 with matals such as Nb, Cr and Ag occuring as
minor elements. Other metals detectected at trace levels were W, Sb, Th, V, Te and Mn. The X-
ray diffraction analysis (XRD) , however , confirmed the originality of the ore and it reveals the
presence of other associated minerals including Fe2O3, -SiO2, Fe3Ti3O10 and Ti3O5.
(ii) Both HCl concetration and temperature have a significant influence on the rutile ore
dissolution. The reaction rate also increases with increasing stirring speed and decreasing particle
diameter. With 4M HCl solution and at a temperature of 80oC using 0.045-0.075mm particle
diameter, about 82.3% of the 10g/L of the rutile ore was activation energy of 42.28kJ/mol has
been calculated for the process while the reaction order with respect to hydrogen ion
concentration follows a first-order mechanism.
(iii) The X-ray diffraction also comfirmed the -quartz (-SiO2) as the main constituent
of the post-leaching residual product, with near absence of titanium and iron in the residue.
(iv) The results of the leaching investigations indicated that the shrinking core model for
spherical particles is applicable. The reaction mechanism for the dissolution is diffusion
controlled with surface chemical reaction as the rate controlling step.
Vol.8, No.10 Dissolution Kinetics and Leaching of Rutile Ore 801
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