chemical engineering research and design 9 0 ( 2 0 1 2 ) 220–228
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Application of Taguchi method for optimization of synthetic rutile nano powder preparation from ilmenite concentrate B.N. Akhgar a , M. Pazouki b,∗ , M. Ranjbar c , A. Hosseinnia b , R. Salarian d a
Mining Engineering Faculty, Sahand University of Technology, Tabriz, Iran Materials and Energy Research Center, MeshkinDasht, Karaj, Iran c Department of Mineral Processing, Engineering Faculty, Shahid Bahonar University, Kerman, Iran d Maziyar University, School of Art and Engineering, Noor, Mazandaran, Iran b
a b s t r a c t In the present research work Taguchi method was applied to investigate the effect of reductive leaching parameters and mechanical pretreatment of ilmenite on nano synthetic rutile synthesis. The parameters such as ilmenite to acid mass ratio, ilmenite to iron powder mass ratio, milling time and initial leaching temperature were selected for optimization of experimental conditions. Consequently, the milling time was the most effective parameter on synthetic rutile preparation compared to the rest of the selected parameters. The optimum conditions obtained were as follows: milling in Argon atmosphere 40 min, initial reaction temperature 100 ◦ C, ilmenite to hydrochloric acid mass ratio 1:9.55 and ilmenite to iron powder mass ratio 1:0.075. The characterization of products indicated that the prepared powder with milling time 40 min, temperature 100 ◦ C, ilmenite to hydrochloric acid mass ratio 1:12.8 and ilmenite to iron powder mass ratio 1:0.05 had particles size of less than 100 nm. The analysis further confirmed that synthetic rutile nano powder had 91.1% TiO2 . The nano powder obtained under the optimized condition had a BET surface area of 54.6 m2 /g. © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Reductive leaching; Mechanical activation; Taguchi method; Synthetic rutile; Nanomaterials
1.
Introduction
The importance of titania (TiO2 ) is due to its application in the manufacturing of paints, pigment, ceramics, papers, and in other related chemical industry (Diebold, 2003). The production of titanium dioxide pigment has been conducted by two main processes, the sulfate process and the dry chlorination process. The ilmenite as a raw material is used in sulfate process which is lengthy, costly and the by product ferrous sulfate is less marketable (Abdel-Aal et al., 2000). The second process (chlorination process) which employs rutile as raw material is more economical and has less waste material (Mackey, 1994). The conversion of ilmenite concentrate into synthetic rutile due to shortage of natural rutile is considered to be interesting substitution. The production of synthetic rutile from ilmenite concentrate can be achieved by several methods (Mahmoud et al., 2004; Li et al., 2008). Some researchers for performing the reductive leaching of ilmenite concentrate used iron powder as a reductive agent to
∗
promote the leaching of ilmenite concentrate in hydrochloric acid (Mahmoud et al., 2004; Lasheen, 2005; El-Hazek et al., 2007). However, dissolution of mechanically activated ilmenite concentrate in various leaching agent for preparation of synthetic rutile was investigated. The achieved results demonstrated that the mechanical pretreatment enhanced the dissolution of ilmenite concentrate due to the increasing lattice strain and surface area of milled ilmenite concentrate (Amer, 2002; Welham and Llewellyn, 1998; Li et al., 2006a,b, 2007, 2008; Sasikumar et al., 2007). There are various parameters affecting the properties of the produced synthetic rutile such as reaction temperature, ilmenite to acid mass ratio, pulp density, and agitation, leaching agent concentrate, ilmenite particle size, leaching time and also related parameters to kind of additive and mechanical activation conditions (Mahmoud et al., 2004; Lasheen, 2005; ElHazek et al., 2007; Amer, 2002; Welham and Llewellyn, 1998; Li et al., 2006a,b, 2007, 2008; Sasikumar et al., 2007). These parameters were investigated by using classical one parameter at a
Corresponding author. Tel.: +98 261 6280036; fax: +98 261 6280030. E-mail addresses:
[email protected] (M. Pazouki),
[email protected] (R. Salarian). Received 18 December 2010; Received in revised form 19 June 2011; Accepted 13 July 2011 0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2011.07.008
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time. However in previous investigations, the leaching mechanism and kinetic studies have been performed (Mahmoud et al., 2004; Amer, 2002; Welham and Llewellyn, 1998; Li et al., 2006a, 2006b; Sasikumar et al., 2007; Van Dyk et al., 2002; Liu et al., 2006). In all of the above mentioned investigations one variable at a series of experiments has been studied to appreciate the effectiveness of parameters on the preparation of synthetic rutile from ilmenite concentrate. In recent years, the statistical experimental designs have been employed to efficient analysis of complex systems (Kim et al., 2005). The application of statistical experimental design provides maximum and reliable data which is used fewer possible experiments. The Taguchi method as an experimental design method can achieve these tasks. In Taguchi method, the experience and knowledge of researcher has a main role in the proper selection of variables and their levels. The main and interaction effect of factors can be investigated by using Taguchi method. The optimum conditions can be defined regarding the desired target of response such as “smaller is better”, “larger is better” or “nominal is the best”. The Taguchi method applied simple tools such as signal to noise ratio (S/N) and analysis of variance (ANOVA) to determine optimal conditions and effect of each factors on main properties. The effect of each selected parameters can be estimated through ANOVA test. However, the maximum S/N ratio values which are calculated regarding to test target for each factor in its selected levels is applied to determine optimum test conditions (Keles, 2009). In the present study, the aim was to investigate the reductive leaching of mechanically activated ilmenite concentrate by hydrochloric acid. With applying the Taguchi method, the effect of four parameters such as ilmenite to acid mass ratio, ilmenite to iron powder mass ratio, milling time and temperature was investigated and the used technique was optimized according to the calculated S/N ratio of parameters. The developments of nano particles and structures synthesis routes gave access to various nanomaterials with different structural and physical properties according to their corresponding bulk material (Djerdj et al., 2008). The recent investigation verified possibility of synthetic rutile nano powder preparation as a photocatalytic material by combination of reductive leaching and mechanical activation (Akhgar et al., 2010).
2.
Experimental
2.1.
Materials and procedure
The Kahnoje ilmenite deposits are available in the southeast province of Iran, which is called Kerman has an estimated reserve about 43 million tonnes. The ilmenite concentrate with particle size −75 m was selected from the above mentioned deposit for mechanical activation. The mechanical activation was conducted in a planetary ball mill under argon atmosphere. The milling operation was performed with rotation speed of 250 rpm and spin rate of 480 rpm. The steel balls with 20 mm diameter were used to fill milling cells by the ball to ore mass ratio of 20:1. The leaching experiments were carried out according to the Taguchi experimental design in a 500 ml glass reactor equipped with a refluxed condenser. A certain amount of 20% HCl solution was heated to the desired temperature in a thermo-statically controlled paraffin bath. The experiments started with addition of 20 g ilmenite and followed by addi-
Ilmenite
Sieving - 75 µm Mechanical Activation
Reductive Leaching
Solid/ Liquid Separation Leach liquor (HCl recycle) Leach residual Dehydration
Calcination
Nano Synthetic Rutile Fig. 1 – A schematic flow sheet of techniques used. tion of iron powder with determinate amount after 20 min. A magnetic stirrer with stirring speed of 400 rpm was applied to mechanical agitation, keeping homogeneous suspension of slurry. The leaching experiments carried out for 6 h and then the slurry was allowed to precipitate and the leach liquor was separated from the leach residual. The dehydration of products was carried out by using benzene isotropic distillation technique (Hosseinnia et al., 2009) and the products calcined at 400 ◦ C. Fig. 1 shows a schematic route of the process. The large consumption of HCl as a disadvantage of this method can be neglected with recycling of used acid (Walpole, 1995; Newman and Balderson, 1993).
2.2.
Taguchi method
In this study, the milling time, ilmenite to hydrochloric acid mass ratio, ilmenite to iron powder mass ratio and the reaction temperature were considered in three levels to optimize process. The milling times were selected as 0, 40 and 360 min and the planned levels for temperature were 85, 100 and 110 ◦ C. However, the ilmenite to hydrochloric acid and ilmenite to iron powder mass ratio were in three levels of 1:7.3, 1:9.55 and 12.8 and 1:0.05, 1:0.075 and 1:0.1, respectively. The used parameters and levels are presented in Table 1. The numbers of 1, 2 and 3 represented the lowest, mid and highest levels, respectively. Four parameters in three levels mean that the L9 (34 ) orthogonal array of Taguchi design have to be considered for performance of experiments in 9 runs. The orthogonal array of L9 was randomly performed and its structure is given in Table 2. The S/N ratio and ANOVA are utilized for designation of optimum conditions.
2.3.
Characterization
The chemical composition of ilmenite concentrate and products of each experiments were indicated by X-ray fluorescence
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Table 1 – Process parameters and their levels used in the experiments. Parameters
Symbol
Level 1
Level 2
Level 3
Milling time (min) Ilmenite/acid mass ratio Ilmenite/iron powder mass ratio Temperature (◦ C)
Mt A Ip T
0 1:7.3 1:0.05 85
40 1:9.55 1:0.075 100
360 1:12.8 1:0.1 110
Table 2 – L9 (34 ) orthogonal array of Taguchi and their S/N ratio value. Exp. no.
Mt
A
Ip
T
y1
y2
y3
yi
1 2 3 4 5 6 7 8 9
1 1 1 2 2 2 3 3 3
1 2 3 1 2 3 1 2 3
1 2 3 2 3 1 3 1 2
1 2 3 3 1 2 2 3 1
34.57 42.76 37.29 88.74 84.12 89.95 78.66 90.77 78.20
37.41 42.22 36.21 88.26 89.19 90.96 80.28 88.81 80.83
33.65 45.19 35.68 92.70 82.39 92.39 81.82 88.32 80.67
35.2 43.4 36.4 89.9 85.2 91.1 80.2 89.3 79.9
S/Ni
T=
(XRF) spectrometry (ARL 8410 SEQUENTIAL). The products were examined three times by XRF test and the mean of TiO2 amount (yi ) in the products was considered as a response in Taguchi method. Measurement of the surface area was performed through a standard BET surface area analyzer (Micrometrics Gimini III 2375, USA). The XRD analysis was applied for examination of ilmenite concentrate, the milled samples and the products. The X-ray diffraction test was performed by Philips DW3710 instrument applying Cu K␣ radiation at 50 kV and 250 mA in the range of 15–75. Investigation of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were carried out by Zeiss Em 10c and Stereoscan 5390 Cambridge 1990 instruments, respectively.
3.
Result and discussion
3.1.
Mechanical activation
Surface area (m2/g)
5
4
3
2
1
0 40
80
120
160
200
*
*
*
*
S/Ni
*
*
* *
C
b
a 20
30
40
50
60
70
2 Theta/degree
Fig. 3 – XRD pattern of (a) unmilled ilmenite, (b) 40 min milled ilmenite and (c) 360 min milled ilmenite.
Fig. 2 illustrates the changes in milled sample’s surface area caused by increasing milling time. As it can be seen, the BET surface area increased from 0.7 m2 /g in unmilled ilmenite concentrate to 4.8 m2 /g in 40 min milled ilmenite and then decreased to about 2.9 m2 /g in 360 min milled samples. With longer milling time, the broken particles begin to form agglomerates which cause the increase of particle size and consequently decrease the surface area (Li et al., 2008; Welham
0
* = ilmenite
30.93 32.75 31.22 39.08 38.61 39.19 38.09 39.02 38.05 326.94
240
280
320
360
MIlling time (min)
Fig. 2 – Variation of surface area with milling time.
400
and Llewellyn, 1998). As shown in Fig. 2, the surface area increased with milling time and then peaked before reducing as result of agglomeration. In order to avoid probable phase changes, the milling operation carried out in the Argon atmosphere. The XRD patterns of ilmenite concentrate and milled powders indicated that all the peaks were matched with the standard XRD pattern of hexagonal FeTiO3 and no phase changes occurred in milled samples (Fig. 3). The XRD patterns of ilmenite concentrate and milled powders demonstrated that the increasing of milling time caused to the broadening and weakening of peaks (Fig. 3). However, the broadening and weakening of peaks can be related to decreasing of crystallinity during the milling leading to rapid dissolution of milled powders (Welham and Llewellyn, 1998). The XRD patterns and BET results verified the mechanical activation increased the surface area and decreased the crystallinity of ilmenite concentrate which were responsible for enhanced reaction between milled powder and leaching agent. The cited results will be reconfirmed with achieved result from Taguchi method in next part of our study.
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Table 3 – ANOVA analysis results for all parameters. Factors
Df
Mt A Ip T Total
2 2 2 2 8
3.2.
SS 4581.96 30.3 35.79 48.98 4697.03
MS 2290.98 15.15 17.89 24.49
Analysis of experimental data
A Taguchi orthogonal array experimental design was applied to optimize reductive leaching of mechanically activated ilmenite concentrate producing highest quantity of TiO2 . The ANOVA test conducted to estimate the effect of each parameter on the characteristic properties, i.e. TiO2 content in this study. The result of ANOVA test indicated the mechanical activation was the most effective parameter than others (Table 3). The signal to noise ratio (S/N) approach was used for analysis of TiO2 . Since the target of the study is preparation of synthetic rutile with highest TiO2 content, the corresponding S/N ratio calculation was performed through “larger is better” approach of Taguchi method (Eq. (1)).
S/N = −10 log
1 1 n y2 n
i=1
(1)
i
where yi is the data obtained from experiments and n is the number of repetition. The TiO2 amount (yi ) and calculated S/N ratio of each experiment are presented in Table 2. The mean S/N ratios for each level of every parameter were computed to realize optimum conditions by Eq. (2) as follows: Mt1 =
Exp.1 + Exp.2 + Exp.3 3
(2)
For example: Mt1 =
A2 =
Exp.1 + Exp.2 + Exp.3 30.93 + 32.75 + 31.22 = = 31.63 3 3
Exp.2 + Exp.5 + Exp.8 32.75 + 38.61 + 39.02 = = 36.79 3 3
Table 5 shows the arrangement of calculated S/N ratios and mean of them in each level for a parameter. The corresponded S/N ratio to a parameter in each level was ordered in each column to calculate the mean of S/N ratio of a parameter in related level. It is obvious that the difference between the maximum and minimum S/N ratios of milling time (Mt) is 7.33, higher than 0.64, 0.65 and 0.81 for other parameters (A, Ip and T, respectively) signifying the effectiveness of milling time. The mean S/N ratio for each level of parameters is shown in Fig. 4. The highest value of mean S/N ratio for a parameter was considered as optimum level of relative parameter. Therefore it is clear that the optimum condition was Mt2, A2, Ip2 and T2. The achieved optimum conditions were discussed Table 4 – ANOVA analysis after pooling. Factors
Df
Mt T Error Total
2 2 4 8
SS 4581.96 48.98 66.09 4697.03
MS 2290.98 24.49 33.04
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independently based on the proposed mechanism of ilmenite concentrate leaching in previous investigations (Van Dyk et al., 2002; Mahmoud et al., 2004). The interactions of selected parameters were not studied in the designed Taguchi method. There is no doubt that the mechanical activation must be considered as a most effective parameter when TiO2 content in leaching products of unmilled ilmenite concentrates are at least. The outcomes of Taguchi method also indicated that 40 min milling time in level 2 preferred to 360 min milling time in level 3. According to BET surface area measures and mentioned outcome in this study, the TiO2 amount of prepared powders depended on surface area which is in contrary with recent investigation while TiO2 amount in their prepared powders depended directly on milling time (Li et al., 2008). The mechanical activation results in several changes in material structure accelerating leaching rate such as lattice strain, lattice defects, structural disorder increment and amorphization of mineral particles (Balaz, 2000; Pourghahramani and Forssberg, 2006b) that all of them increased with milling time in ilmenite concentrate (Amer, 2002; Welham and Llewellyn, 1998; Li et al., 2006a, 2006b, 2007, 2008; Sasikumar et al., 2007). In other hand, the surface area of milled ilmenite decreased when the broken particle agglomerated (after reaching a maximum value in early stage of milling) in longer milling time slowing down reactions rate. The competitive changes of ilmenite structure and surface area are in contrast with each other affecting the leachability of milled ilmenite concentrate in longer milling time. The products in experiments of 4–6 with 40 min milling time as pretreatment contained more TiO2 in their compositions than the products of experiments leached after 360 min milling time (7–9). In addition to the XRF test results (Table 2 and 6), a violet color of related leach liquor, i.e. experiments 7–9 comparing with the yellow-red color of leach liquor in the experiments 4–6 indicated more Ti loss in leaching of 360 min milled ilmenite concentrate. In spite of surface area decreasing, the changes of ilmenite structure brought more Ti from milled ilmenite concentrate to the leach liquor in the leached samples with 360 min milling, the extracted Ti remained in solution and were not hydrolyzed to TiO2 leading to less TiO2 content in the products of 7–9 than the products of 4–6. In the case of optimum leaching temperature similar to previous investigation 100 ◦ C in level 2 was selected by applying Taguchi method (Li et al., 2008). In the experiments which were conducted with initial temperature of 110 ◦ C in level 3, hard-controlled initial temperature inside the reactor increased due to the exothermic reaction between ilmenite concentrate and hydrochloric acid, which caused evaporation of HCl and then the effectiveness of leaching agent was reduced on ilmenite concentrate in 110 ◦ C of level 2. As final point; the ilmenite to hydrochloric acid (20%) and ilmenite to iron powder mass ratio optimal levels were taken in 1:9.55 and 1:0.075, respectively. These two parameters have lowest effectiveness on TiO2 production with compared to other variables and can be pooled to error (Table 4). The stoichiometry of HCl and Fe powder was increased in the proposed reactions (Mahmoud et al., 2004) due to the variation of the ilmenite to hydrochloric acid (20%) and ilmenite to iron powder mass ratio, from 1:7.3 to 1:12.8 and from 1:0.05 to 1:0.1, respectively. It was reported that a higher HCl stoichiometry increases the TiO2 loss in the leach liquor (Mahmoud et al., 2004; Lasheen, 2005; Li et al., 2008), thus the selection of ilmenite to hydrochloric acid mass ratio in mid level of A2 is not unreasonable as Taguchi method outcomes rec-
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Mean of SN ratio
40
35
30 1
Mt
2
3
Levels
Mean of SN ratio
40
35
30 1
A
2
3
Levels
Mean of SN ratio
40
35
30 1
Ip
2
3
Levels
Mean of SN ratio
40
35
30
T
1
2
3
Levels
Fig. 4 – Plots of process parameters effect (S/N ratio). ommend it. The parameter of ilmenite to iron powder mass ratio (Ip) was optimized in med level (Ip2) similar to other parameters. The iron powder addition influences the leaching of ilmenite concentrate through different ways. The HCl concentrate decreased less with smaller amount of Fe powder
addition and as well not more Fe+3 would be remained in solution leading to product contamination by unconverted Fe+3 , while the Fe powder addition helps Ti+3 to stay longer in solution as a reductive agent to convert Fe+3 to Fe+2 , also the iron powder creates new surfaces by breaking up ilmenite grain
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Table 5 – The arrangement of calculated S/N ratios and mean of S/N ratios in each level to compute optimum conditions.
S/N S/N S/N Mean in each level Max − Min
Mt1
Mt2
Mt3
A1
A2
A3
Ip1
Ip2
Ip3
T1
T2
30.93 32.75 31.22 31.63
39.08 38.61 39.19 38.96 7.33
38.09 39.02 38.05 38.39
30.93 39.08 38.09 36.03
32.75 38.61 39.02 36.79 0.64
31.22 39.19 38.05 36.15
30.93 39.19 39.02 36.38
32.75 39.08 38.05 36.62 0.65
31.22 38.61 38.09 35.97
30.93 38.61 38.05 35.87
32.75 39.19 38.09 36.68 0.81
structure during the reductive leaching in favor of ilmenite dissolution (Mahmoud et al., 2004; Lasheen, 2005). In low and high level of the iron powder addition (lp1 and lp3) one of the mentioned phenomenons is dominant with their side effects reducing product quality. Finally, the S/N ratio of optimized process was calculated by using Eq. (3) to predict the amount of TiO2 in the product for optimum condition by Eq. (3), where represents optimum value and T is referred to the total levels of each parameter in S/N ratios. Due to the effectiveness of milling time and initial reaction temperature, Mt2 and T2 were used to estimate the amount of TiO2 in optimal experiment and the A2 and Ip2 were excluded due to their less influence.
T T T T + Mt2 − + A2 − + Ip2 − n n n n −326.94 + 38.96 + 36.68 = 39.31 = 9
=
+ T2 −
T n
(3)
Using the achieved optimal level of parameters, the maximum TiO2 amount was predicted by Eq. (3) where, T value was 326.93 and Mt2 and T2 were mean of S/N ratios in level 2 for milling time (Mt2) and temperature (T2), respectively. The predicted TiO2 amount was 92.35% corresponded to 39.31. The confirmation experiment was conducted under optimum condition to verify the Taguchi results. The prepared powder under optimum condition contained 92.8% TiO2 as main compound. The confirmation test result and predicted TiO2 amount were close together revealing the results of designed Taguchi method were valid and also, the influence of uncontrolled factors (noise factor) such as operator error was ignorable due to the precised experimental runs.
3.3.
Characterizations
Although nanosized titania can be denoted as one of the most valuable photocatalyst with wide application fields, the synthesis of new materials which have elements such as iron in their composition are important due to influence of iron on reactivity of these catalysts (Javio et al., 1999). In addition, the preparation of nanopowders such as synthetic rutile similar to iron-doped TiO2 directly from raw material such as ilmenite concentrate has economically attractive issues. Therefore, the prepared products in all of the nine experiments were characterized to examine the formation of nanosized synthetic rutile which composed from TiO2 as a main component with small amount of Fe2 O3 . The X-ray diffraction (XRD) patterns revealed that major phase of ilmenite concentrate was hexagonal structure of FeTiO3 (Fig. 3). Table 6 provides information about chemical composition of ilmenite concentrate and products of experiments 4, 6 and 8. Comparing with other ilmenite concentrate in the world, the used concentrates in this investigation have low TiO2 content and high impurity contents such as Fe2 O3 and SiO2 .
T3 31.22 39.08 39.02 36.44
The chemical composition of products was analyzed through XRF test after dehydration followed by calcination in 400 ◦ C. The XRF results indicated that the products had main impurities such as Fe2 O3 , SiO2 and traces of other impurities such as MgO, CaO and Al2 O3 . The SiO2 and Fe2 O3 amount increased and decreased in leach residuals, respectively. The compound with less than 0.1% was presented as blank tables. The X-ray diffraction (XRD) patterns of calcined products are presented in Fig. 5. The XRD patterns indicated the ilmenite phase remained in the products of experiments 1–3. Therefore, most of ilmenite concentrate did not participate in the occurred reactions between ilmenite and hydrochloric acid. In the case of experiments 4–9 where the mechanical activation was carried out as pretreatment, the XRD pattern confirmed the formation of rutile phase without traces of ilmenite phase. The results matched achievements of Taguchi method that mechanical activations had most significant effect in compare with other selected parameters. The formation of rutile phase instead of anatase phase is related to the small particle size of products. It is known that the stability of rutile and anatase phases depends on the particle size and also the stability of rutile phase is more noticeable in extremely small particles (Li et al., 2008). The existence of ilmenite phase in experiments 1 and 3 reveals that nanosized synthetic rutile cannot be produced in the related conditions. Therefore, the prepared powders in experiments 4–9 were chosen for further investigation with SEM in next stage. Fig. 6 depicts the SEM micrographs of prepared powders in experiments 4–9. The SEM micrograph of experiments 5 and 6 indicated the finest particles were produced during the relative condition. Although the prepared powder through experiments 4 and 7–9 had nanosized particles but their SEM micrographs exposed that some of their particles were more than 100 nm in size. Therefore, the produced powder of experiments 5 and 6 are considered for TEM microscopy examination in order to prove for nano size preparation of the synthetic rutile. The TEM images of these powders certify the formation of nanosized synthetic rutile with particles size which were less than 100 nm. The particles were formed in different crystalline shape and morphology. In comparison with produced powder in experiment 5, the prepared nanopowder through experiment 6 had smaller and more uniform morphology (Fig. 7). The BET surface area of nanosized synthetic rutile prepared through experiment 6 was 54.6 m2 /g. The high surface area of product 6 provided further evidence for formation of nanoparticles. The nanoparticles were created because of accelerated dissolution of the ilmenite concentrate followed by rapid hydrolysis of dissolved titanium (Li et al., 2008). The combination of mechanical activation pretreatment and iron powder addition was responsible for increasing the reactivity of ilmenite concentrate because of the creation new surface and new crystal defect on the ilmenite particle and also due to breaking up the grain structure by addition of iron powder
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Table 6 – Chemical composition of Kahnoje ilmenite concentrate and product of experiments 4, 6 and 8.
Ilmenite concentrate Experiment no. 4 Experiment no. 6 Experiment no. 8
*= ilmenite
Fe2 O3 %
TiO2 %
SiO2 %
MgO%
CaO%
Al2 O3 %
MnO%
44.6 2.1 1.4 2.7
32.9 89.9 91.1 89.3
5.9 6.5 6.7 6.3
3.6 0.1 – 0.1
2.2 0.2 – 0.3
2.5 0.2 0.3 0.2
2.1 – – 0.1
*
*= ilmenite
Exp. 1
Exp. 2
* *
*
*
*
*
* *
10
20
30
40
*
50
*
60
10
70
20
30
2Theta/degree
40
50
60
70
2Theta/degree
= rutile
*= ilmenite
*
Exp. 3
*
Exp. 4
* * *
10
20
30
40
50
*
60
70
10
20
30
2Theta/degree
= rutile
20
30
40
50
60
70
10
20
30
40
50
60
70
50
60
70
10
Exp. 8
20
30
2Theta/degree
40
2Theta/degree
= rutile
10
40
= rutile
Exp. 7
30
70
2Theta/degree
= rutile
20
60
Exp. 6
2Theta/degree
10
50
= rutile
Exp. 5
10
40
2Theta/degree
Exp. 9
20
30
40
50
60
70
2Theta/degree
Fig. 5 – XRD patterns of powders produced in experiments 1–9.
50
60
70
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Fig. 6 – SEM micrographs of synthetic rutile produced powders in experiments 4–9.
Fig. 7 – TEM image of nanosized synthetic rutile prepared in experiments 5 and 6.
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during the ilmenite reductive leaching (Mahmoud et al., 2004; Li et al., 2008). The nanosized synthetic rutile prepared in experiment 6 with main constituent of TiO2 can be considered as heterogeneous photocatalysis. Depending on circumstance of iron impurities being in nanosized synthetic rutile crystalline structure, new properties like iron-doped TiO2 nano particles were anticipated for nanosized synthetic rutile as new nanopowder.
4.
Conclusion
In this research, the Taguchi’s L9 (34 ) experimental design was applied to set the optimal conditions of mechanically activated ilmenite reductive leaching for preparation of synthetic rutile with highest TiO2 content from ilmenite concentrate. By ANOVA test results, the milling time was more effective parameter on quantity of TiO2 in synthetic rutile than other selected parameters. In addition, the optimal conditions achieved by using S/N ratios were as follows; milling in Argon atmosphere for 40 min, initial reaction temperature: 100 ◦ C, ilmenite to 20% hydrochloric acid mass ratio: 1:9.55 and ilmenite to iron powder mass ratio: 1:0.075. The obtained result from Taguchi method was controlled by performance of an experiment under achieved optimum conditions. The TiO2 amount in prepared synthetic rutile of confirmation experiment was 92.8% close to calculated prediction. Based on the XRD and BET results of milled powders, it was verified that mechanical activation accelerated the dissolution of milled powder by increasing the surface area and decreasing the crystallinity of ilmenite concentrate in comparison with unmilled ilmenite concentrate. The XRD patterns of calcined products (prepared during the designed experiments of Taguchi method) revealed without mechanical activation pretreatment, parts of ilmenite concentrate did not participate in the occurred reactions between ilmenite and hydrochloric acid. However, the XRD patterns indicated rutile as major phase formed in prepared powders from mechanically activated ilmenite. The TEM and SEM micrographs proved the nanosized synthetic rutile produced in experiments 6. The prepared nanosize synthetic rutile has 91.1% TiO2 , 1.4% Fe2 O3 , 6.7% SiO2 and trace amounts of impurities such as CaO, MgO, and Al2 O3 . The TEM analysis of experiments 6 product presented its particle sizes were less than 100 nm. As final result, the BET surface area of prepared powder in Exp. 6 measured was about 54.6 m2 /g. In present study, the nano synthetic rutile was prepared among some of Taguchi experimental design products and more investigation was suggested. 1. Since the physical and chemical properties of nanoparticles were differing from the corresponding bulk material, the nanosized synthetic rutile was recommended for characterization of new physical and chemical properties. 2. Fe-doped TiO2 and other composition of titania nano particles has wide usage in industry. The investigation for indicating of crystalline structure and composing phase of synthetic rutile nanopowders will be worthy to attention.
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