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Effect of mechanical activation on the dissolution of Panzhihua ilmenite
Minerals Engineering 19 (2006) 1430–1438 This article is also available online at: www.elsevier.com/locate/mineng
Effect of mechanical activation on the dissolution of Panzhihua ilmenite Chun Li a
a,*
, Bin Liang a, Ling-hong Guo b, Zi-bin Wu
a
Multi-phases Mass Transfer and Reaction Engineering Laboratory, Department of Chemical Engineering, Sichuan University, 24, South one Cycle Road, Chengdu 610065, PR China b Analytic and Testing Center, Sichuan University, Chengdu 610065, PR China Received 24 October 2005; accepted 21 February 2006 Available online 19 May 2006
Abstract Dissolution of mechanically activated Panzhihua ilmenite was carried out at 100 °C with a 50% sulfuric acid solution. The particle size, specific surface area, crystallite size, lattice strain and mechano-chemical reaction of milled ilmenite were measured. The cleavage induced by energetic milling occurred more preferentially along the a-axis of ilmenite unit cell. It was found that the defects formed in milling process, especially the lattice strain in the c-axis of ilmenite unit cell, greatly enhanced the leaching reaction. At low temperatures, the leaching reaction is likely to start along the direction of the basal plane (0 0 0 1) of milled ilmenite. With the mechanically activated ilmenite, leaching conversion at 100 °C reached 82.1%. However, the leaching kinetic tests revealed that the ball milling only partially activated the ilmenite. The initial rapid dissolution is due to the leaching of both the fine powders and highly reactive crystal faces. In the later stage, the dissolution exhibited essentially the same behavior as non-activated ilmenite. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ilmenite; Mechanical activation; Leaching; Kinetics; Mechanism
1. Introduction Panzhihua ilmenite accounts for more than 90% of titanium reserves of China. After the removal of vanadium and iron, the ilmenite concentrate contains TiO2 6 48 wt.%, Fe2O3 6 wt.%, and MgO 6 wt.%. Because of its high impurity contents, Panzhihua ilmenite is not suitable for the chlorination process to produce TiO2 pigment. The ilmenite concentrate is used in the sulfate process to manufacture TiO2 pigment. In the sulfate process, ilmenite is dissolved with concentrated sulfuric acid (about 85 wt.%) at temperatures up to 200 °C. The resultant aqueous solution is subject to iron removal before titania precursor is precipitated. The precursor is then calcined to produce TiO2 pigment. More than 90% of TiO2 pigment plants in China are operated with the sulfate process. However, the sulfate pro*
Corresponding author. Tel.: +86 28 85408056; fax: +86 28 85461108. E-mail address: [email protected] (C. Li).
0892-6875/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.02.005
cess faces severe environmental challenges. The digestion reaction of ilmenite with concentrated sulfuric acid is highly exothermic which can cause catastrophe if not properly controlled (Liang et al., 2005). In addition, the dilute spent acid (20 wt.%) formed during hydrolysis is not presently recycled to the process, which causes not only environmental problems but also the waste of sulfur resource. Leaching ilmenite with dilute acid can ease the flue gas pollution due to low reaction temperature, and also reduce the waste acid release by partially recycling the dilute acid. However, the reaction rate is very slow. NL corporation (Rahm and Cole, 1981) reported that 48 h reaction time were required for digesting ilmenite with 20–65% sulfuric acid in a multi-stage batch reactor series to achieve a Ti extraction ratio of 95%. She et al. (1998) obtained only 68% of Ti extraction when digesting Panzhihua ilmenite with a 30–50% sulfuric acid solution in a continuously operated fluidized bed. Jing et al. (2003) proposed a multi-step leaching operation with 65% sulfuric acid. However, it was still limited by the low reactivity.
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Mechanical activation, usually carried out by energetic milling, was reported to significantly accelerate the dissolution rate of solid minerals by increasing their chemical reactivity (Ficeriova´ et al., 2002; Bala´zˇ et al., 2003; Welham, 2001; Amer, 2002; Chen et al., 1999; Sasikumar et al., 2004; Welham and Llewellyn, 1998). However, contradictory results about the mechanical activation of ilmenite were reported. Chen et al. (1999) observed that ilmenite was gradually converted to pseudo-rutile (Fe2Ti3O9) during milling in air. However, Sasikumar et al. (2004) reported that the pseudo-rutile gradually disappeared and no mechanochemical oxidation of Fe2+ occurred with milling. Welham and Llewellyn (1998) noted that after 25 h milling the a-parameter of ilmenite unit cell decreased while the c-parameter remained unchanged. They suggested that the milling caused strain preferentially along the a-axis. On the contrary, Sasikumar et al. (2004) observed the cparameter firstly increased linearly with milling time up to 240 min whereas the a-parameter was kept constant in the first 90 min and then increased, which implied the initial strain along the c-axis. The leaching process of mechanically activated ilmenite, as reported by Welham and Llewellyn (1998), includes two stages. At the beginning, the leaching reaction is fast with low apparent activation energy (Ea = 15 kJ/mol). In the late stage, the apparent activation energy increases up to 70 kJ/mol. They inferred that the initial reaction was linked with the dissolution of the crystal faces with higher chemical reactivity. However, they did not provide sufficient evidence in support of their claim. Sasikumar et al. (2004) also described the similar leaching phenomenon. They did not distinguish the activation energies between the initial and late stages and found that the activation energies changed in 53.9–62.0 kJ/mol depending on the milling time. To date only literatures (Amer, 2002; Chen et al., 1999; Sasikumar et al., 2004; Welham and Llewellyn, 1998) are found concerning the effect of mechanical activation on structure and morphology of beach sand type ilmenite and its dissolution. In this work, we studied the leaching of the Panzhihua ilmenite of rock-type origin by dilute sulfuric acid with mechanical activation. The objective was to examine the effect of mechanical activation on leaching performance. Structure and morphology of the ore were also characterized. 2. Experimental 2.1. Material The ilmenite was obtained from Titanium Company of Panzhihua Steel & Iron (Group) Corporation, Sichuan, China. It is in a size range of 150 + 100 lm. Its chemical composition, obtained with an X-ray fluorescence Spectrometer (Phillips DW/480), is shown in Table 1. XRD results (detected by a Phillips X’pert Pro. MPD) showed that the major mineral constitution was hexagonal structured FeTiO3.
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Table 1 Chemical composition of the ilmenite used (wt.%) FeO
TiO2
Fe2O3
MgO
SiO2
Al2O3
MnO2
34.21
47.25
5.56
6.23
2.75
1.49
0.61
2.2. Mechanical activation A vertical planetary ball mill (QM-1SP2, Nanjing, China) with a rotation speed of 200 rpm and a spin rate of 500 rpm was employed in the mechanical activation of ilmenite. The porcelain cells were about 100 ml and balls used were 8 mm in diameter. The mass ratio of ball/ilmenite was 20:1 in the milling operation. The ilmenite, milled under either atmospheric or vacuum conditions (residual pressure 6 16.3 kPa), was characterized to detect the structural changes and leached to measure its reactivity. 2.3. Leaching experiments Milled samples were leached in a 500 ml four-necked glass reactor. An agitator, a condenser and a thermometer were fitted into its openings. The fourth opening was used to feed reactants and withdraw the slurry samples. The reactants were heated using an oil bath which controlled the temperature to within ±2 °C. In each experiment, 400 ml 50% sulfuric acid solution was preheated to the temperature required before 4 g milled sample was added. The agitation was kept constant at 500 ± 10 rpm during the entire leaching operation. A comparison test was conducted without agitation at ambient temperature (25–30 °C), in which 5 g milled sample reacted with 100 ml 50% sulfuric acid solution in a 300 ml sealed glass reactor. The slurry was sampled during the leaching process to measure the titanium conversion. The sampled slurry was filtered and washed before titrated with a reduction–oxidation titration method (Barksdale, 1966). The residual solid was characterized. 2.4. Characterization The ilmenite ore, milled powder and leaching residue were characterized by XRD, infrared (Fourier transform) spectroscopy, thermal analysis, particle size analysis and surface area measurement. XRD experiments were performed using XRD (X’Pert Pro MPD, Philips, Netherlands). The voltage and anode current used were 40 kV and 40 mA, respectively. The CuKa = 0.15405 nm and continuously scanning mode with 0.02 interval and 0.25 s of set time were used to collect the XRD patterns of samples. Further, parallel X-ray incident beams of hybrid mirror with divergence slit of 1/8° and diffracted beam of the parallel plate collimator with Soller slit 0.04 Rad and prepositional detector were used in all XRD experiments. The structure refinement of FeTiO3 was performed on Cerius2 platform (MSI company, USA). The Rietveld method was embedded in DWBS90 software
C. Li et al. / Minerals Engineering 19 (2006) 1430–1438
package in Cerius2 (Thompson et al., 1987). The original crystal model was constructed based on the data with Cerius2. The principle of Rietveld is refining the parameters of original crystal model to fit the XRD experimental data with the Netwon–Raphson algorithm. The parameters to be refined in the present study were more than 40, but only lattice strain and crystallite size were selected to characterize the milling process. Measurement of vibration spectra was performed using a Nicolet MX-1E FTIR spectrometer. Thermal analysis experiments were carried out on a simultaneous TG/ DTA analyzer (Seiko, Japan, EXSTAR6000), with a heating rate of 10 K/min and an air flow rate of 300 ml/min. Surface area measurement was conducted using a multipoint BET technique (Quantachrome, America, NOVA3000), and the particle size analysis was carried out using a laser diffraction analyzer (China, JL-1178).
100 90 80
crystallite size/nm
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70 60 50
a-axis
40 30
c-axis
20 0
2
4
6
8
10
activation time/h Fig. 2. Variation of the crystallite size with activation time.
3. Results 3.1. Structural changes 3.1.1. X-ray diffraction From the XRF results, the 6.23% MgO was detected in the concentrate. The XRD experiment verified that 6.23% MgO mechanically mixed with the ore was detectable. However, the MgO of the unmilled and milled ores was not detected in all XRD experiments. Thus, the Mg atoms in the concentrate must have replaced the Fe atoms in the crystal structure of FeTiO3 to form solid solution. The slight difference of interplannar distance between the ore and standard FeTiO3 from PDF also indicated that other kinds of atoms enter the cell of FeTiO3 and result in expansion.
600 min
Fig. 1 illustrates the XRD patterns of the ore samples milled for different times. All peaks match the standard XRD pattern of hexagonal structure FeTiO3. No new crystalline phase was observed. From the XRD patterns, the crystallite size (D) and lattice strain (e) were estimated by the method of Rietvald structure refinement. The variations of D and e with milling time are shown in Figs. 2 and 3. The results show that the crystallite sizes decreased and their lattice strains increased, with milling time. The initial crystallite size in c-axis was larger than that in a-axis. The crystallite sizes are reduced sharply in the initial stage of milling. For the ore milled longer than 20 min, the crystallite size in c-axis is smaller than that in a-axis. Thus, the energetic milling resulted in more cleavage along the a-axis although ilmenite does not have a recognized cleavage plane (Wang and Pan, 1982). In addition, the lattice strain in a-axis of the ilmenite unit cell varies more significantly than that in c-axis.
480 min 0.30
Intensity/CPS
a-axis 0.25
lattice strain/%
120 min
60 min
0.20
c-axis
0.15 0.10 0.05
unmilled 0.00 20
30
40
50
60
70
80
2Theta/deg Fig. 1. XRD of the samples milled for different times in a planetary mill.
0
2
4
6
8
10
activation time/h Fig. 3. Variation of the lattice strain with activation time.
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6
5
589.2
240 min
339.4
120 min
574.7 336.9
60 min
562.4 332.7 554.4460.9
313.4 528.4 447.4
3000
2000
1000
0
wavenumbers (cm-1)
3
2
1
0
100
200
300
400
500
600
milling time /min Fig. 6. Variation of specific surface area with milling time.
Fig. 4. IR patterns of the unmilled ore and samples milled for different times.
3.1.2. IR Fig. 4 illustrates the IR spectra of different samples. The ilmenite characteristic bands at 444.7 and 313.4 cm1 gradually disappeared after milling, and the band at 528.4 cm1 significantly shifted toward high wave number. Thus, mechanical milling enhanced the energy level split of molecules (Li et al., 1997), which increased the reactivity of ilmenite. Of course, the band variations can also be caused by other physical–chemical changes, such as the mechanochemical oxidation of the ilmenite. However, the standard IR spectra for the derivatives of ilmenite are not available. 3.2. Morphologic changes 3.2.1. Particle size analysis The particle size distributions of the unmilled sample and samples milled for various times are illustrated in
100
80
% undersize
4
0
unmilled
4000
BET surface area /m2 g-1
600 min
Fig. 5. The original ilmenite concentrate is composed of the particles with a d50 around 140 lm, in which only 20% of the particles are below 100 lm. After 30 min milling the distribution dramatically narrowed and the d50 decreased from 140 lm to 1.3 lm. Longer milling times up to 600 min essentially had a minor effect on the distribution. The d50 for 60 and 600 min slowly increased to 1.7 lm and 2.0 lm, respectively. About 20% of the largest particles with a size range of 6 20 lm was not affected by the extended milling. 3.2.2. Surface area analysis The BET surface area of the samples, as shown in Fig. 6, increased significantly with milling time with a maximum value of 5.3 m2/g achieved in 30 min of milling, beyond which the area began to decrease. The value at 600 min milling was only 1.7 m2/g. The change in the surface area agreed well with that in the d50, indicating that the size reduction was completed in the first 30 min. After that time, the rate of the re-welding (agglomeration) of particles exceeded that of the particle breakages. This is typical for milled solids. Godoeˇ˜ıkoka´ had also reported that the size reduction only occurred in the first 15 min in a planetary ball mill (Godocˇ´ıkova´ and Bala´zˇ, 2002).
60
3.3. Mechanochemical oxidation 2
40
1 4
20
3 0 0.1
1
10
100
size/μm Fig. 5. Particle size distributions of the unmilled ore and samples milled for different times. 1—unmilled, 2—30 min, 3—60 min and 4—600 min.
Chen (1997) conducted a thorough study on the mechanically activated oxidation of natural ilmenite sands in air. The ilmenite was gradually converted to thermally metastable Fe2Ti3O9 and c-Fe2O3 phases and this process was completed in 100 h of ball milling. No further reactions were observed during a prolonged milling or under a higher milling intensity. In this study thermogravimetric method was used to characterize the mechanochemical oxidation of samples. Mass increase may be observed in the TG experiments
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experiment, the pre-oxidation ratio of the sample milled under vacuum condition was only 3.1%.
4
1 3.5
3.4. Leaching experiments
% weight gain
3
2
Leaching curves in 50% sulfuric acid at 80 °C are shown in Fig. 8. Compared with the 4.3% Ti conversion in 120 min for the original ilmenite, 55.6%, 60.4%, 67.5% and 76.2% of titanium conversions were obtained for the samples milled for 60, 120, 240 and 600 min, respectively. The results illustrate that mechanical activation greatly enhanced the leaching reaction. Fig. 9 presents the leaching conversions at different temperatures of the sample milled in air for 1 h. Temperature significantly affected the leaching rate. At 100 °C, the
2.5
3 2 1.5
4 1 0.5 0
0
500
1000
1500
temperature/°C Fig. 7. Thermogravimetric plots of the unmilled ore and samples milled for various times. 1—unmilled, 2—60 min, 3—240 min and 4—600 min.
90 80
6
70
Milling time (min)
Oxidation temperature range (°C)
TG weight gain (%)
Calculated FeTiO3 oxidation ratios (%)
0 60 60 (under vacuum) 240 600
745–934 374–661 368–656
3.805 2.876 3.683
24.3 3.1
397–670 439–695
2.393 1.386
37.0 63.5
5
60
% Ti extracted
Table 2 Thermogravimetric data of samples milled for different times and their pre-oxidation ratios during milling operation
4
50
3 40
2 30 20 10
1 0 0
20
40
because the Fe contained in the ore was oxidized to Fe . As shown in Fig. 7, the milled samples were more active in oxidation than the original ilmenite ore. The starting temperature of oxidation reduced by 371 °C after milling ilmenite for 60 min (Table 2). Further milling caused starting temperature rising, which agrees with the increase of d50 (in Fig. 5). But the final temperatures for complete oxidation were very close for the milled samples, which is due to the same size distribution in the large particle range. According to the iron content in ilmenite (see Table 1), the theoretical mass increase due to absorbing oxygen from air in the oxidation of Fe2+ to Fe3+ is estimated to be 3.8%. The reaction can be expressed as (Chen, 1997): 6FeTiO3 þ 3=2O2 ¼ 2Fe2 Ti3 O9 þ Fe2 O3 The TG result (3.805%) of ilmenite is consistent with the theoretical value. Compared with the original ilmenite sample, the milled samples showed less mass increase in the TG process. Thus, part of the iron contained in the milled samples was pre-oxidized in the milling operation. From the TG mass change, the percentages of pre-oxidation are estimated to be 24.3% and 63.5% for 60 and 600 min milling, respectively. In addition, based on the mass increase in TG
60
80
100
120
leaching time/min
3+
Fig. 8. Effect of milling time and atmosphere on leaching of ilmenite at 80 °C. 1—unmilled, 2—60 min, 3—60 min under vacuum, 4—120 min, 5—240 min and 6—600 min.
90
100°C
80 70
% Ti extracted
2+
60
80°C
50
70°C
40 30
60°C
20
50°C
10 0 0
20
40
60
80
100
120
leaching time/min Fig. 9. Effect of temperature on leaching of 60 min milled ilmenite.
C. Li et al. / Minerals Engineering 19 (2006) 1430–1438
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5.5
120 min 50
5.0
mean particle size /μm
%Ti extracted
40
60 min 30
20
4.5
4.0
3.5
10
0
3.0
0
2
4
6
8
0
leaching time/day
20
40
60
80
100
120
leaching time/min
Fig. 10. Effect of milling time on leaching ilmenite at ambient temperature.
Fig. 11. Variation of mean particle size with leaching time for sample milled for 60 min and leached at 80 °C.
leaching conversion reached 82.1% in 120 min, which is commercially attractive. However, at a low temperature the leaching conversion was very low, for example, the leaching conversion at 50 °C is only 23.3%. Leaching the milled ilmenite at ambient temperature, as showed in Fig. 10, was low although the leaching time was as long as 8 days. It was 42% for the sample milled 1 hour and 50% for the sample milled 2 h. From these figures, one can find the most significant change in leaching conversion occurred in the initial stage. In the later stage, the slopes of leaching curves for milled samples are very close, which means they had the same reaction rate.
increased with the leaching conversion. The increase of activation energy indicates that large particles have lower reactivity than the fine particles. However, the values at high leaching ratios are unreasonably larger than the activation energy for unactivated Panzhihua ilmenite determined by Xu and Huang (1993) (44.7 kJ/mol) and (Li et al., 2005) (47.5 kJ/mol). Liang et al. (2005) reported an activation energy of 72.6 kJ/mol. However, it should be noted that the parameter was obtained with a concentrated sulfuric acid of 85 wt.% and at a higher leaching temperature up to 200 °C. Therefore, application of the iso-conversion model in the present study does not render results consistent with the literature. As described above, the specific surface area of milled samples reduced with increasing milling time after 30 min (Fig. 6). However, their leaching rate did not reduce with reduction of the surface area (Fig. 8). Thus, the fast dissolution in the initial period is not only associated with the large surface area of fine particles, but also the mechanical activation. To describe the leaching kinetics in the later stage, the variation of surface area with leaching conversion must be considered. The mass loss and the specific surface area of the sample milled for 1 h and leached with different titanium conversions were measured and the residual surface area was calculated. Comparing the leaching data in Fig. 9 with the surface area obtained, we find that the relationship between them can be expressed as following:
3.5. Leaching kinetics 3.5.1. Kinetic modeling Particle size of leaching residue during leaching process was measured at different reaction times. The mean particles size (d50) increased monotonically with time when the 60 min milled sample was leached at 80 °C (see Fig. 11). The results can be interpreted as the more rapid dissolution of small particles since the fine particles have high reactivity due to their large surface energy per unit volume. Similar leaching behavior for milled minerals was also observed elsewhere (Welham and Llewellyn, 1998). The simplified shrinking core model (Levenspiel, 1999), which hypotheses the grain size unchangeable or gradually decreasing (depending on if there is a solid product to form on the unreacted core) in liquid–solid reaction, is not suitable to describe the leaching process. The iso-conversion model (Sewry and Brown, 2002) had also been examined. Fitting of the leaching kinetic data (in Fig. 9) in the temperature range of 50–80 °C to the model yielded the apparent activation energies of 34.4, 82.0 and 96.2 kJ/mol at the leach ratios of 10%, 20% and 30%, respectively. As described previously, the particle size
s ¼ a þ becx
ð1Þ
where s is the surface area (m2) of the residue when 1 g of milled sample was leached for a time period of t, the parameters a, b, and c are 1.184, 4.841, and 9.346, respectively, e is the base of natural logarithms, and x is leaching conversion.
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Assume the dissolution in the late stage was chemically limited, kinetic equation can be written as: a0 dx=dt ¼ kscns
0.20 0.18
ð2Þ
ð1=acÞ ln½ecx =ða þ becx Þ ¼ cns k t=a0 þ c0
0.16 0.14
lattice strain/%
where a0 is the FeTiO3 content of ilmenite, k is the reaction rate constant, cs is the sulfuric acid concentration which is constant because excess amount was used, n is the order of reaction with respect to sulfuric acid. Substitution of Eq. (1) into Eq. (2) and integration of both sides lead to:
0.08
0.04
where c0 is the integration constant. Let parameter P = (1/a c)ln[ecx/(a + becx)], k 0 ¼ cns k=a0 , then the dissolution kinetic model for the later stage can be simplified as:
0.02
c-axis
0.00 0
20
3.6. Dissolution mechanism The 60 min milled sample was leached at 80 °C. The leaching residues were examined by XRD. The lattice
40
60
80
100
leaching time/min
ð4Þ
3.5.2. Activation energy Fitting the experimental data in the temperature range of 50–80 °C in Fig. 9 to the model (Eq. (4)) showed an excellent agreement (Fig. 12). From the apparent reaction rate constant determined by the fitting, the Arrhenius activation energy for the stage was estimated to be 48.1 kJ/ mol. It is within the range typical of chemical control. In the model, P = c0 at t = 0. As shown in Fig. 12, however, the P value at t = 0 varied with leaching temperature. It means the model cannot be extrapolated to the initial stage. In addition, with increasing leaching temperature, the occurrence of chemical reaction limited stage was postponed. To roughly estimate the initial activation energy, the average leaching rate was calculated from the conversion at 10 min. The initial Arrhenius activation energy was about 28.4 kJ/mol.
Fig. 13. Variation of lattice strain with leaching time for sample milled for 60 min and leached at 80 °C.
strains were calculated from the XRD patterns by the method of Rietveld structure refinement and presented in Fig. 13. Both the lattice strains in the a-axis and c-axis reduced rapidly in the initial leaching period. The strain in a-axis dropped down to the base line in 10 min, while that in c-axis continuously decreased in the first 60 min. After that, the c-axis strain of leaching residue lowered to the value close to that of unmilled ilmenite sample (Fig. 3). The strain in the a-axis, however, still maintained in a higher level than that of the original ilmenite during extended leaching. A comparison experiment was conducted as leaching the 60 min milled ilmenite sample at ambient temperature without any agitation. The leaching residues were also measured by XRD, and the lattice strains calculated. As shown in Fig. 14, the lattice strain in a-axis kept constant in an even higher level than that with the dissolution at
0.22
0.55
80°C
a-axis
0.20
0.50
0.18 0.16
lattice strain/%
0.45
parameter P
a-axis
0.10
0.06
ð3Þ
P ¼ k 0 t þ c0
0.12
70°C
0.40 0.35
60°C
0.30
0.12 0.10 0.08 0.06
50°C
0.25
0.14
c-axis
0.04 0.20
0.02
0 0
20
40
60
80
100
120
leaching time/min Fig. 12. Fit of the model Eq. (4) to the leaching data (in Fig. 9).
2
4
6
8
leaching time/day Fig. 14. Variation of lattice strain with leaching time for sample milled for 60 min and leached at ambient temperature time.
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80 °C, but the strain in c-axis decreased with leaching time. The result indicates that the chemical reactivity on different crystal face is different from each other. The leaching reaction is more sensitive to the strain in the c-axis. 4. Discussion Both IR and XRD analyses demonstrated that milling operation can change the crystalline structure of ilmenite. The breakage of ilmenite crystal may cause defects on the crystal faces, and increase in the reactivity of ilmenite. Based on the surface area measurements and the leaching results of the milled ilmenite, one can infer that the leaching rate increased monotonically with milling time although the surface area decreased when the sample was milled for a time over 30 min. Thus, an energetic milling enhances the dissolution of ilmenite by not only increasing its surface area but also elevating its chemical activity. This conclusion was also arrived by Bala´zˇ (2003). The thermogravimetric experiments confirmed the occurrence of oxidation during the mechanical activation of ilmenite, even though the XRD analysis failed to directly detect Fe2Ti3O9 product phase, which is also in support of the report (Chen et al., 1999) on the phase with a poorly crystallized structure. Barton and McCornnel (1979) have reported that FeTiO3 phase dissolved more efficiently than Fe2Ti3O9 in sulfuric acid solution. However, our leaching experiments did not show much difference between samples milled in air and in vacuum (Fig. 8). The result might indicate that the Fe2Ti3O9 phase dispersed in FeTiO3 can quickly dissolve in the leaching process. Thus, milling Panzhihua ilmenite in air has less effect on the mechanical activation efficiency. Welham and Llewellyn inferred that the pressure generated during the impact of balls onto the powder would tend to break the ilmenite along the (0 0 0 1) and (1 0 1 1) planes (Welham and Llewellyn, 1998). It is confirmed in the present study. From the XRD analysis, the cleavage of ilmenite occurred more preferentially along the a-axis. Since (0 0 0 1) crystal face is parallel to the a-axis, more (0 0 0 1) crystal faces are expected to expose out in the milled samples. Both Barton and McCornnel (1979) and Duncan and Metson (1982) reported that the dissolution of nonactivated ilmenite was more rapid in the basal (0 0 0 1) and (1 0 1 1) planes. We also observed that any normal displacement between (0 0 0 1) crystal faces [i.e., the lattice strain in the c-direction] affects the reaction rate stronger than that between (1 0 0 0) or (0 1 0 0) crystal faces. The leaching reaction is likely to start from the (0 0 0 1) crystal face at a lower temperature. Only at high temperatures, the selectivity of leaching on different crystal faces is decreased. The leaching kinetic curves showed that the leaching process included two stages. The initial dissolution stage was rapid and had a low activation energy (28.4 kJ/mol). The low activation energy presents the character of a diffusion-limited process. It is clearly attributed to the dissolution of both the fine particles and highly reactive crystal
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faces (e.g., (0 0 0 1)). In the later stage, the apparent activation energy approaches the value of unactivated ilmenite due to the consumption of the highly reactive fine particles and crystal faces. Thus, the ball mill only partially activated the ilmenite, which is in agreement with the results obtained by Li et al. (2005) with a tumbling ball mill. Therefore, the mechanical activation prior to leaching mainly enhances the dissolution kinetics of early stage. However, a simultaneous milling and leaching with a stirring ball mill significantly improved the whole dissolution kinetics of Panzhihua ilmenite (Li et al., 2006). The structural defects induced by energetic ball milling include both effective and ineffective defects. The effective defects, i.e., the strain in the c-axis of ilmenite unit cell, strongly affects the leaching rate, while the ineffective defects, i.e., the strain in the a-axis, have minimal effect on the leaching rate. 5. Conclusion Mechanical activation significantly accelerated the dissolution of Panzhihua ilmenite in a 50% sulfuric acid solution. As high as 82.1% of conversion was obtained by leaching the 60 min milled ilmenite at 100 °C for 2 h. The leaching rate was apparently linked with the lattice strain of ilmenite crystal. The dissolution reaction responded more sensitively to the strain in the c-axis than that in the a-axis. At low temperature, the dissolution reaction is likely to start along the direction of (0 0 0 1) crystal face. Mechanical milling prior to leaching only partially activated the ilmenite. In the initial period, the activated part [i.e., the fine particles and highly reactive crystal faces] of ilmenite rapidly dissolved and the apparent activation energy was 28.4 kJ/mol, which showed the characteristics of a diffusion-limited process. In the later period, the leaching activation energy increased to approach the value obtained by leaching original ilmenite. It was considered to be the leaching of unactivated part. References Amer, A.M., 2002. Alkaline pressure leaching of mechanically activated Rosetta ilmenite concentrate. Hydrometallurgy 67, 125–133. Bala´zˇ, P., 2003. Mechanical activation in hydrometallurgy. International Journal of Mineral Processing 72, 341–354. Bala´zˇ, P., Ficeriova´, J., Leon, C.V., 2003. Silver leaching from a mechanochemically pretreated complex sulfide concentrate. Hydrometallurgy 70, 113–119. Barksdale, J., 1966. Titanium, Its Occurrence, Chemistry and Technology, second ed. Ronald Press, New York. Barton, A.F., McCornnel, S.R., 1979. Rotating disc dissolution rates of ionic solids. Part 3. Natural and synthetic ilmenites. Transaction of Faraday Society 75, 971–983. Chen, Y., 1997. Low-temperature oxidation of ilmenite (FeTiO3) induced by high energy ball milling at room temperature. Journal of Alloys and Compounds 257, 156–160. Chen, Y., Williams, J.S., Campbell, S.J., Wang, G.M., 1999. Increased dissolution of ilmenite induced by high-energy ball milling. Materials Science and Engineering A 271, 485–490.
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Duncan, J.F., Metson, J.B., 1982. Acid attack on New Zealand ilmenite. I. The mechanism of dissolution. New Zealand Journal of Science 25, 103–109. Ficeriova´, J., Bala´zˇ, P., Boldizˇa´rova´, E., 2002. Thiosulfate leaching of gold from a mechanically activated CuPbZn concentrate. Hydrometallurgy 67, 37–43. Godocˇ´ıkova´, E., Bala´zˇ, P., 2002. Spectroscopic study of the surface oxidation of mechanically activated sulfides. Applied Surface Science 200, 36–47. Jing, J.L., Zhang, Q.Z., Qiu, L.Y., Liang, B., 2003. An Investigation of Liquid Phase Digestion of Ilmenite for Sulfate Process TiO2 Pigment Production. Chemical Reaction Engineering and Process 19 (4), 337– 343 (in Chinese). Levenspiel, O., 1999. Chemical Reaction Engineering, third ed. John Wiley & Sons, New York. Liang, B., Li, C., Zhang, C.G., Zhang, Y.K., 2005. Leaching kinetics of Panzhihua ilmenite in sulfuric acid. Hydrometallurgy 76 (3–4), 173– 179. Li, H.G., Zhao, Z.W., Zhao, T.C., 1997. Effect of experimental facility style on the activation of mineral. Journal of Central South University of Technology 28 (2), 130–133 (in Chinese). Li, C., Liang, B., Liang, X.M., 2005. Leaching Kinetics of Mechanically Activated Ilmenite Ore. Journal of Sichuan University 37 (1), 35–38 (in Chinese).
Li, C., Liang, B., Chen, S.P., 2006. Milling dissolution of Panzhihua ilmenite with dilute sulfuric acid. Hydrometallurgy, in press. Rahm, J.A., Cole, D., 1981. US Patent 4288415. Sasikumar, C., Rao, D.S., Srikanth, S., Ravikumar, B., 2004. Effect of mechanical activation on the kinetics of sulfuric leaching of beach sand ilmenite from Orissa, India. Hydrometallurgy 75, 189–204. Sewry, J.D., Brown, M.E., 2002. ‘‘Model-free’’ kinetic analysis? Thermochimica Acta 390, 217–225. She, Z.H., Xu, S., Fan, J.Q., 1998. Decomposing Titanium Concentrate with Acid in Liquid Phase by Fluidization. Complex Utilization of Mineral Resources 31 (2), 22–26 (in Chinese). Thompson, P., Cox, D.E., Hastings, J.B., 1987. Rietveld Refinement of Debye–Scherrer Synchrotron X-ray Data from Al2O3. Journal of Applied Crystallography 20, 79–83. Wang, P., Pan, Z.L., 1982. The System of Mineralogy, vol. 1. Geology Press, Beijing. Welham, N.J., 2001. Enhanced dissolution of tantalite/columbite following milling. International Journal of Mineral Processing 61, 145– 154. Welham, N.J., Llewellyn, D.J., 1998. Mechanical Enhancement of the Dissolution of Ilmenite. Minerals Engineering 11 (9), 827–841. Xu, S., Huang, Z.S., 1993. The Kinetics of Panzhihua Ilmenite Leaching with Sulfuric Acid. Journal of Mining and Metallurgy Engineering 13, 44–48 (in Chinese).
Effect of mechanical activation on the dissolution of Panzhihua ilmenite Chun Li a
a,*
, Bin Liang a, Ling-hong Guo b, Zi-bin Wu
a
Multi-phases Mass Transfer and Reaction Engineering Laboratory, Department of Chemical Engineering, Sichuan University, 24, South one Cycle Road, Chengdu 610065, PR China b Analytic and Testing Center, Sichuan University, Chengdu 610065, PR China Received 24 October 2005; accepted 21 February 2006 Available online 19 May 2006
Abstract Dissolution of mechanically activated Panzhihua ilmenite was carried out at 100 °C with a 50% sulfuric acid solution. The particle size, specific surface area, crystallite size, lattice strain and mechano-chemical reaction of milled ilmenite were measured. The cleavage induced by energetic milling occurred more preferentially along the a-axis of ilmenite unit cell. It was found that the defects formed in milling process, especially the lattice strain in the c-axis of ilmenite unit cell, greatly enhanced the leaching reaction. At low temperatures, the leaching reaction is likely to start along the direction of the basal plane (0 0 0 1) of milled ilmenite. With the mechanically activated ilmenite, leaching conversion at 100 °C reached 82.1%. However, the leaching kinetic tests revealed that the ball milling only partially activated the ilmenite. The initial rapid dissolution is due to the leaching of both the fine powders and highly reactive crystal faces. In the later stage, the dissolution exhibited essentially the same behavior as non-activated ilmenite. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ilmenite; Mechanical activation; Leaching; Kinetics; Mechanism
1. Introduction Panzhihua ilmenite accounts for more than 90% of titanium reserves of China. After the removal of vanadium and iron, the ilmenite concentrate contains TiO2 6 48 wt.%, Fe2O3 6 wt.%, and MgO 6 wt.%. Because of its high impurity contents, Panzhihua ilmenite is not suitable for the chlorination process to produce TiO2 pigment. The ilmenite concentrate is used in the sulfate process to manufacture TiO2 pigment. In the sulfate process, ilmenite is dissolved with concentrated sulfuric acid (about 85 wt.%) at temperatures up to 200 °C. The resultant aqueous solution is subject to iron removal before titania precursor is precipitated. The precursor is then calcined to produce TiO2 pigment. More than 90% of TiO2 pigment plants in China are operated with the sulfate process. However, the sulfate pro*
Corresponding author. Tel.: +86 28 85408056; fax: +86 28 85461108. E-mail address: [email protected] (C. Li).
0892-6875/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2006.02.005
cess faces severe environmental challenges. The digestion reaction of ilmenite with concentrated sulfuric acid is highly exothermic which can cause catastrophe if not properly controlled (Liang et al., 2005). In addition, the dilute spent acid (20 wt.%) formed during hydrolysis is not presently recycled to the process, which causes not only environmental problems but also the waste of sulfur resource. Leaching ilmenite with dilute acid can ease the flue gas pollution due to low reaction temperature, and also reduce the waste acid release by partially recycling the dilute acid. However, the reaction rate is very slow. NL corporation (Rahm and Cole, 1981) reported that 48 h reaction time were required for digesting ilmenite with 20–65% sulfuric acid in a multi-stage batch reactor series to achieve a Ti extraction ratio of 95%. She et al. (1998) obtained only 68% of Ti extraction when digesting Panzhihua ilmenite with a 30–50% sulfuric acid solution in a continuously operated fluidized bed. Jing et al. (2003) proposed a multi-step leaching operation with 65% sulfuric acid. However, it was still limited by the low reactivity.
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Mechanical activation, usually carried out by energetic milling, was reported to significantly accelerate the dissolution rate of solid minerals by increasing their chemical reactivity (Ficeriova´ et al., 2002; Bala´zˇ et al., 2003; Welham, 2001; Amer, 2002; Chen et al., 1999; Sasikumar et al., 2004; Welham and Llewellyn, 1998). However, contradictory results about the mechanical activation of ilmenite were reported. Chen et al. (1999) observed that ilmenite was gradually converted to pseudo-rutile (Fe2Ti3O9) during milling in air. However, Sasikumar et al. (2004) reported that the pseudo-rutile gradually disappeared and no mechanochemical oxidation of Fe2+ occurred with milling. Welham and Llewellyn (1998) noted that after 25 h milling the a-parameter of ilmenite unit cell decreased while the c-parameter remained unchanged. They suggested that the milling caused strain preferentially along the a-axis. On the contrary, Sasikumar et al. (2004) observed the cparameter firstly increased linearly with milling time up to 240 min whereas the a-parameter was kept constant in the first 90 min and then increased, which implied the initial strain along the c-axis. The leaching process of mechanically activated ilmenite, as reported by Welham and Llewellyn (1998), includes two stages. At the beginning, the leaching reaction is fast with low apparent activation energy (Ea = 15 kJ/mol). In the late stage, the apparent activation energy increases up to 70 kJ/mol. They inferred that the initial reaction was linked with the dissolution of the crystal faces with higher chemical reactivity. However, they did not provide sufficient evidence in support of their claim. Sasikumar et al. (2004) also described the similar leaching phenomenon. They did not distinguish the activation energies between the initial and late stages and found that the activation energies changed in 53.9–62.0 kJ/mol depending on the milling time. To date only literatures (Amer, 2002; Chen et al., 1999; Sasikumar et al., 2004; Welham and Llewellyn, 1998) are found concerning the effect of mechanical activation on structure and morphology of beach sand type ilmenite and its dissolution. In this work, we studied the leaching of the Panzhihua ilmenite of rock-type origin by dilute sulfuric acid with mechanical activation. The objective was to examine the effect of mechanical activation on leaching performance. Structure and morphology of the ore were also characterized. 2. Experimental 2.1. Material The ilmenite was obtained from Titanium Company of Panzhihua Steel & Iron (Group) Corporation, Sichuan, China. It is in a size range of 150 + 100 lm. Its chemical composition, obtained with an X-ray fluorescence Spectrometer (Phillips DW/480), is shown in Table 1. XRD results (detected by a Phillips X’pert Pro. MPD) showed that the major mineral constitution was hexagonal structured FeTiO3.
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Table 1 Chemical composition of the ilmenite used (wt.%) FeO
TiO2
Fe2O3
MgO
SiO2
Al2O3
MnO2
34.21
47.25
5.56
6.23
2.75
1.49
0.61
2.2. Mechanical activation A vertical planetary ball mill (QM-1SP2, Nanjing, China) with a rotation speed of 200 rpm and a spin rate of 500 rpm was employed in the mechanical activation of ilmenite. The porcelain cells were about 100 ml and balls used were 8 mm in diameter. The mass ratio of ball/ilmenite was 20:1 in the milling operation. The ilmenite, milled under either atmospheric or vacuum conditions (residual pressure 6 16.3 kPa), was characterized to detect the structural changes and leached to measure its reactivity. 2.3. Leaching experiments Milled samples were leached in a 500 ml four-necked glass reactor. An agitator, a condenser and a thermometer were fitted into its openings. The fourth opening was used to feed reactants and withdraw the slurry samples. The reactants were heated using an oil bath which controlled the temperature to within ±2 °C. In each experiment, 400 ml 50% sulfuric acid solution was preheated to the temperature required before 4 g milled sample was added. The agitation was kept constant at 500 ± 10 rpm during the entire leaching operation. A comparison test was conducted without agitation at ambient temperature (25–30 °C), in which 5 g milled sample reacted with 100 ml 50% sulfuric acid solution in a 300 ml sealed glass reactor. The slurry was sampled during the leaching process to measure the titanium conversion. The sampled slurry was filtered and washed before titrated with a reduction–oxidation titration method (Barksdale, 1966). The residual solid was characterized. 2.4. Characterization The ilmenite ore, milled powder and leaching residue were characterized by XRD, infrared (Fourier transform) spectroscopy, thermal analysis, particle size analysis and surface area measurement. XRD experiments were performed using XRD (X’Pert Pro MPD, Philips, Netherlands). The voltage and anode current used were 40 kV and 40 mA, respectively. The CuKa = 0.15405 nm and continuously scanning mode with 0.02 interval and 0.25 s of set time were used to collect the XRD patterns of samples. Further, parallel X-ray incident beams of hybrid mirror with divergence slit of 1/8° and diffracted beam of the parallel plate collimator with Soller slit 0.04 Rad and prepositional detector were used in all XRD experiments. The structure refinement of FeTiO3 was performed on Cerius2 platform (MSI company, USA). The Rietveld method was embedded in DWBS90 software
C. Li et al. / Minerals Engineering 19 (2006) 1430–1438
package in Cerius2 (Thompson et al., 1987). The original crystal model was constructed based on the data with Cerius2. The principle of Rietveld is refining the parameters of original crystal model to fit the XRD experimental data with the Netwon–Raphson algorithm. The parameters to be refined in the present study were more than 40, but only lattice strain and crystallite size were selected to characterize the milling process. Measurement of vibration spectra was performed using a Nicolet MX-1E FTIR spectrometer. Thermal analysis experiments were carried out on a simultaneous TG/ DTA analyzer (Seiko, Japan, EXSTAR6000), with a heating rate of 10 K/min and an air flow rate of 300 ml/min. Surface area measurement was conducted using a multipoint BET technique (Quantachrome, America, NOVA3000), and the particle size analysis was carried out using a laser diffraction analyzer (China, JL-1178).
100 90 80
crystallite size/nm
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70 60 50
a-axis
40 30
c-axis
20 0
2
4
6
8
10
activation time/h Fig. 2. Variation of the crystallite size with activation time.
3. Results 3.1. Structural changes 3.1.1. X-ray diffraction From the XRF results, the 6.23% MgO was detected in the concentrate. The XRD experiment verified that 6.23% MgO mechanically mixed with the ore was detectable. However, the MgO of the unmilled and milled ores was not detected in all XRD experiments. Thus, the Mg atoms in the concentrate must have replaced the Fe atoms in the crystal structure of FeTiO3 to form solid solution. The slight difference of interplannar distance between the ore and standard FeTiO3 from PDF also indicated that other kinds of atoms enter the cell of FeTiO3 and result in expansion.
600 min
Fig. 1 illustrates the XRD patterns of the ore samples milled for different times. All peaks match the standard XRD pattern of hexagonal structure FeTiO3. No new crystalline phase was observed. From the XRD patterns, the crystallite size (D) and lattice strain (e) were estimated by the method of Rietvald structure refinement. The variations of D and e with milling time are shown in Figs. 2 and 3. The results show that the crystallite sizes decreased and their lattice strains increased, with milling time. The initial crystallite size in c-axis was larger than that in a-axis. The crystallite sizes are reduced sharply in the initial stage of milling. For the ore milled longer than 20 min, the crystallite size in c-axis is smaller than that in a-axis. Thus, the energetic milling resulted in more cleavage along the a-axis although ilmenite does not have a recognized cleavage plane (Wang and Pan, 1982). In addition, the lattice strain in a-axis of the ilmenite unit cell varies more significantly than that in c-axis.
480 min 0.30
Intensity/CPS
a-axis 0.25
lattice strain/%
120 min
60 min
0.20
c-axis
0.15 0.10 0.05
unmilled 0.00 20
30
40
50
60
70
80
2Theta/deg Fig. 1. XRD of the samples milled for different times in a planetary mill.
0
2
4
6
8
10
activation time/h Fig. 3. Variation of the lattice strain with activation time.
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6
5
589.2
240 min
339.4
120 min
574.7 336.9
60 min
562.4 332.7 554.4460.9
313.4 528.4 447.4
3000
2000
1000
0
wavenumbers (cm-1)
3
2
1
0
100
200
300
400
500
600
milling time /min Fig. 6. Variation of specific surface area with milling time.
Fig. 4. IR patterns of the unmilled ore and samples milled for different times.
3.1.2. IR Fig. 4 illustrates the IR spectra of different samples. The ilmenite characteristic bands at 444.7 and 313.4 cm1 gradually disappeared after milling, and the band at 528.4 cm1 significantly shifted toward high wave number. Thus, mechanical milling enhanced the energy level split of molecules (Li et al., 1997), which increased the reactivity of ilmenite. Of course, the band variations can also be caused by other physical–chemical changes, such as the mechanochemical oxidation of the ilmenite. However, the standard IR spectra for the derivatives of ilmenite are not available. 3.2. Morphologic changes 3.2.1. Particle size analysis The particle size distributions of the unmilled sample and samples milled for various times are illustrated in
100
80
% undersize
4
0
unmilled
4000
BET surface area /m2 g-1
600 min
Fig. 5. The original ilmenite concentrate is composed of the particles with a d50 around 140 lm, in which only 20% of the particles are below 100 lm. After 30 min milling the distribution dramatically narrowed and the d50 decreased from 140 lm to 1.3 lm. Longer milling times up to 600 min essentially had a minor effect on the distribution. The d50 for 60 and 600 min slowly increased to 1.7 lm and 2.0 lm, respectively. About 20% of the largest particles with a size range of 6 20 lm was not affected by the extended milling. 3.2.2. Surface area analysis The BET surface area of the samples, as shown in Fig. 6, increased significantly with milling time with a maximum value of 5.3 m2/g achieved in 30 min of milling, beyond which the area began to decrease. The value at 600 min milling was only 1.7 m2/g. The change in the surface area agreed well with that in the d50, indicating that the size reduction was completed in the first 30 min. After that time, the rate of the re-welding (agglomeration) of particles exceeded that of the particle breakages. This is typical for milled solids. Godoeˇ˜ıkoka´ had also reported that the size reduction only occurred in the first 15 min in a planetary ball mill (Godocˇ´ıkova´ and Bala´zˇ, 2002).
60
3.3. Mechanochemical oxidation 2
40
1 4
20
3 0 0.1
1
10
100
size/μm Fig. 5. Particle size distributions of the unmilled ore and samples milled for different times. 1—unmilled, 2—30 min, 3—60 min and 4—600 min.
Chen (1997) conducted a thorough study on the mechanically activated oxidation of natural ilmenite sands in air. The ilmenite was gradually converted to thermally metastable Fe2Ti3O9 and c-Fe2O3 phases and this process was completed in 100 h of ball milling. No further reactions were observed during a prolonged milling or under a higher milling intensity. In this study thermogravimetric method was used to characterize the mechanochemical oxidation of samples. Mass increase may be observed in the TG experiments
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experiment, the pre-oxidation ratio of the sample milled under vacuum condition was only 3.1%.
4
1 3.5
3.4. Leaching experiments
% weight gain
3
2
Leaching curves in 50% sulfuric acid at 80 °C are shown in Fig. 8. Compared with the 4.3% Ti conversion in 120 min for the original ilmenite, 55.6%, 60.4%, 67.5% and 76.2% of titanium conversions were obtained for the samples milled for 60, 120, 240 and 600 min, respectively. The results illustrate that mechanical activation greatly enhanced the leaching reaction. Fig. 9 presents the leaching conversions at different temperatures of the sample milled in air for 1 h. Temperature significantly affected the leaching rate. At 100 °C, the
2.5
3 2 1.5
4 1 0.5 0
0
500
1000
1500
temperature/°C Fig. 7. Thermogravimetric plots of the unmilled ore and samples milled for various times. 1—unmilled, 2—60 min, 3—240 min and 4—600 min.
90 80
6
70
Milling time (min)
Oxidation temperature range (°C)
TG weight gain (%)
Calculated FeTiO3 oxidation ratios (%)
0 60 60 (under vacuum) 240 600
745–934 374–661 368–656
3.805 2.876 3.683
24.3 3.1
397–670 439–695
2.393 1.386
37.0 63.5
5
60
% Ti extracted
Table 2 Thermogravimetric data of samples milled for different times and their pre-oxidation ratios during milling operation
4
50
3 40
2 30 20 10
1 0 0
20
40
because the Fe contained in the ore was oxidized to Fe . As shown in Fig. 7, the milled samples were more active in oxidation than the original ilmenite ore. The starting temperature of oxidation reduced by 371 °C after milling ilmenite for 60 min (Table 2). Further milling caused starting temperature rising, which agrees with the increase of d50 (in Fig. 5). But the final temperatures for complete oxidation were very close for the milled samples, which is due to the same size distribution in the large particle range. According to the iron content in ilmenite (see Table 1), the theoretical mass increase due to absorbing oxygen from air in the oxidation of Fe2+ to Fe3+ is estimated to be 3.8%. The reaction can be expressed as (Chen, 1997): 6FeTiO3 þ 3=2O2 ¼ 2Fe2 Ti3 O9 þ Fe2 O3 The TG result (3.805%) of ilmenite is consistent with the theoretical value. Compared with the original ilmenite sample, the milled samples showed less mass increase in the TG process. Thus, part of the iron contained in the milled samples was pre-oxidized in the milling operation. From the TG mass change, the percentages of pre-oxidation are estimated to be 24.3% and 63.5% for 60 and 600 min milling, respectively. In addition, based on the mass increase in TG
60
80
100
120
leaching time/min
3+
Fig. 8. Effect of milling time and atmosphere on leaching of ilmenite at 80 °C. 1—unmilled, 2—60 min, 3—60 min under vacuum, 4—120 min, 5—240 min and 6—600 min.
90
100°C
80 70
% Ti extracted
2+
60
80°C
50
70°C
40 30
60°C
20
50°C
10 0 0
20
40
60
80
100
120
leaching time/min Fig. 9. Effect of temperature on leaching of 60 min milled ilmenite.
C. Li et al. / Minerals Engineering 19 (2006) 1430–1438
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5.5
120 min 50
5.0
mean particle size /μm
%Ti extracted
40
60 min 30
20
4.5
4.0
3.5
10
0
3.0
0
2
4
6
8
0
leaching time/day
20
40
60
80
100
120
leaching time/min
Fig. 10. Effect of milling time on leaching ilmenite at ambient temperature.
Fig. 11. Variation of mean particle size with leaching time for sample milled for 60 min and leached at 80 °C.
leaching conversion reached 82.1% in 120 min, which is commercially attractive. However, at a low temperature the leaching conversion was very low, for example, the leaching conversion at 50 °C is only 23.3%. Leaching the milled ilmenite at ambient temperature, as showed in Fig. 10, was low although the leaching time was as long as 8 days. It was 42% for the sample milled 1 hour and 50% for the sample milled 2 h. From these figures, one can find the most significant change in leaching conversion occurred in the initial stage. In the later stage, the slopes of leaching curves for milled samples are very close, which means they had the same reaction rate.
increased with the leaching conversion. The increase of activation energy indicates that large particles have lower reactivity than the fine particles. However, the values at high leaching ratios are unreasonably larger than the activation energy for unactivated Panzhihua ilmenite determined by Xu and Huang (1993) (44.7 kJ/mol) and (Li et al., 2005) (47.5 kJ/mol). Liang et al. (2005) reported an activation energy of 72.6 kJ/mol. However, it should be noted that the parameter was obtained with a concentrated sulfuric acid of 85 wt.% and at a higher leaching temperature up to 200 °C. Therefore, application of the iso-conversion model in the present study does not render results consistent with the literature. As described above, the specific surface area of milled samples reduced with increasing milling time after 30 min (Fig. 6). However, their leaching rate did not reduce with reduction of the surface area (Fig. 8). Thus, the fast dissolution in the initial period is not only associated with the large surface area of fine particles, but also the mechanical activation. To describe the leaching kinetics in the later stage, the variation of surface area with leaching conversion must be considered. The mass loss and the specific surface area of the sample milled for 1 h and leached with different titanium conversions were measured and the residual surface area was calculated. Comparing the leaching data in Fig. 9 with the surface area obtained, we find that the relationship between them can be expressed as following:
3.5. Leaching kinetics 3.5.1. Kinetic modeling Particle size of leaching residue during leaching process was measured at different reaction times. The mean particles size (d50) increased monotonically with time when the 60 min milled sample was leached at 80 °C (see Fig. 11). The results can be interpreted as the more rapid dissolution of small particles since the fine particles have high reactivity due to their large surface energy per unit volume. Similar leaching behavior for milled minerals was also observed elsewhere (Welham and Llewellyn, 1998). The simplified shrinking core model (Levenspiel, 1999), which hypotheses the grain size unchangeable or gradually decreasing (depending on if there is a solid product to form on the unreacted core) in liquid–solid reaction, is not suitable to describe the leaching process. The iso-conversion model (Sewry and Brown, 2002) had also been examined. Fitting of the leaching kinetic data (in Fig. 9) in the temperature range of 50–80 °C to the model yielded the apparent activation energies of 34.4, 82.0 and 96.2 kJ/mol at the leach ratios of 10%, 20% and 30%, respectively. As described previously, the particle size
s ¼ a þ becx
ð1Þ
where s is the surface area (m2) of the residue when 1 g of milled sample was leached for a time period of t, the parameters a, b, and c are 1.184, 4.841, and 9.346, respectively, e is the base of natural logarithms, and x is leaching conversion.
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Assume the dissolution in the late stage was chemically limited, kinetic equation can be written as: a0 dx=dt ¼ kscns
0.20 0.18
ð2Þ
ð1=acÞ ln½ecx =ða þ becx Þ ¼ cns k t=a0 þ c0
0.16 0.14
lattice strain/%
where a0 is the FeTiO3 content of ilmenite, k is the reaction rate constant, cs is the sulfuric acid concentration which is constant because excess amount was used, n is the order of reaction with respect to sulfuric acid. Substitution of Eq. (1) into Eq. (2) and integration of both sides lead to:
0.08
0.04
where c0 is the integration constant. Let parameter P = (1/a c)ln[ecx/(a + becx)], k 0 ¼ cns k=a0 , then the dissolution kinetic model for the later stage can be simplified as:
0.02
c-axis
0.00 0
20
3.6. Dissolution mechanism The 60 min milled sample was leached at 80 °C. The leaching residues were examined by XRD. The lattice
40
60
80
100
leaching time/min
ð4Þ
3.5.2. Activation energy Fitting the experimental data in the temperature range of 50–80 °C in Fig. 9 to the model (Eq. (4)) showed an excellent agreement (Fig. 12). From the apparent reaction rate constant determined by the fitting, the Arrhenius activation energy for the stage was estimated to be 48.1 kJ/ mol. It is within the range typical of chemical control. In the model, P = c0 at t = 0. As shown in Fig. 12, however, the P value at t = 0 varied with leaching temperature. It means the model cannot be extrapolated to the initial stage. In addition, with increasing leaching temperature, the occurrence of chemical reaction limited stage was postponed. To roughly estimate the initial activation energy, the average leaching rate was calculated from the conversion at 10 min. The initial Arrhenius activation energy was about 28.4 kJ/mol.
Fig. 13. Variation of lattice strain with leaching time for sample milled for 60 min and leached at 80 °C.
strains were calculated from the XRD patterns by the method of Rietveld structure refinement and presented in Fig. 13. Both the lattice strains in the a-axis and c-axis reduced rapidly in the initial leaching period. The strain in a-axis dropped down to the base line in 10 min, while that in c-axis continuously decreased in the first 60 min. After that, the c-axis strain of leaching residue lowered to the value close to that of unmilled ilmenite sample (Fig. 3). The strain in the a-axis, however, still maintained in a higher level than that of the original ilmenite during extended leaching. A comparison experiment was conducted as leaching the 60 min milled ilmenite sample at ambient temperature without any agitation. The leaching residues were also measured by XRD, and the lattice strains calculated. As shown in Fig. 14, the lattice strain in a-axis kept constant in an even higher level than that with the dissolution at
0.22
0.55
80°C
a-axis
0.20
0.50
0.18 0.16
lattice strain/%
0.45
parameter P
a-axis
0.10
0.06
ð3Þ
P ¼ k 0 t þ c0
0.12
70°C
0.40 0.35
60°C
0.30
0.12 0.10 0.08 0.06
50°C
0.25
0.14
c-axis
0.04 0.20
0.02
0 0
20
40
60
80
100
120
leaching time/min Fig. 12. Fit of the model Eq. (4) to the leaching data (in Fig. 9).
2
4
6
8
leaching time/day Fig. 14. Variation of lattice strain with leaching time for sample milled for 60 min and leached at ambient temperature time.
C. Li et al. / Minerals Engineering 19 (2006) 1430–1438
80 °C, but the strain in c-axis decreased with leaching time. The result indicates that the chemical reactivity on different crystal face is different from each other. The leaching reaction is more sensitive to the strain in the c-axis. 4. Discussion Both IR and XRD analyses demonstrated that milling operation can change the crystalline structure of ilmenite. The breakage of ilmenite crystal may cause defects on the crystal faces, and increase in the reactivity of ilmenite. Based on the surface area measurements and the leaching results of the milled ilmenite, one can infer that the leaching rate increased monotonically with milling time although the surface area decreased when the sample was milled for a time over 30 min. Thus, an energetic milling enhances the dissolution of ilmenite by not only increasing its surface area but also elevating its chemical activity. This conclusion was also arrived by Bala´zˇ (2003). The thermogravimetric experiments confirmed the occurrence of oxidation during the mechanical activation of ilmenite, even though the XRD analysis failed to directly detect Fe2Ti3O9 product phase, which is also in support of the report (Chen et al., 1999) on the phase with a poorly crystallized structure. Barton and McCornnel (1979) have reported that FeTiO3 phase dissolved more efficiently than Fe2Ti3O9 in sulfuric acid solution. However, our leaching experiments did not show much difference between samples milled in air and in vacuum (Fig. 8). The result might indicate that the Fe2Ti3O9 phase dispersed in FeTiO3 can quickly dissolve in the leaching process. Thus, milling Panzhihua ilmenite in air has less effect on the mechanical activation efficiency. Welham and Llewellyn inferred that the pressure generated during the impact of balls onto the powder would tend to break the ilmenite along the (0 0 0 1) and (1 0 1 1) planes (Welham and Llewellyn, 1998). It is confirmed in the present study. From the XRD analysis, the cleavage of ilmenite occurred more preferentially along the a-axis. Since (0 0 0 1) crystal face is parallel to the a-axis, more (0 0 0 1) crystal faces are expected to expose out in the milled samples. Both Barton and McCornnel (1979) and Duncan and Metson (1982) reported that the dissolution of nonactivated ilmenite was more rapid in the basal (0 0 0 1) and (1 0 1 1) planes. We also observed that any normal displacement between (0 0 0 1) crystal faces [i.e., the lattice strain in the c-direction] affects the reaction rate stronger than that between (1 0 0 0) or (0 1 0 0) crystal faces. The leaching reaction is likely to start from the (0 0 0 1) crystal face at a lower temperature. Only at high temperatures, the selectivity of leaching on different crystal faces is decreased. The leaching kinetic curves showed that the leaching process included two stages. The initial dissolution stage was rapid and had a low activation energy (28.4 kJ/mol). The low activation energy presents the character of a diffusion-limited process. It is clearly attributed to the dissolution of both the fine particles and highly reactive crystal
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faces (e.g., (0 0 0 1)). In the later stage, the apparent activation energy approaches the value of unactivated ilmenite due to the consumption of the highly reactive fine particles and crystal faces. Thus, the ball mill only partially activated the ilmenite, which is in agreement with the results obtained by Li et al. (2005) with a tumbling ball mill. Therefore, the mechanical activation prior to leaching mainly enhances the dissolution kinetics of early stage. However, a simultaneous milling and leaching with a stirring ball mill significantly improved the whole dissolution kinetics of Panzhihua ilmenite (Li et al., 2006). The structural defects induced by energetic ball milling include both effective and ineffective defects. The effective defects, i.e., the strain in the c-axis of ilmenite unit cell, strongly affects the leaching rate, while the ineffective defects, i.e., the strain in the a-axis, have minimal effect on the leaching rate. 5. Conclusion Mechanical activation significantly accelerated the dissolution of Panzhihua ilmenite in a 50% sulfuric acid solution. As high as 82.1% of conversion was obtained by leaching the 60 min milled ilmenite at 100 °C for 2 h. The leaching rate was apparently linked with the lattice strain of ilmenite crystal. The dissolution reaction responded more sensitively to the strain in the c-axis than that in the a-axis. At low temperature, the dissolution reaction is likely to start along the direction of (0 0 0 1) crystal face. Mechanical milling prior to leaching only partially activated the ilmenite. In the initial period, the activated part [i.e., the fine particles and highly reactive crystal faces] of ilmenite rapidly dissolved and the apparent activation energy was 28.4 kJ/mol, which showed the characteristics of a diffusion-limited process. In the later period, the leaching activation energy increased to approach the value obtained by leaching original ilmenite. It was considered to be the leaching of unactivated part. References Amer, A.M., 2002. Alkaline pressure leaching of mechanically activated Rosetta ilmenite concentrate. Hydrometallurgy 67, 125–133. Bala´zˇ, P., 2003. Mechanical activation in hydrometallurgy. International Journal of Mineral Processing 72, 341–354. Bala´zˇ, P., Ficeriova´, J., Leon, C.V., 2003. Silver leaching from a mechanochemically pretreated complex sulfide concentrate. Hydrometallurgy 70, 113–119. Barksdale, J., 1966. Titanium, Its Occurrence, Chemistry and Technology, second ed. Ronald Press, New York. Barton, A.F., McCornnel, S.R., 1979. Rotating disc dissolution rates of ionic solids. Part 3. Natural and synthetic ilmenites. Transaction of Faraday Society 75, 971–983. Chen, Y., 1997. Low-temperature oxidation of ilmenite (FeTiO3) induced by high energy ball milling at room temperature. Journal of Alloys and Compounds 257, 156–160. Chen, Y., Williams, J.S., Campbell, S.J., Wang, G.M., 1999. Increased dissolution of ilmenite induced by high-energy ball milling. Materials Science and Engineering A 271, 485–490.
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C. Li et al. / Minerals Engineering 19 (2006) 1430–1438
Duncan, J.F., Metson, J.B., 1982. Acid attack on New Zealand ilmenite. I. The mechanism of dissolution. New Zealand Journal of Science 25, 103–109. Ficeriova´, J., Bala´zˇ, P., Boldizˇa´rova´, E., 2002. Thiosulfate leaching of gold from a mechanically activated CuPbZn concentrate. Hydrometallurgy 67, 37–43. Godocˇ´ıkova´, E., Bala´zˇ, P., 2002. Spectroscopic study of the surface oxidation of mechanically activated sulfides. Applied Surface Science 200, 36–47. Jing, J.L., Zhang, Q.Z., Qiu, L.Y., Liang, B., 2003. An Investigation of Liquid Phase Digestion of Ilmenite for Sulfate Process TiO2 Pigment Production. Chemical Reaction Engineering and Process 19 (4), 337– 343 (in Chinese). Levenspiel, O., 1999. Chemical Reaction Engineering, third ed. John Wiley & Sons, New York. Liang, B., Li, C., Zhang, C.G., Zhang, Y.K., 2005. Leaching kinetics of Panzhihua ilmenite in sulfuric acid. Hydrometallurgy 76 (3–4), 173– 179. Li, H.G., Zhao, Z.W., Zhao, T.C., 1997. Effect of experimental facility style on the activation of mineral. Journal of Central South University of Technology 28 (2), 130–133 (in Chinese). Li, C., Liang, B., Liang, X.M., 2005. Leaching Kinetics of Mechanically Activated Ilmenite Ore. Journal of Sichuan University 37 (1), 35–38 (in Chinese).
Li, C., Liang, B., Chen, S.P., 2006. Milling dissolution of Panzhihua ilmenite with dilute sulfuric acid. Hydrometallurgy, in press. Rahm, J.A., Cole, D., 1981. US Patent 4288415. Sasikumar, C., Rao, D.S., Srikanth, S., Ravikumar, B., 2004. Effect of mechanical activation on the kinetics of sulfuric leaching of beach sand ilmenite from Orissa, India. Hydrometallurgy 75, 189–204. Sewry, J.D., Brown, M.E., 2002. ‘‘Model-free’’ kinetic analysis? Thermochimica Acta 390, 217–225. She, Z.H., Xu, S., Fan, J.Q., 1998. Decomposing Titanium Concentrate with Acid in Liquid Phase by Fluidization. Complex Utilization of Mineral Resources 31 (2), 22–26 (in Chinese). Thompson, P., Cox, D.E., Hastings, J.B., 1987. Rietveld Refinement of Debye–Scherrer Synchrotron X-ray Data from Al2O3. Journal of Applied Crystallography 20, 79–83. Wang, P., Pan, Z.L., 1982. The System of Mineralogy, vol. 1. Geology Press, Beijing. Welham, N.J., 2001. Enhanced dissolution of tantalite/columbite following milling. International Journal of Mineral Processing 61, 145– 154. Welham, N.J., Llewellyn, D.J., 1998. Mechanical Enhancement of the Dissolution of Ilmenite. Minerals Engineering 11 (9), 827–841. Xu, S., Huang, Z.S., 1993. The Kinetics of Panzhihua Ilmenite Leaching with Sulfuric Acid. Journal of Mining and Metallurgy Engineering 13, 44–48 (in Chinese).