A Study on the Electrochemical Dissolution of Ilmenite Fraction of Beach Sand in Sulphuric Acid
Solution
M. Rostom Ali*, Ranjit Kumar Biswas and Md. Golam Zakaria
Department of Applied Chemistry and Chemical Engineering
Rajshahi University Rajshahi, Bangladesh [email protected], [email protected]
Abstract— Electrochemical techniques have been used to investigate the dissolution of ilmenite fraction of beach sand of Bangladesh in H2SO4 solutions at various temperatures. It is seen from the cyclic voltammetric studies that the dissolution of ilmenite is very difficult without the addition of carbon powder in ilmenite. The effects of ilmenite-carbon ratio, acid concentration and temperature on cyclic voltammograms have been investigated to understand the process. The investigated results show that the dissolution rate of ilmenite is low at low applied reduction potentials and temperatures. However, the dissolution rate is increased at more negative applied reduction potentials and higher temperatures. The dissolution rate is also increased on increasing acid concentration up to 1 mol dm-3; and at more acid concentration and higher reduction potential, it is decreased due to the increase of H2 evolution which eventually decreases the active surface area of pellet by adsorption. The activation energy (Ea) is 50±10 kJ mol-1 in the higher temperature region (htr) and
15±5 kJ mol-1 in the lower temperature region (ltr). The value of Ea suggests the process to be diffusion controlled at ltr and chemically controlled at htr.
Keywords— Ilmenite; Cyclic voltammetry; Electrochemical dissolution; Activation energy; Sulphuric acid
I. INTRODUCTION
An estimated 3.5 million ton of heavy mineral deposit has been discovered in the sand of Chittagong-Cox’s Bazar-Teknaf beach and off-shore islands of Moheskhali, Kutubdia, and Nizhum-Dwip of Bangladesh. This deposit includes ilmenite, rutile, magnetite/hematite, zircon, garnet and monazite [1]. The Pilot Plant of BAEC at Kalatoli, Cox’s Bazar has fractionated the heavy fraction of beach sand into the above cited minerals.
The complete dissolution of ilmenite or leaching out of iron from ilmenite to form synthetic rutile is a very difficult task.
Acid leaching (complete or fractional) is not so effective at moderate condition excepting HF [2]. A thorough study on the dissolution of ilmenite with HF has demonstrated that 5.5 mol L-1 HF at its boiling temperature under atmospheric pressure can dissolve as much as 81% Ti and 26% Fe present in ilmenite at solid to liquid (S/L) ratio of 0.05 g/mL and in 300 min.
However, this method suffers from inconvenience in HF- handling, considerable long leaching time and involvement of too many steps to recover the desired product.
Dilute HCl is not so effective for ilmenite dissolution [2]. It has been reported that the pressure leaching at 150 oC with ~12 mol L-1 HCl can leach out iron from ilmenite to get the feed (synthetic rutile) of the chlorination process [3]. Leaching of ilmenite by 6 mol L-1 HCl + 0.5 mol L-1 C2H5OH/C6H5OH has been thoroughly investigated by Habib et al. [4]. The ethanol mixed reagent can dissolve as much as 88% Ti and 95% Fe;
whereas, the phenol mixed reagent can dissolve only 81% Ti and 85% Fe in comparison to 74% Ti and 81.5% Fe dissolutions in 6 mol L-1 HCl alone at 110 oC and S/L = 0.02 Kg/L in 7 h. The dissolution behaviors of ilmenite from different sources by HCl have been reported by several workers [5-10]. Moreover, the dissolution of ilmenite on roasting with LPG-pyrolysed products followed by leaching with 6 mol L-1 HCl and 3 mol L-1 H2SO4 has been investigated [11]. At optimized conditions, 85% Ti and 80.1% Fe dissolutions in 6 mol L-1 HCl, and 52.3% Ti and 60.8% Fe dissolutions in 3 mol L-1 H2SO4 occur. On the other hand, 93%
Ti and 95% Fe dissolutions in 6 mol L-1 HCl can be achieved on roasting ilmenite by reformed product of LPG-H2O mixture in presence of nickel powder [12]. Consequently, it appears that there is no method for complete dissolution of ilmenite or complete leaching out of Fe from ilmenite. The electrochemical dissolution of Fe from reduced ilmenite has been investigated by some workers [13-15] and reported that anodic dissolution of iron and cathodic reduction of oxygen occur in NH4Cl solution within 30-150 oC. Zhang and Nicol [16] have reported the reduction and dissolution of ilmenite in 450 g/L H2SO4 medium at elevated temperature by an electrochemical study (above 0.3 V, dissolution of ilmenite occurs due to reduction of hematite and at negative potentials, the dissolution increases for reduction of ilmenite to Ti(III)).
Ilmenite (p-type semiconductor) possesses electrical conductivity ranging from 0.016 to 0.38 Ω-1 cm-1 depending of crystal orientation and hematite content [17]. Thus electrochemical studies can provide useful information on
0 50 100 150 200
20 40 60 80
Position, 2/O
Intensity, I(a.u.)
12 3
4
5
6 7
8 910
11 13
1415 16 2,3,4,6,8,13,15,16- FeTiO3
5- Fe2O3 9,13,14,16- TiO2 3,6- (Fe, Mg) Ti2O5
3,7,9,10- Fe2TiO5 8,10,11,12- FeCr2O4 1- SiO2
12
kinetics and mechanism of ilmenite dissolution. However, limited information is available due to non-availability of pure ilmenite.
As pointed out earlier, the electrical conductivity of ilmenite depends on the hematite content of the sample, this report discusses on the results obtained on the cyclic voltammetric analysis of Bangladeshi ilmenite concentrate produced by BAEC Pilot Plant for heavy mineral exploration centre at Kalatoli, Cox’s Bazar.
II. EXPERIMENTAL
A. Materials
Ilmenite fraction was collected from BAEC’s pilot plant at Cox’s Bazar. According to BAEC, the composition of the received sample was 38.5% TiO2, 25.62% Fe2O3, 29.75% FeO, 1.2% SiO2, 0.03% P2O5, 1.2% MnO2 and 1.08% Cr2O3. The ilmenite sand was dry-ground and sieved to collect particles of size <53 µm. Carbon was collected from waste dry cells.
Carbon was similarly processed. All the other chemicals were of A.R. grade (E. Merck–BDH) products and used without further purifications.
B. Preparation of Ilmenite Working Electrodes (pellet) Working electrodes were prepared as follows: A homogeneous paste was prepared by mixing powders of ilmenite and graphite with liquid paraffin in definite ratios by mass and placed in a steel holder of 3.3 cm depth and 1.4 cm diameter. A copper wire (10 cm long) was placed inside the mass of the holder. The holder was then placed in a pellet making machine to form a pellet on applying 40 ton pressure.
The pellet was covered by Araldite epoxy resin exposing the working surface of area 1.54 cm2. The electrode surface was polished with 2000 grade SiC paper and rinsed with deionized water before each electrochemical measurement.
C. Electrochemical measurements
Electrochemical investigations including potential step chronoamperometry, chronopotentiometry and cyclic voltammetry were carried out using a Hokudo Denko HAB- 151 Potentiostat/Galvanostat, Tokyo, Japan, equipped with a potential sweeper. Data were recorded in a computer through data acquisition system (USA) using WinDaq software. A three electrode system consisting of a working electrode, a platinum (50100.1 mm3) counter electrode and a silver wire quasi-reference electrode were used in all electrochemical studies. The counter electrode was cleaned electrochemically in 1.5 mol L-1 H2SO4–1.5 mol L-1 H3PO4 mixture, and rinsed with deionised water and acetone and dried prior to use.
Voltammograms were obtained at 30 to 85 oC with scan rate ranging from 5 to 100 mV s-1. A silver wire (immersed in H2SO4 solution) was used as quasi-reference electrode in all experiments. All potentials in this work are quoted with respect to this AgAg(I) reference electrode.
D. Elcetrochemical dissolution of ilmenite fraction
Electrochemical dissolution of ilmenite fraction was carried out in 0.30 3.00 mol L-1 H2SO4 solution under constant potential methods in the range from -0.4 V to -1.2 V at temperatures ranging from 30 to 85oC. Following each dissolution experiment, the resultant solution was analysed for Ti(IV) and Fe2+/Fe3+ contents. Titanium and iron contents in solutions were analysed colorimetrically using a UV–visible Spectrophotometer (UV-1650 PC, Shimadzu, Japan). In case of Ti(IV), H2SO4-H3PO4-H2O2 method at 420 nm was [19a];
whereas, Fe(III) was estimated by the NH4SCN method at 480 nm [19b]. The later analysis was carried out without and after HNO3 oxidation for determining Fe2+/Fe3+ contents.
III. RESULTS AND DISCUSSION
A. Characterization of Ilmenite Fraction by XRD and EDAX X-ray diffraction analysis has been carried out with a Philips PW 1716 diffractometer using Cu K radiation (40 kV, 30 mA, scan speed: 0.01 o/s) to identify the compounds/phases existing in ilmenite sample. The acquired diffraction pattern is shown in Fig. 1. The automatching indicates the presence of SiO2, FeTiO3, Fe2O3, TiO2, (Fe, Mg)Ti2O5, Fe2TiO5 and FeCr2O4 in ilmenite. XRD and chemical analysis data indicate that it contains about 62.8% FeTiO3, 16.35% Fe2TiO5 and 14.72% Fe2O3 (as free and also as FeCr2O4 and (Fe, Mg)Ti2O5).
B. Cyclic Voltammetry of Ilmenite-C-Paraffin mixture Pellet in H2SO4
The cyclic voltammogram recorded on a pellet of 2:1:0.1 wt.
ratio of ilmenite, carbon and paraffin mixture electrode in 1 M H2SO4 solution at 30 oC with a scan rate of 10 mV s-1 is shown in Fig. 2. The rest potential is -0.10 V vs. AgAg(I) reference electrode. The scan towards negative direction (in the second cycle) consists of first and second reduction waves C1 and C2
with the current starting to increase at 0.30 V and -0.62 V, respectively. An additional reduction wave C3 is observed with the current increase starting at -0.97 V. The reverse scan consists of first oxidation peak Pa at 0.48 V. Additional
Fig. 1 X-ray diffraction pattern of the ilmenite fraction of beach sand of Bangladesh.
-70 -50 -30 -10 10 30
-1.8 -1 -0.2 0.6 1.4
Ilme:C:Para = 1:0:0.1 Ilme:C:Para = 0:1:0.1 Ilme:C:Para = 2:1:0.1
C1
C2
Pa
Current density, i/ mA cm-2
Potential, E/ V vs. AgAg+
2H2O 4H+ + O2 + 4e- 4H++ 2e-2H2
C3
-50 -30 -10 10 30
-1.5 -0.5 0.5 1.5
Reverse potential = -1.38 V Reverse potential = -0.25 V Reverse potential = -0.89 V
C1 C2
Pa
Current density, i/ mA cm-2
Potential, E/ V vs. AgAg+ C3
-70 -50 -30 -10 10 30
-1.6 -0.8 0 0.8 1.6
Ilme:C:Para = 2:1:0.1 Ilme:C:Para = 4:3:0.1 Ilme:C:Para = 1:1:0.1
C1 C2
Pa
Current density, i/ mA cm-2
Potential, E/ V vs. AgAg+ C3
-65 -45 -25 -5 15
-1.4 -0.6 0.2 1
C1 C2
Pa
Current density, i/ mA cm-2
Potential, E/ V vs. AgAg+ C3 0.3 mol L-1H2SO4
1.0 mol L-1 H2SO4 3.0 mol L-1 H2SO4
oxidation wave is observed with the current increase starting at 1.15 V. The first reduction wave C1 is absent in the forward scan of the first cycle. However in the first cycle, the magnitude of the current densities in the second reduction wave C2 is same while it is decreased in the first oxidation peak Pa.
Compared with the voltammogram obtained in the absence of ilmenite (dashed curve in Fig. 2), the reduction wave appearing at -0.92 V corresponds to the reduction of H+ in H2SO4 acid solution, while the oxidation wave appearing at 1.13 V corresponds to the evolution of O2 according to the following reactions:
2H+ + 2e- H2 (at -0.95 V) (1) 2H2O 4H+ + O2 + 4e- (at 1.15 V) (2) The voltammogram in absence of carbon (dotted curve in Fig.
2) does not have any reduction and oxidation wave. This indicates that reduction of ilmenite is not possible in absence of carbon due to its very low conductivity. The solid curve in
Fig. 2 indicates that the 1st and 2nd reduction waves are responsible for the reduction of ilmenite; whereas, the oxidation peak corresponds to the oxidation of the reduced species of ilmenite.
Figure 3 shows the effect of cathodic sweeping potentials on the cyclic voltammograms recorded on a pellet of 2:1:0.1 wt. ratio of ilmenite, carbon and paraffin mixed electrode in 1 mol L-1 H2SO4 solution at 30 oC with a scan rate of 10 mV s-1. It is readily seen that both 1st and 2nd reduction waves C1 and C2 correspond to the 1st oxidation peak Pa. The 1st cathodic peak at about -0.06 V is probably involving the reduction of Fe3+ to Fe2+ either in solution or in solid phase. The 2nd reduction wave is possibly associated with the reduction of FeTiO3 and Fe2O3 fractions to TiO+ and Fe2+; and the 1st oxidation peak is attributed to oxidation of Fe2+ to Fe3+ in solution:
Fig. 2 Cyclic voltammograms recorded on a pellet of ilmenite, carbon and paraffin mixed electrode in 1 mol L-1 H2SO4 at 30 oC with a scan rate of 10 mV s-1.
Fig. 3 Effect of sweeping potential on the cyclic voltammograms recorded on a pellet of ilmenite, carbon and paraffin mixed electrode in 1 mol L-1 H2SO4 at 30 oC with a scan rate of 10 mV s-1.
Fig. 4 Effect of ilmenite and carbon ratio on the cyclic voltammograms (2nd cycle) recorded on a pellet of ilmenite, carbon and paraffin mixed electrode in 1 mol L-1 H2SO4 at 30oC with a scan rate of 10 mV s-1.
Fig. 5 Effect of H2SO4 concentration on the cyclic voltammograms (2nd cycle) recorded on a pellet of 2:1:0.1 ratio of lilmenite:carbon: paraffin mixed electrode at 30oC with a scan rate of 10 mV s-1.
-90 -60 -30 0 30 60
-1.5 -0.5 0.5 1.5
C1
C2 Pa
Current density, i/ mA cm-2
Potential, E/ V vs. AgAg+ C3
30 oC 60 oC 80 oC
0 20 40 60 80 100
0 1 2 3 4
Dissol. Pot. = -1.0 V, Fe Dissol. Pot. = -1.0 V, Ti Dissol. Pot. = -0.8 V, Fe Dissol. Pot. = -0.8 V, Ti
Time, t/hr Rate108, / mol cm-2s-1
0 20 40 60 80
25 45 65 85
Dissol. Pot. = -1.2 V Dissol. Pot. = -1.0 V Dissol. Pot. = -0.8 V Dissol. Pot. = -0.6 V Dissol. Pot. = -0.4 V Dissol. Pot. = -1.2 V Dissol. Pot. = -1.0 V Dissol. Pot. = -0.8 V Dissol. Pot. = -0.6 V Dissol. Pot. = -0.4 V
Temperature, T/oC Rate108, / mol cm-2s-1
0 20 40 60 80
0.2 1.2 2.2 3.2
-1.2 V -1.0 V -0.8 V -0.6 V -0.4 V -1.2 V -1.0 V -0.8 V -0.6 V -0.4 V
Conc. of H2SO4, C/ mol L-1 Rate108, / mol cm-2s-1
Fe3+ + e- Fe2+ (at C1) (3)
FeTiO3 + 2H+ + e- Fe2+ + TiO+ + 2OH- (at C2) (4)
Fe2O3 + 3H+ + 2e- 2Fe2+ + 3OH- (at C2) (5)
Fe2+ Fe3+ + e- (at Pa) (6) Similar results have been reported by Zhang and Nicol [16]
for the electrochemical reduction and dissolution of ilmenite in H2SO4 solution. The dissolved solution obtained at a constant potential dissolution method (-1.0 V) contains Fe2+, Fe3+ and Ti4+ ions.
The effect of ilmenite, carbon and paraffin ratio in pellet on the cyclic voltammogram (2nd cycle) recorded in 1 mol L-1 H2SO4 acid solution at 30 oC with a scan rate of 10 mV s-1 is shown in Fig. 4. It is readily seen that magnitudes of the 1st and 2nd reduction peaks and also the oxidation peak current densities are increased with the increase of ilmenite content in the pellet. It indicates that 2:1:0.1 wt. ratio of ilmenite-C- Paraffin in pellet will give higher dissolution rate of ilmenite fraction by constant potential and constant current methods.
Figure 5 shows the effect of H2SO4 concentration on the cyclic voltammograms (2nd cycle) recorded on a 2:1:0.1 (wt.
ratio) ilmenite-C-paraffin mixed electrode at 30 oC with a scan rate of 10 mV s-1. The magnitude of the reduction and oxidation peak current densities in 1 mol L-1 H2SO4 is higher than those in lower (0.3 mol L-1) and higher (3 mol L-1) [H2SO4], which indicates that 1 mol L-1 H2SO4 is suitable to obtain higher dissolution rate.
The effect of temperature on cyclic voltammogram (2nd cycle) recorded on a 2:1:0.1 (wt. ratio) ilmenite-C-paraffin mixed electrode in 1 mol L-1 H2SO4 with a scan rate of 10 mV s-1 is shown in Fig. 6. Magnitudes of the reduction and oxidation peaks’ C.Ds are increased with rise of temperature.
It is therefore concluded that higher dissolution rate can be obtained from 2:1:0.1 (wt. ratio) ilmenite-C-paraffin mixed electrodes in 1 mol L-1 H2SO4 at 80 oC.
C. Electrochemical Dissolution Study
Variations of dissolution rate of ilmenite with time in 1 mol L-1 H2SO4 at 80 oC from 2:1:0.1 (wt. ratio) ilmenite-C-
Fig. 6 Effect of temperature on the cyclic voltammograms (2nd cycle) recorded on a pellet of 2:1:0.1 ratio of lilmenite:carbon: paraffin mixed electrode in 1 mol L-1 H2SO4 with a scan rate of 10 mV s-1.
Fig. 7 Effect of time on the dissolution rate of iron and titanium from 2:1:0.1 wt. ratio of ilmenite, carbon and paraffin mixed in 1 mol L-1 H2SO4
at 30 oC. Applied dissolution potential: -0.8 V and -1.0 V.
Fig. 8 Effect of temperature on the dissolution rate of iron and titanium from 2:1:0.1 wt. ratio of ilmenite, carbon and paraffin mixed in 1 mol L-1 H2SO4 at different applied dissolution potential.
Fig. 9 Effect of H2SO4 concentration on the dissolution rate of iron and titanium from 2:1:0.1 wt. ratio of ilmenite, carbon and paraffin mixed at 80 oC. Applied dissolution potential: -0.8 V and -1.0 V.
Open symbol for Ti and closed symbol for Fe
-8.5 -7.5 -6.5
0.0028 0.003 0.0032 0.0034
-1.2 V, Fe -1.0 V, Fe -0.8 V, Fe -0.6 V, Fe -0.4 V, Fe -1.2 V, Ti -1.0 V, Ti -0.8 V, Ti -0.6 V, Ti -0.4 V, Ti
1/T, K-1 log/ mol cm-2s-1
paraffin mixed pellet under constant potential method are shown in Fig. 7. The dissolution potentials are -1.0 V and -0.8 V vs. AgAg+ reference electrode. Initially, the dissolution rate of ilmenite fraction is sharply decreased with the increase of time up to 1 h and then the decreasing rate is significantly slow. The dissolution rate of iron is about 5 times to that of titanium at both the applied dissolution potentials for a fixed time. Therefore, dissolution time of 1 h has been selected in subsequent electrochemical studies.
Figure 8 shows the effect of temperature on the dissolution rate of ilmenite in 1 mol L-1 H2SO4 from 2:1:0.1 ilmenite-C- paraffin mixed pellet at different applied potential method.
Dissolutions of ilmenite have been carried out in the potential range of -0.40 V to -1.20 V and the temperature range of 30 oC to 80 oC. At constant potential, the dissolution rate is increased with the increase of temperature. The increasing rate is slow up to 60 oC and then sharply increased with the rise of temperature. The ratio of the rate of iron dissolution to that of titanium at all applied dissolution potentials and temperature is about 5.0.
The effects of H2SO4 concentration on the dissolution rate of ilmenite fraction at 80 oC from 2:1:0.1 ilmenite-C-Paraffin mixed pellet under constant potential method are shown in Fig. 9. At low applied reduction potentials (-0.4 V -0.6 V), the dissolution rate is first increased up to 1 mol L-1 H2SO4
solution and then the increasing rate is very slow. However at a more negative applied reduction potentials (>-0.6 V), the dissolution rate is first sharply increased up to 1 mol L-1 H2SO4 solution and then it is decreased appreciably with the rise of acid concentration. At higher H2SO4 acid concentration (>1.0 mol L-1) and more negative applied reduction potentials (<-0.8 V), the dissolution rate of ilmenite fraction is decreased due to the increase of hydrogen gas evolution which eventually decreases the active surface area of ilmenite pellet by adsorption.
The Arrhenius plot for the dissolution rate of ilmenite fraction (log vs. 1/T ) at various dissolution potentials are
shown in Fig. 10. The experimental data for a set of experimental parameter do not fall on a straight line; rather curve is obtained. The slopes of the tangential lines at higher temperature region give activation energy (Ea) of 50±10 kJ mol-1 and those at lower temperature region give Ea of 15±5 kJ mol-1. The values of Ea suggest the existence of a diffusion film which is destroyed at higher temperature. At lower temperature region, the process is diffusion controlled and with the rise of temperature, the process becomes chemically controlled.
IV. CONCLUSIONS
The dissolution of ilmenite fraction could not be obtained in H2SO4 acid solution without the addition of carbon in ilmenite fraction. The dissolution rate of ilmenite (FeTiO3) and hematite (Fe2O3) is low at low applied reduction potentials (<-0.60 V) and temperatures (<60 oC). At more negative potentials and higher temperatures, the dissolution rate of ilmenite is increased through increasing rate of reduction of ilmenite to Fe2+ and TiO+. The dissolution rate is also increased on increasing acid concentration up to 1 mol L-1; and at higher acid concentration and higher reduction potentials, it is decreased due to the increase of H2 evolution which eventually decreases the active surface area of ilmenite pellet by adsorption. The hematite phase in the mineral has a higher dissolution rate under reductive conditions than the ilmenite.
The values of Ea in the higher and lower temperature regions are 50±10 kJ mol-1 and 15±5 kJ mol-1, respectively. The values of Ea suggest the existence of a diffusion film at lower temperature which is destroyed at higher temperature. At lower temperature region, the process is diffusion controlled and with the rise of temperature the process becomes chemically controlled.
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