- Email: [email protected]
Preparation of nano-titanium dioxide from ilmenite using sulfuric acid-decomposition by liquid phase method
Powder Technology 287 (2016) 256–263
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Preparation of nano-titanium dioxide from ilmenite using sulfuric acid-decomposition by liquid phase method Zenghe Li, Zhencui Wang, Ge Li ⁎ Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 27 July 2015 Accepted 5 September 2015 Available online 12 September 2015 Keywords: Ilmenite H2SO4 Liquid phase method Direct hydrolysis Nano-TiO2
a b s t r a c t In this study, high purity nano-TiO2 was prepared from Panzhihua ilmenite using sulfuric acid-decomposition by liquid phase method. The effects of H2SO4 molar volume, reaction time, initial pre-heating temperature and H2SO4 concentration on the decomposition rate of ilmenite were investigated in detail. The results showed that the decomposition rate reached 95.21% under the optimal conditions. The resulting titanium solution was qualified with 121 g/L Ti4+. Since the initial pH value of hydrolysis system was below 3, the precipitation of metatitanic acid was obtained by direct hydrolysis of the titanium solution. On the other hand, the wellcrystallized anatase and rutile were achieved at different calcination temperatures. The purity of the asprepared TiO2 was above 99% with an average particle size of about 40–100 nm. The ilmenite ore, intermediates and final products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray fluorescence spectrometer (EDS), X-ray fluorescence spectroscopy (XRF), thermal gravimetric analysis (TG), inductive coupled plasma emission spectrometer (ICP) and spectrophotometer. The results demonstrated that the process was suitable for industrial production because it was inexpensive, environment-friendly and promising in the preparation of high purity nano-TiO2. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nano-TiO2 is widely used in photocatalysis, solar cells, sensors, coatings and other areas of chemical industry, owing to its nontoxicity, chemical stability, excellent color effect and strong ability for UVshielding [1]. Currently, there are many methods to produce nano-TiO2, such as chemical vapor deposition (CVD), oxidation of titanium tetrachloride, sol–gel technique, and thermal decomposition or hydrolysis of titanium alkoxides [2–5]. The titanium sources in these processes are expensive. Moreover, the reactions often require rigorous conditions. Therefore, these processes are restricted in the further applications. As is known to all, ilmenite is cheap, non-toxic and widely distributed in the world, primarily in North America (USA, Canada), South America (Brazil), Australia, Europe (Norway, Ukraine), Africa (South Africa, Mozambique) and Asia (China, India). Panzhihua is one of the important mineral deposits in China [6]. Thus, it would be very desirable for Panzhihua ilmenite to adapt to the industrial preparation of nano-TiO2. Nano-TiO2 is commercially manufactured by sulfuric aciddecomposition method and chlorination method. The chlorination method is complex, and it requires high quality ilmenite feedstock. Therefore, sulfuric acid-decomposition method is still an important
⁎ Corresponding author. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.powtec.2015.09.008 0032-5910/© 2015 Elsevier B.V. All rights reserved.
way for the preparation of nano-TiO2 from Panzhihua ilmenite, all of which are associated with magnetite and high-level CaO and MgO. Sulfuric acid-decomposition method can be divided into solid-phase method and liquid-phase method via the way of decomposition. Several researches have published the sulfuric acid-decomposition treatment to produce TiO2 from ilmenite by solid-phase method [7,8]. However, it requires high reaction temperature (above 200 °C) and high concentration of H2SO4 (above 15.5 M). Moreover, it discharges large amount of waste gas such as SO2 and SO3, leading to severe environmental problems. However, liquid-phase method needs lower acid concentration below 15.5 M and lower reaction temperature at the range of 130– 160 °C. The reaction is more moderate and it's beneficial for reducing waste gas emission. Besides, it is easy to achieve continuous production without the need of the leaching process. Therefore, it is necessary and urgent to study the liquid-phase process deeply to simplify the sulfate process and reduce environmental pollution. In this paper, nano-TiO2 was prepared from Panzhihua ilmenite using sulfuric acid-decomposition by liquid phase method. The factors affecting the decomposition rate were investigated. The initial pH value of the resulting TiOSO4 solution was below 3, which could not reach the acid precipitation of Mg2+ and Al3 + during the hydrolysis. Therefore, it effectively prevented the small amount of impurities precipitating. H2TiO3 precipitate was obtained by direct hydrolysis without the addition of any surfactant. The well-crystallized nano-TiO2 with high purity was obtained via calcination from the as-prepared H2TiO3
Z. Li et al. / Powder Technology 287 (2016) 256–263
257
2.3. Preparation of nano-TiO2 from ilmenite
Fig. 1. XRD pattern of the ilmenite ore.
precipitate. In addition, the effects of calcination temperature on polymorphs and particle size of TiO2 were investigated.
This process was carried out in a 250 ml three-necked flask employed with a thermometer, stirrer and reflux condenser. Firstly, the concentrated H2SO4 was heated to certain temperature, and then 20 g of ilmenite was added to the solution under continuous stirring. After 1 h, in order to keep the reaction in the liquid condition, 50 ml of 1.0 mol/L dilute H2SO4 at 90 °C was added to the reaction system. This article mainly focused on the effects of H2SO4 molar volume, reaction time, initial pre-heating temperature, and H2SO4 concentration on the decomposition rate of ilmenite. The existence of Fe3+ would reduce the purity of TiO2, thus, affecting the whiteness of the product. Therefore, it should be reduced to Fe2+ via the addition of an appropriate amount of iron powder late into the reaction. Titanium solution was obtained by filtration, then crystallized at 5 °C for two days. Finally, FeSO4 · 7H2O crystal and TiOSO4 filtrate solution were obtained by filtration. The titanium concentration in the solution was analyzed by a spectrophotometer. The decomposition rate of ilmenite was calculated by the following formula [5]: x% ¼ ðV C Ti =m wTi %Þ 100%
ð1Þ
2. Experimental 2.1. Materials and analysis All the chemical reagents employed were of analytical grade, and deionized water was used throughout. The ilmenite used in the research was supplied by titanium company of Panzhihua Honor and Trade Corporation. Its particle size was about 100 μm. Iron powder was purchased from Beijing Chemical Company. Its effective iron content was 98% and the particle size was about 80 μm. X-ray diffraction (XRD) characterization was carried out with a diffractometer D/Max-RA (Rigaku, Japan) using Cu Kα radiation with a step size of 0.02° and a scan range from 10° to 80°. SEM images were taken from the S-4700 scanning electron microscope (Hitachi, Japan), which operated at 20 kV. TEM images were taken from the H-800 transmission electron microscope (Hitachi, Japan), which operated at 200 kV. HRTEM images were taken from the JEM-3010 high-resolution transmission electron microscope (Hitachi, Japan). JEOL J-3010 (Japan) energy dispersive X-ray spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF) system was used for chemical composition analysis. Thermal analysis experiments were carried out on a simultaneous thermogravimetric and differential thermal analyzer (Diamond TG/DTA, PERKINELMER) with a heating rate of 10 °C/min and an air flow rate of 300 ml/min. ICP analysis was performed on ICPS-7500 inductive coupled plasma emission spectrometer (Shimadzu, Japan). The concentration of titanium in the solution was analyzed by 722 N spectrophotometer (China).
2.2. Characterization of the ilmenite ore XRD and chemical composition for the ilmenite ore used in this study were shown in Fig. 1 and Table 1, respectively. Fig. 1 indicated that the ore mainly contained ilmenite (FeTiO3, PDF NO.29-0733), which is consistent with its chemical composition analysis (Table 1). Based on the mass balance principle, the mineral content of the ore included 86.79 wt.% ilmenite, 5.32 wt.% rutile, 4.40 wt.% quartz, 3.11 wt.% hematite and 0.38 wt.% microcline [9].
where x% denotes the decomposition rate of ilmenite, CTi denotes the titanium concentration in the titanium solution (g · L−1), m denotes the total mass of the ilmenite (g), wTi% denotes the mass fraction of soluble titanium in the ilmenite, and V denotes the volume of titanium solution (L). The resulting concentrated TiOSO4 solution was used for hydrolysis in a 250 ml three-necked flask employed with a thermometer, stirrer and reflux condenser. Firstly, 25 ml of deionized water was added to the three-necked flask, and 5 ml of TiOSO4 solution was slowly dropped to the reaction system under continuous stirring at room temperature. Then the solution was heated to 90 °Cand kept for 3 h. The precipitate was obtained during aging at room temperature for 6 h, filtered, washed with deionized water and ethanol, and then dried at 80 °C. The asprepared H2TiO3 precipitate was calcined at various temperatures in a tubular furnace for 4 h to synthesize nano-TiO2. The experimental procedure was shown in Fig. 2. 3. Results and discussion 3.1. Decomposition of ilmenite with H2SO4 The ilmenite ore was composed of a variety of minerals, and the decomposition of ilmenite with H2SO4 was a complex heterogeneous reaction. The main reactions were expressed by Eqs. (2)–(6) [9]: FeTiO3 ðsÞ þ 4Hþ ðaqÞ ¼ TiO2þ ðaqÞ þ Fe2þ ðaqÞ þ 2H2 O
ð2Þ
Fe2 O3 ðsÞ þ 6Hþ ðaqÞ ¼ 2 Fe3þ ðaqÞ þ 3H2 OðlÞ
ð3Þ
MgOðsÞ þ 2Hþ ðaqÞ ¼ Mg2þ ðaqÞ þ H2 OðlÞ
ð4Þ
Al2 O3 ðsÞ þ 6Hþ ðaqÞ ¼ 2Al ðaqÞ þ 3H2 OðlÞ
ð5Þ
CaOðsÞ þ 2Hþ ðaqÞ ¼ Ca2þ ðaqÞ þ H2 OðlÞ:
ð6Þ
3þ
Table 1 The chemical composition analysis of ilmenite (wt.%). Composition
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Wt.%
4.78
43.6
2.10
3.11
34.45
0.69
4.73
1.15
0.20
0.13
0.05
5.01
258
Z. Li et al. / Powder Technology 287 (2016) 256–263
Fig. 4. Decomposition rate of ilmenite under different H2SO4 molar volumes.
increase the cost, but also make the acidity too strong for subsequent hydrolysis, leading to slow hydrolyzation. Therefore, the volume of 0.32 mol was used in the following experiments.
Fig. 2. The experimental procedure of preparation of nano-TiO2 from ilmenite.
The effects of H2SO4 molar volume, reaction time, initial preheating temperature and H2SO4 concentration on the rate of decomposition were investigated, respectively. 3.1.1. Effect of H2SO4 molar volume According to the stoichiometric ratio, the decomposition of 20 g ilmenite required 0.30 mol H2SO4. Therefore, when the reaction time was 120 min, the initial preheat temperature was 150 °C, and the H2SO4 concentration was 13.5 mol/L; sulfuric acid molar volume of 0.24 mol, 0.28 mol, 0.30 mol, 0.32 mol, 0.34 mol, and 0.36 mol were adopted to investigate the influence of H2SO4 molar volume on the decomposition rate of ilmenite. Figs. 3 and 4 were the XRD patterns of residue and the decomposition rate of ilmenite under different H2SO4 molar volumes, respectively. As can be seen from Fig. 3, the main diffraction peak of FeTiO3 in the residue (2θ = 33°) almost disappeared when H2SO4 molar volume reached 0.32 mol, indicating that the reaction tended to be complete. The results in Fig. 4 showed that the decomposition rate of ilmenite increased with the increasing of H2SO4 molar volume from 0.24 mol to 0.32 mol, while little change appeared from 0.32 mol to 0.36 mol. In addition, too much H2SO4 would not only
Fig. 3. XRD patterns of residue under different H2SO4 molar volumes.
3.1.2. Effect of reaction time The effect of reaction time was studied in the range of 30–150 min. Figs. 5 and 6 were the XRD patterns of residue and decomposition rate of ilmenite under different reaction times. The results illustrated that the diffraction peak intensity of FeTiO3 decreased significantly and the decomposition rate of ilmenite increased with an increase in the
Fig. 5. XRD patterns of residue under different reaction times.
Fig. 6. Decomposition rate of ilmenite under different reaction times.
Z. Li et al. / Powder Technology 287 (2016) 256–263
259
Fig. 7. XRD patterns of residue under different initial preheating temperatures. Fig. 9. XRD patterns of residue under different sulfuric acid concentrations.
reaction time from 30 to 120 min. The decomposition rate reached 93.32% in 120 min. In a reaction time range of 120–150 min, a small variation of the decomposition rate was observed, it maintained between 93.32% and 93.21%. It would have an adverse effect upon the decomposition of ilmenite if the increasing of the reaction time was excessive, because TiOSO4 solution was likely to hydrolyze in advance. Therefore, 120 min would represent as the optimum reaction time.
3.1.3. Effect of initial preheating temperature The activation energy of ilmenite with H2SO4 was too high to allow the occurring of the reaction at room temperature. Therefore, it was necessary to preheat to a certain temperature. The effect of initial preheating temperature on the decomposition rate of ilmenite was studied in the range of 100–160 °C. Figs. 7 and 8 were the XRD patterns of residue and decomposition rate of ilmenite under different initial preheating temperatures, respectively. It could be seen that the initial preheating temperature had a significant effect on the decomposition rate. The diffraction peak intensity of FeTiO3 was significantly weakened with an increase in initial preheat temperature. Meanwhile, the decomposition rate increased sharply and reached 93.32% at 150 °C. In a temperature range of 150–160 °C, a slight increase of decomposition rate from 93.32% to 95.21% was observed; the decomposition rate tended to be stable. These results were due to the fact that the elevating temperature may enhance surface reaction rate, and reduce the viscosity of the liquid phase [10]. It could reduce the ion diffusion resistance and enhance mass transfer driving force. Therefore, 160 °C would be considered as the optimum temperature.
Fig. 8. Decomposition rate of ilmenite under different initial preheating temperatures.
3.1.4. Effect of H2SO4 concentration In order to examine the effect of the H2SO4 concentration on the decomposition of ilmenite, several experiments were carried out using H2SO4 concentrations varying from 11.5 to 15.5 mol/L. Figs. 9 and 10 were the XRD patterns of residue and decomposition rate of ilmenite under different H2SO4 concentrations, respectively. As shown in Figs. 9 and 10, it was clear to see that the decomposition rate of ilmenite increased with the increasing of H2SO4 concentration from 11.5 to 13.5 mol/L. These results were due to the contact probability between ilmenite surface and H+, SO2− increased with an increase in the con4 centration of H2SO4, strengthening the solid surface force and dipole interaction between H+ and SO24 −. Thus the decomposition rate of ilmenite accelerated. However, the decomposition rate was almost unchanged when H2SO4 concentration was from 13.5 to 14.5 mol/L, and then decreased with the further increasing of acid concentration. The low decomposition rate at higher acid concentration was probably due to the reduction of proton activity and the formation of a product layer over the unreacted core. Therefore, the excessively high acid concentration would not improve the decomposition rate. Moreover, it would impose a heavy burden upon the H2SO4 regeneration system. An acid concentration of 13.5 mol/L was suggested as the optimum condition and the decomposition rate reached 95.21%. The ilmenite did not fully decompose; this may have been affected by the mass transfer resistance of the product layer in the reaction. The EDS pattern of residue was shown in Fig. 11. It could be seen that the residue contained major impurities, such as Ti, Fe, Si, Ca, Al, and Mg. It indicated that most of the impurities were removed during the decomposition process. The resulting titanium solution was qualified with 121 g/L Ti4+.
Fig. 10. Decomposition rate of ilmenite under different sulfuric acid concentrations.
260
Z. Li et al. / Powder Technology 287 (2016) 256–263
3.2. Removing iron from titanium solution The titanium solution with certain iron ions may reduce the quality of as-prepared TiO2. Thus the Fe3+ should be reduced to Fe2+ using iron powder as a reducing agent. The ferrous sulfate could be separated after cooling and crystallization. The reaction was expressed as the following equation: 2Fe3þ þ Fe → 3 Fe2þ :
ð7Þ
The amount of iron powder was a key indicator to the reduction of Fe3 +. Table 2 showed the reduction effect of Fe3 + with different amounts of iron powder. It could be seen that when iron powder with an amount of 0.50 g (2.27 times that of the theoretical amount), KSCN solution in titanium solution could not turn to red, indicating that Fe3+ tended to reduce to Fe2+ completely. The XRD pattern of the byproduct was shown in Fig. 12. It could be seen that the by-product contained FeSO4 · 7H2O and FeSO4 · 4H2O. This was due to the fact that FeSO4 · 7H2O was prone to lose three crystal water to form FeSO4 · 4H2O in preservation. The obtained by-product could be widely used in medicine, dye, wastewater treatment and other fields.
Fig. 11. EDS pattern of residue.
Table 2 Reduction effect of Fe3+ under different amounts of iron powder. The amount of iron powder (g)
0.22
0.25
0.30
0.35
0.40
0.45
0.50
Color of KSCN solution
Red
Red
Red
Red
Red
Red
Colorless
3.3. Hydrolysis of TiOSO4 solution to prepare H2TiO3 Hydrolysis of TiOSO4 solution was shown as the following equation: TiO2þ þ 2H2 O → H2 TiO3 ↓ þ 2Hþ :
Fig. 12. XRD pattern of the by-product.
ð8Þ
On one hand, the hydrolysis of TiOSO4 solution was a process of H+ release. H+ could inhibit hydrolysis at strong acidity, thus, polymerization of the resulting titanium sol particles would be suppressed due to the repulsion of electric double layer, and the stable sol system could be obtained. Moreover, the polarity difference may inhibit TiOSO4 dissolved in an organic solvent system. On the other hand, the surfactant solubility in water was low; it could not self-assemble into larger micelles, instead of interacting with TiOSO4 solution in the form of small aggregation micelles. The products with poor structural order would be obtained. Therefore, TiOSO4 solution was hydrolyzed directly to prepare H2TiO3 without the addition of any surfactant in this study. For the hydrolysis system, some researches have published that TiO2 was prepared in alkaline systems, while few experiments were carried out under high acidity [11–14]. Since the initial pH value of the hydrolysis system was below 3, which couldn't reach the acidity precipitation of cations such as Mg2+ and Al3+ during hydrolysis. Thus, it effectively
Fig. 13. XRD and EDS patterns of H2TiO3.
Z. Li et al. / Powder Technology 287 (2016) 256–263
261
Table 3 The chemical composition analysis of H2TiO3 (wt.%). Composition
TiO2
Al2O3
MgO
CaO
SO3
Wt.%
93.129
0.108
0.023
0.93
5.81
prevented the small amount of impurities precipitating, and provided the conditions to obtain TiO2 with high purity. Fig. 13(a) displayed the XRD pattern of H2TiO3; it could be seen that the diffraction peak was relatively wide, illustrating that H2TiO3 was poor crystallized. As the EDS pattern is shown in Fig. 13(b), almost all impurities except sulfur were absent in the H2TiO3 precipitate. This was due to the fact that the precipitated hydrated TiO2 in the hydrolysis of TiOSO4 solution could adsorb a large number of sulfate ions present as impurities in H2TiO3. In order to further confirm that the elements existed in the as-prepared H2TiO3 precipitate, the chemical composition analysis by XRF was carried out. As shown in Table 3, in addition to sulfur element, Al, Mg, and Ca were also detected. A TG experiment of the H2TiO3 was conducted and the result was presented in Fig. 14. The curve could be divided into three stages. The initial weight loss (13.5%) occurred in the temperature ranged from the ambient temperature to 200 °C. The stage was attributed to the evaporation of the physically adsorbed water. The second stage was from 200 °C to 400 °C with a mass loss of 9% owing to the evaporation of the chemisorbed water. The third stage with a mass loss of 7.5% occurred above 500 °C as a result of the H2SO4 desorption; it was reasonably consistent with the result of the chemical composition analysis by XRF (5.81% SO3≌ 7.12% H2SO4). It indicated that the active sulfate could firmly bond at the surface of TiO2 at 500 °C during calcination, which was consistent with the results of the reported literature [15]. In order to remove sulfate effectively, calcination temperature above 500 °C was chosen for the experiments. 3.4. Calcination to prepare nano-TiO2 The composition of the resulting H2TiO3 sample could be described as TiO2 · xH2O · ySO3. In order to remove the crystal water and H2SO4 from the sample, H2TiO3 precipitate was subsequently calcined at 600 °C, 850 °C and 950 °C for 4 h, respectively. Nano-TiO2 with different polymorphs and sizes was prepared successively. The reaction during calcination was described as the following equation: TiO2 xSO3 yH2 O → ΔTiO2 þ xSO3 ↑ þ yH2 O:
ð9Þ
Fig. 15 displayed the XRD patterns of TiO2 under different calcination temperatures. According to the peak indexes of Jade 5.0, the anatase (PDF NO.65-5714) with a crystallinity of 93.89% was obtained under calcination temperature of 600 °C, the rutile (PDF NO.65-1119) with a
Fig. 14. TG trace of H2TiO3.
Fig. 15. XRD patterns of as-prepared TiO2 under different calcination temperatures: (a) 600 °C, (b) 850 °C, (c) 950 °C.
crystallinity of 96.82% was obtained under calcination temperature of 950 °C, a mixture of anatase and rutile obtained under calcination temperature of 850 °C. Therefore, the well-crystallized TiO2 with different polymorphs could be obtained from different calcination temperatures. The morphology images (SEM, TEM, HRTEM) of the as-prepared TiO2 calcined at 600 °C, 850 °C and 950 °C were illustrated in Fig. 16. As could be seen from Fig. 16(a, b, c), the TiO2 calcined at 600 °C consisted of spherical particles with a typical size of about 40 nm. It could be observed from Fig. 16(d, e, f); the TiO2 calcined at 850 °C was obtained in the form of aggregates, consisting of a larger particle size (~60 nm) and spherical morphology. As can be seen from Fig. 16(g, h, i), it was clear that the TiO2 calcined at 950 °C consisted of non-uniform particles with irregular morphology, meanwhile, the particles were heavily aggregated. The EDS pattern of the as-prepared anatase was shown in Fig. 17. It indicated that no impurities such as Al, Mg, Ca, and Mn could be detected. In order to further confirm the purity of product, ICP analysis was carried out. As shown in Table 4, the trace amount of Al, Mg, and Ca was detected in the result of ICP although there were no impurities detected in EDS. The sulfate ions adsorbed on the surface of H2TiO3 were removed via calcination [16],the as-prepared TiO2 with a purity above 99% was obtained. Therefore, it could be concluded that maintaining the initial pH value of the hydrolysis system below 3 could effectively prevent the small amount of other impurities precipitating. It was a feasible way to improve the purity of the TiO2 product. 4. Conclusion In this study, nano-TiO2 with different polymorphs was prepared from Panzhihua ilmenite using sulfuric acid-decomposition by liquid phase method. The main conclusions were as follows: (1) These four factors including H2SO4 molar volume, reaction time, initial pre-heating temperature and H2SO4 concentration have a significant effect on the decomposition rate of ilmenite. The decomposition rate increased with the increasing of sulfuric acid molar volume from 0.28 mol to 0.32 mol, while little change appeared from 0.32 mol to 0.36 mol. It reached 93.32% using the H2SO4 molar volume of 0.32 mol. The decomposition rate increased from 30 min to 120 min, and kept stably later than 120 min. The decomposition rate increased with an increase in the initial preheating temperature from 100 °C to 160 °C and reached 95.21% at 160 °C. The decomposition rate increased with the increasing of H2SO4 concentration from 11.5 mol/L to 13.5 mol/L. However, a sharp decrease was observed when the concentration was greater than 14.5 mol/L. When ilmenite reacted with 13.5 mol/L H2SO4 solution at 160 °C for 120 min, it
262
Z. Li et al. / Powder Technology 287 (2016) 256–263
Fig. 16. Morphology images of the as-prepared TiO2: anatase (a, b and c); anatase–rutile mixture (d, e and f); rutile (g, h and i).
required 0.32 mol H2SO4, and the decomposition rate could be achieved at 95.21%. These would be considered as the optimal experimental conditions. (2) Fe3+ in the titanium solution was reduced to Fe2+ via adding an appropriate amount of iron powder late into the reaction, then Fe2 + was removed after cooling and crystallization. The result
showed that it required 0.5 g iron powder to reduce Fe3 + for 20 g ilmenite. (3) The initial pH value of the hydrolysis system below 3 was beneficial for improving the purity of H2TiO3 during hydrolysis. (4) The spherical anatase with a particle size of about 40 nm was obtained after being calcined at 600 °C for 4 h. The anatase was homogeneous with a crystallinity of 93.89% and a purity of 99.938%. A mixture of anatase and rutile with a particle size of about 60 nm was obtained by calcination at 850 °C. The sample was spherical with agglomeration. The rutile with a crystallinity of 96.82% was obtained by calcination at 950 °C, exhibiting nonuniform particles with irregular morphology. The particles were heavily aggregated. Based on the above results, we have proposed an environmentfriendly, economical and simple method to prepare nano-TiO2 from ilmenite, which was promising for the large industrialized production in wastewater treatment, cosmetics and coatings and other areas of chemical industry. Acknowledgments The authors gratefully acknowledge support from the Fundamental Research Funds for the China Central Universities (2013ZY1348).
Fig. 17. EDS pattern of anatase.
References Table 4 Major chemical composition of the as-prepared anatase. Elements
Ti
Fe
Si
Al
Mg
Ca
As-prepared anatase (wt.%)
99.938
0
0
0.018
0.003
0.041
Mn 0
[1] H. Rafik, I. Mahmoud, T. Mohamed, B. Abdenacer, TiO2 films photocatalytic activity improvements by swift heavy ions irradiation, Radiat. Phys. Chem. 101 (2014) 1–7. [2] A. Molea, V. Popescu, N.A. Rowson, A.M. Dinescu, Influence of pH on the formulation of TiO2 nano-crystalline powders with high photocatalytic activity, Powder Technol. 253 (2014) 22–28.
Z. Li et al. / Powder Technology 287 (2016) 256–263 [3] A. Molea, V. Popescu, N.A. Rowson, Effects of I-doping content on the structural, optical and photocatalytic activity of TiO2 nanocrystalline, Powder Technol. 230 (2012) 203–211. [4] H.H. Tseng, W.W. Lee, M.C. Wei, B.S. Huang, M.C. Hsieh, P.Y. Cheng, Synthesis of TiO2/SBA-15 photocatalyst for the azo dye decolorization through the polyol method, Chem. Eng. J. 210 (2012) 529–538. [5] F. Li, H. Li, L.X. Guan, M.M. Yao, Nanocrystalline CO2+/F-codoped TiO2–SiO2 composite films for environmental applications, Chem. Eng. J. 252 (2014) 1–10. [6] T. Tao, A.M. Glushenkov, Q.Y. Chen, H.P. Hu, D. Zhou, H.Z. Zhang, B. Markus, S. Liu, R. Amal, Y. Chen, Porous TiO2 with a controllable bimodal pore size distribution from natural ilmenite, CrystEngComm 13 (2011) 1322. [7] X.H. Xiong, Z.X. Wang, F.X. Wu, X.H. Li, H.J. Guo, Preparation of TiO2 from ilmenite using sulfuric acid decomposition of the titania residue combined with separation of Fe3+ with EDTA during hydrolysis, Adv. Powder Technol. 24 (2013) 60–67. [8] J. Yu, Y. Chen, A.M. Glushenkov, Titanium oxide nanorods extracted from ilmenite sands, Cryst. Growth Des. 9 (2009) 1240–1244. [9] Q. Wang, L. Jiang, F. Ni, T. Wang, The separation and concentration of titanium (IV) from sulfuric acid solution with trioctylamine, Ind. Eng. Chem. Res. 51 (2012) 3430–3435. [10] S.C. Zhang, M.J. Nicol, Kinetics of the decomposition of ilmenite in sulfuric acid solutions under reducing conditions, Hydrometallurgy 103 (2010) 196–204.
263
[11] I. Szilagyi, E. Konigsberger, P.M. May, Characterization of chemical speciation of titanyl sulfate solutions for production of titanium dioxide precipitates, Inorg. Chem. 48 (2009) 2200–2204. [12] C.X. Tian, S.H. Huang, Y. Yang, Anatase TiO2 white pigment production from unenriched industrial titanyl sulfate solution via short sulfate process, Dyes Pigments 96 (2013) 609–613. [13] K.M. Khalil, R.M. Elkhatib, T.T. Ali, H.A. Mahmoud, A.A. Elsamahy, Titania nanoparticles by acidic peptization of xerogel formed by hydrolysis of titanium(IV) isopropoxide under atmospheric humidity conditions, Powder Technol. 24 (2013) 156–162. [14] Z.C. Xu, Q. Li, S.A. Gao, J.K. Shang, As(III) removal by hydrous titanium dioxide prepared from one-step hydrolysis of aqueous TiCl4 solution, Water Res. 44 (2010) 5713–5721. [15] C. Li, B. Liang, H. Song, J.Q. Xu, X.Q. Wang, Preparation of porous rutile titania from ilmenite by mechanical activation and subsequent sulfuric acid leaching, Microporous Mesoporous Mater. 115 (2008) 293–300. [16] T. Tao, Q.Y. Chen, H.P. Hu, Z.L. Yin, Y. Chen, TiO2 nanoparticles prepared by hydrochloric acid leaching of mechanically activated and carbothermic reduced ilmenite, Trans. Nonferrous Metals Soc. China 22 (2012) 1232–1238.
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Preparation of nano-titanium dioxide from ilmenite using sulfuric acid-decomposition by liquid phase method Zenghe Li, Zhencui Wang, Ge Li ⁎ Faculty of Science, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 21 August 2014 Received in revised form 27 July 2015 Accepted 5 September 2015 Available online 12 September 2015 Keywords: Ilmenite H2SO4 Liquid phase method Direct hydrolysis Nano-TiO2
a b s t r a c t In this study, high purity nano-TiO2 was prepared from Panzhihua ilmenite using sulfuric acid-decomposition by liquid phase method. The effects of H2SO4 molar volume, reaction time, initial pre-heating temperature and H2SO4 concentration on the decomposition rate of ilmenite were investigated in detail. The results showed that the decomposition rate reached 95.21% under the optimal conditions. The resulting titanium solution was qualified with 121 g/L Ti4+. Since the initial pH value of hydrolysis system was below 3, the precipitation of metatitanic acid was obtained by direct hydrolysis of the titanium solution. On the other hand, the wellcrystallized anatase and rutile were achieved at different calcination temperatures. The purity of the asprepared TiO2 was above 99% with an average particle size of about 40–100 nm. The ilmenite ore, intermediates and final products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray fluorescence spectrometer (EDS), X-ray fluorescence spectroscopy (XRF), thermal gravimetric analysis (TG), inductive coupled plasma emission spectrometer (ICP) and spectrophotometer. The results demonstrated that the process was suitable for industrial production because it was inexpensive, environment-friendly and promising in the preparation of high purity nano-TiO2. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nano-TiO2 is widely used in photocatalysis, solar cells, sensors, coatings and other areas of chemical industry, owing to its nontoxicity, chemical stability, excellent color effect and strong ability for UVshielding [1]. Currently, there are many methods to produce nano-TiO2, such as chemical vapor deposition (CVD), oxidation of titanium tetrachloride, sol–gel technique, and thermal decomposition or hydrolysis of titanium alkoxides [2–5]. The titanium sources in these processes are expensive. Moreover, the reactions often require rigorous conditions. Therefore, these processes are restricted in the further applications. As is known to all, ilmenite is cheap, non-toxic and widely distributed in the world, primarily in North America (USA, Canada), South America (Brazil), Australia, Europe (Norway, Ukraine), Africa (South Africa, Mozambique) and Asia (China, India). Panzhihua is one of the important mineral deposits in China [6]. Thus, it would be very desirable for Panzhihua ilmenite to adapt to the industrial preparation of nano-TiO2. Nano-TiO2 is commercially manufactured by sulfuric aciddecomposition method and chlorination method. The chlorination method is complex, and it requires high quality ilmenite feedstock. Therefore, sulfuric acid-decomposition method is still an important
⁎ Corresponding author. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.powtec.2015.09.008 0032-5910/© 2015 Elsevier B.V. All rights reserved.
way for the preparation of nano-TiO2 from Panzhihua ilmenite, all of which are associated with magnetite and high-level CaO and MgO. Sulfuric acid-decomposition method can be divided into solid-phase method and liquid-phase method via the way of decomposition. Several researches have published the sulfuric acid-decomposition treatment to produce TiO2 from ilmenite by solid-phase method [7,8]. However, it requires high reaction temperature (above 200 °C) and high concentration of H2SO4 (above 15.5 M). Moreover, it discharges large amount of waste gas such as SO2 and SO3, leading to severe environmental problems. However, liquid-phase method needs lower acid concentration below 15.5 M and lower reaction temperature at the range of 130– 160 °C. The reaction is more moderate and it's beneficial for reducing waste gas emission. Besides, it is easy to achieve continuous production without the need of the leaching process. Therefore, it is necessary and urgent to study the liquid-phase process deeply to simplify the sulfate process and reduce environmental pollution. In this paper, nano-TiO2 was prepared from Panzhihua ilmenite using sulfuric acid-decomposition by liquid phase method. The factors affecting the decomposition rate were investigated. The initial pH value of the resulting TiOSO4 solution was below 3, which could not reach the acid precipitation of Mg2+ and Al3 + during the hydrolysis. Therefore, it effectively prevented the small amount of impurities precipitating. H2TiO3 precipitate was obtained by direct hydrolysis without the addition of any surfactant. The well-crystallized nano-TiO2 with high purity was obtained via calcination from the as-prepared H2TiO3
Z. Li et al. / Powder Technology 287 (2016) 256–263
257
2.3. Preparation of nano-TiO2 from ilmenite
Fig. 1. XRD pattern of the ilmenite ore.
precipitate. In addition, the effects of calcination temperature on polymorphs and particle size of TiO2 were investigated.
This process was carried out in a 250 ml three-necked flask employed with a thermometer, stirrer and reflux condenser. Firstly, the concentrated H2SO4 was heated to certain temperature, and then 20 g of ilmenite was added to the solution under continuous stirring. After 1 h, in order to keep the reaction in the liquid condition, 50 ml of 1.0 mol/L dilute H2SO4 at 90 °C was added to the reaction system. This article mainly focused on the effects of H2SO4 molar volume, reaction time, initial pre-heating temperature, and H2SO4 concentration on the decomposition rate of ilmenite. The existence of Fe3+ would reduce the purity of TiO2, thus, affecting the whiteness of the product. Therefore, it should be reduced to Fe2+ via the addition of an appropriate amount of iron powder late into the reaction. Titanium solution was obtained by filtration, then crystallized at 5 °C for two days. Finally, FeSO4 · 7H2O crystal and TiOSO4 filtrate solution were obtained by filtration. The titanium concentration in the solution was analyzed by a spectrophotometer. The decomposition rate of ilmenite was calculated by the following formula [5]: x% ¼ ðV C Ti =m wTi %Þ 100%
ð1Þ
2. Experimental 2.1. Materials and analysis All the chemical reagents employed were of analytical grade, and deionized water was used throughout. The ilmenite used in the research was supplied by titanium company of Panzhihua Honor and Trade Corporation. Its particle size was about 100 μm. Iron powder was purchased from Beijing Chemical Company. Its effective iron content was 98% and the particle size was about 80 μm. X-ray diffraction (XRD) characterization was carried out with a diffractometer D/Max-RA (Rigaku, Japan) using Cu Kα radiation with a step size of 0.02° and a scan range from 10° to 80°. SEM images were taken from the S-4700 scanning electron microscope (Hitachi, Japan), which operated at 20 kV. TEM images were taken from the H-800 transmission electron microscope (Hitachi, Japan), which operated at 200 kV. HRTEM images were taken from the JEM-3010 high-resolution transmission electron microscope (Hitachi, Japan). JEOL J-3010 (Japan) energy dispersive X-ray spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF) system was used for chemical composition analysis. Thermal analysis experiments were carried out on a simultaneous thermogravimetric and differential thermal analyzer (Diamond TG/DTA, PERKINELMER) with a heating rate of 10 °C/min and an air flow rate of 300 ml/min. ICP analysis was performed on ICPS-7500 inductive coupled plasma emission spectrometer (Shimadzu, Japan). The concentration of titanium in the solution was analyzed by 722 N spectrophotometer (China).
2.2. Characterization of the ilmenite ore XRD and chemical composition for the ilmenite ore used in this study were shown in Fig. 1 and Table 1, respectively. Fig. 1 indicated that the ore mainly contained ilmenite (FeTiO3, PDF NO.29-0733), which is consistent with its chemical composition analysis (Table 1). Based on the mass balance principle, the mineral content of the ore included 86.79 wt.% ilmenite, 5.32 wt.% rutile, 4.40 wt.% quartz, 3.11 wt.% hematite and 0.38 wt.% microcline [9].
where x% denotes the decomposition rate of ilmenite, CTi denotes the titanium concentration in the titanium solution (g · L−1), m denotes the total mass of the ilmenite (g), wTi% denotes the mass fraction of soluble titanium in the ilmenite, and V denotes the volume of titanium solution (L). The resulting concentrated TiOSO4 solution was used for hydrolysis in a 250 ml three-necked flask employed with a thermometer, stirrer and reflux condenser. Firstly, 25 ml of deionized water was added to the three-necked flask, and 5 ml of TiOSO4 solution was slowly dropped to the reaction system under continuous stirring at room temperature. Then the solution was heated to 90 °Cand kept for 3 h. The precipitate was obtained during aging at room temperature for 6 h, filtered, washed with deionized water and ethanol, and then dried at 80 °C. The asprepared H2TiO3 precipitate was calcined at various temperatures in a tubular furnace for 4 h to synthesize nano-TiO2. The experimental procedure was shown in Fig. 2. 3. Results and discussion 3.1. Decomposition of ilmenite with H2SO4 The ilmenite ore was composed of a variety of minerals, and the decomposition of ilmenite with H2SO4 was a complex heterogeneous reaction. The main reactions were expressed by Eqs. (2)–(6) [9]: FeTiO3 ðsÞ þ 4Hþ ðaqÞ ¼ TiO2þ ðaqÞ þ Fe2þ ðaqÞ þ 2H2 O
ð2Þ
Fe2 O3 ðsÞ þ 6Hþ ðaqÞ ¼ 2 Fe3þ ðaqÞ þ 3H2 OðlÞ
ð3Þ
MgOðsÞ þ 2Hþ ðaqÞ ¼ Mg2þ ðaqÞ þ H2 OðlÞ
ð4Þ
Al2 O3 ðsÞ þ 6Hþ ðaqÞ ¼ 2Al ðaqÞ þ 3H2 OðlÞ
ð5Þ
CaOðsÞ þ 2Hþ ðaqÞ ¼ Ca2þ ðaqÞ þ H2 OðlÞ:
ð6Þ
3þ
Table 1 The chemical composition analysis of ilmenite (wt.%). Composition
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Wt.%
4.78
43.6
2.10
3.11
34.45
0.69
4.73
1.15
0.20
0.13
0.05
5.01
258
Z. Li et al. / Powder Technology 287 (2016) 256–263
Fig. 4. Decomposition rate of ilmenite under different H2SO4 molar volumes.
increase the cost, but also make the acidity too strong for subsequent hydrolysis, leading to slow hydrolyzation. Therefore, the volume of 0.32 mol was used in the following experiments.
Fig. 2. The experimental procedure of preparation of nano-TiO2 from ilmenite.
The effects of H2SO4 molar volume, reaction time, initial preheating temperature and H2SO4 concentration on the rate of decomposition were investigated, respectively. 3.1.1. Effect of H2SO4 molar volume According to the stoichiometric ratio, the decomposition of 20 g ilmenite required 0.30 mol H2SO4. Therefore, when the reaction time was 120 min, the initial preheat temperature was 150 °C, and the H2SO4 concentration was 13.5 mol/L; sulfuric acid molar volume of 0.24 mol, 0.28 mol, 0.30 mol, 0.32 mol, 0.34 mol, and 0.36 mol were adopted to investigate the influence of H2SO4 molar volume on the decomposition rate of ilmenite. Figs. 3 and 4 were the XRD patterns of residue and the decomposition rate of ilmenite under different H2SO4 molar volumes, respectively. As can be seen from Fig. 3, the main diffraction peak of FeTiO3 in the residue (2θ = 33°) almost disappeared when H2SO4 molar volume reached 0.32 mol, indicating that the reaction tended to be complete. The results in Fig. 4 showed that the decomposition rate of ilmenite increased with the increasing of H2SO4 molar volume from 0.24 mol to 0.32 mol, while little change appeared from 0.32 mol to 0.36 mol. In addition, too much H2SO4 would not only
Fig. 3. XRD patterns of residue under different H2SO4 molar volumes.
3.1.2. Effect of reaction time The effect of reaction time was studied in the range of 30–150 min. Figs. 5 and 6 were the XRD patterns of residue and decomposition rate of ilmenite under different reaction times. The results illustrated that the diffraction peak intensity of FeTiO3 decreased significantly and the decomposition rate of ilmenite increased with an increase in the
Fig. 5. XRD patterns of residue under different reaction times.
Fig. 6. Decomposition rate of ilmenite under different reaction times.
Z. Li et al. / Powder Technology 287 (2016) 256–263
259
Fig. 7. XRD patterns of residue under different initial preheating temperatures. Fig. 9. XRD patterns of residue under different sulfuric acid concentrations.
reaction time from 30 to 120 min. The decomposition rate reached 93.32% in 120 min. In a reaction time range of 120–150 min, a small variation of the decomposition rate was observed, it maintained between 93.32% and 93.21%. It would have an adverse effect upon the decomposition of ilmenite if the increasing of the reaction time was excessive, because TiOSO4 solution was likely to hydrolyze in advance. Therefore, 120 min would represent as the optimum reaction time.
3.1.3. Effect of initial preheating temperature The activation energy of ilmenite with H2SO4 was too high to allow the occurring of the reaction at room temperature. Therefore, it was necessary to preheat to a certain temperature. The effect of initial preheating temperature on the decomposition rate of ilmenite was studied in the range of 100–160 °C. Figs. 7 and 8 were the XRD patterns of residue and decomposition rate of ilmenite under different initial preheating temperatures, respectively. It could be seen that the initial preheating temperature had a significant effect on the decomposition rate. The diffraction peak intensity of FeTiO3 was significantly weakened with an increase in initial preheat temperature. Meanwhile, the decomposition rate increased sharply and reached 93.32% at 150 °C. In a temperature range of 150–160 °C, a slight increase of decomposition rate from 93.32% to 95.21% was observed; the decomposition rate tended to be stable. These results were due to the fact that the elevating temperature may enhance surface reaction rate, and reduce the viscosity of the liquid phase [10]. It could reduce the ion diffusion resistance and enhance mass transfer driving force. Therefore, 160 °C would be considered as the optimum temperature.
Fig. 8. Decomposition rate of ilmenite under different initial preheating temperatures.
3.1.4. Effect of H2SO4 concentration In order to examine the effect of the H2SO4 concentration on the decomposition of ilmenite, several experiments were carried out using H2SO4 concentrations varying from 11.5 to 15.5 mol/L. Figs. 9 and 10 were the XRD patterns of residue and decomposition rate of ilmenite under different H2SO4 concentrations, respectively. As shown in Figs. 9 and 10, it was clear to see that the decomposition rate of ilmenite increased with the increasing of H2SO4 concentration from 11.5 to 13.5 mol/L. These results were due to the contact probability between ilmenite surface and H+, SO2− increased with an increase in the con4 centration of H2SO4, strengthening the solid surface force and dipole interaction between H+ and SO24 −. Thus the decomposition rate of ilmenite accelerated. However, the decomposition rate was almost unchanged when H2SO4 concentration was from 13.5 to 14.5 mol/L, and then decreased with the further increasing of acid concentration. The low decomposition rate at higher acid concentration was probably due to the reduction of proton activity and the formation of a product layer over the unreacted core. Therefore, the excessively high acid concentration would not improve the decomposition rate. Moreover, it would impose a heavy burden upon the H2SO4 regeneration system. An acid concentration of 13.5 mol/L was suggested as the optimum condition and the decomposition rate reached 95.21%. The ilmenite did not fully decompose; this may have been affected by the mass transfer resistance of the product layer in the reaction. The EDS pattern of residue was shown in Fig. 11. It could be seen that the residue contained major impurities, such as Ti, Fe, Si, Ca, Al, and Mg. It indicated that most of the impurities were removed during the decomposition process. The resulting titanium solution was qualified with 121 g/L Ti4+.
Fig. 10. Decomposition rate of ilmenite under different sulfuric acid concentrations.
260
Z. Li et al. / Powder Technology 287 (2016) 256–263
3.2. Removing iron from titanium solution The titanium solution with certain iron ions may reduce the quality of as-prepared TiO2. Thus the Fe3+ should be reduced to Fe2+ using iron powder as a reducing agent. The ferrous sulfate could be separated after cooling and crystallization. The reaction was expressed as the following equation: 2Fe3þ þ Fe → 3 Fe2þ :
ð7Þ
The amount of iron powder was a key indicator to the reduction of Fe3 +. Table 2 showed the reduction effect of Fe3 + with different amounts of iron powder. It could be seen that when iron powder with an amount of 0.50 g (2.27 times that of the theoretical amount), KSCN solution in titanium solution could not turn to red, indicating that Fe3+ tended to reduce to Fe2+ completely. The XRD pattern of the byproduct was shown in Fig. 12. It could be seen that the by-product contained FeSO4 · 7H2O and FeSO4 · 4H2O. This was due to the fact that FeSO4 · 7H2O was prone to lose three crystal water to form FeSO4 · 4H2O in preservation. The obtained by-product could be widely used in medicine, dye, wastewater treatment and other fields.
Fig. 11. EDS pattern of residue.
Table 2 Reduction effect of Fe3+ under different amounts of iron powder. The amount of iron powder (g)
0.22
0.25
0.30
0.35
0.40
0.45
0.50
Color of KSCN solution
Red
Red
Red
Red
Red
Red
Colorless
3.3. Hydrolysis of TiOSO4 solution to prepare H2TiO3 Hydrolysis of TiOSO4 solution was shown as the following equation: TiO2þ þ 2H2 O → H2 TiO3 ↓ þ 2Hþ :
Fig. 12. XRD pattern of the by-product.
ð8Þ
On one hand, the hydrolysis of TiOSO4 solution was a process of H+ release. H+ could inhibit hydrolysis at strong acidity, thus, polymerization of the resulting titanium sol particles would be suppressed due to the repulsion of electric double layer, and the stable sol system could be obtained. Moreover, the polarity difference may inhibit TiOSO4 dissolved in an organic solvent system. On the other hand, the surfactant solubility in water was low; it could not self-assemble into larger micelles, instead of interacting with TiOSO4 solution in the form of small aggregation micelles. The products with poor structural order would be obtained. Therefore, TiOSO4 solution was hydrolyzed directly to prepare H2TiO3 without the addition of any surfactant in this study. For the hydrolysis system, some researches have published that TiO2 was prepared in alkaline systems, while few experiments were carried out under high acidity [11–14]. Since the initial pH value of the hydrolysis system was below 3, which couldn't reach the acidity precipitation of cations such as Mg2+ and Al3+ during hydrolysis. Thus, it effectively
Fig. 13. XRD and EDS patterns of H2TiO3.
Z. Li et al. / Powder Technology 287 (2016) 256–263
261
Table 3 The chemical composition analysis of H2TiO3 (wt.%). Composition
TiO2
Al2O3
MgO
CaO
SO3
Wt.%
93.129
0.108
0.023
0.93
5.81
prevented the small amount of impurities precipitating, and provided the conditions to obtain TiO2 with high purity. Fig. 13(a) displayed the XRD pattern of H2TiO3; it could be seen that the diffraction peak was relatively wide, illustrating that H2TiO3 was poor crystallized. As the EDS pattern is shown in Fig. 13(b), almost all impurities except sulfur were absent in the H2TiO3 precipitate. This was due to the fact that the precipitated hydrated TiO2 in the hydrolysis of TiOSO4 solution could adsorb a large number of sulfate ions present as impurities in H2TiO3. In order to further confirm that the elements existed in the as-prepared H2TiO3 precipitate, the chemical composition analysis by XRF was carried out. As shown in Table 3, in addition to sulfur element, Al, Mg, and Ca were also detected. A TG experiment of the H2TiO3 was conducted and the result was presented in Fig. 14. The curve could be divided into three stages. The initial weight loss (13.5%) occurred in the temperature ranged from the ambient temperature to 200 °C. The stage was attributed to the evaporation of the physically adsorbed water. The second stage was from 200 °C to 400 °C with a mass loss of 9% owing to the evaporation of the chemisorbed water. The third stage with a mass loss of 7.5% occurred above 500 °C as a result of the H2SO4 desorption; it was reasonably consistent with the result of the chemical composition analysis by XRF (5.81% SO3≌ 7.12% H2SO4). It indicated that the active sulfate could firmly bond at the surface of TiO2 at 500 °C during calcination, which was consistent with the results of the reported literature [15]. In order to remove sulfate effectively, calcination temperature above 500 °C was chosen for the experiments. 3.4. Calcination to prepare nano-TiO2 The composition of the resulting H2TiO3 sample could be described as TiO2 · xH2O · ySO3. In order to remove the crystal water and H2SO4 from the sample, H2TiO3 precipitate was subsequently calcined at 600 °C, 850 °C and 950 °C for 4 h, respectively. Nano-TiO2 with different polymorphs and sizes was prepared successively. The reaction during calcination was described as the following equation: TiO2 xSO3 yH2 O → ΔTiO2 þ xSO3 ↑ þ yH2 O:
ð9Þ
Fig. 15 displayed the XRD patterns of TiO2 under different calcination temperatures. According to the peak indexes of Jade 5.0, the anatase (PDF NO.65-5714) with a crystallinity of 93.89% was obtained under calcination temperature of 600 °C, the rutile (PDF NO.65-1119) with a
Fig. 14. TG trace of H2TiO3.
Fig. 15. XRD patterns of as-prepared TiO2 under different calcination temperatures: (a) 600 °C, (b) 850 °C, (c) 950 °C.
crystallinity of 96.82% was obtained under calcination temperature of 950 °C, a mixture of anatase and rutile obtained under calcination temperature of 850 °C. Therefore, the well-crystallized TiO2 with different polymorphs could be obtained from different calcination temperatures. The morphology images (SEM, TEM, HRTEM) of the as-prepared TiO2 calcined at 600 °C, 850 °C and 950 °C were illustrated in Fig. 16. As could be seen from Fig. 16(a, b, c), the TiO2 calcined at 600 °C consisted of spherical particles with a typical size of about 40 nm. It could be observed from Fig. 16(d, e, f); the TiO2 calcined at 850 °C was obtained in the form of aggregates, consisting of a larger particle size (~60 nm) and spherical morphology. As can be seen from Fig. 16(g, h, i), it was clear that the TiO2 calcined at 950 °C consisted of non-uniform particles with irregular morphology, meanwhile, the particles were heavily aggregated. The EDS pattern of the as-prepared anatase was shown in Fig. 17. It indicated that no impurities such as Al, Mg, Ca, and Mn could be detected. In order to further confirm the purity of product, ICP analysis was carried out. As shown in Table 4, the trace amount of Al, Mg, and Ca was detected in the result of ICP although there were no impurities detected in EDS. The sulfate ions adsorbed on the surface of H2TiO3 were removed via calcination [16],the as-prepared TiO2 with a purity above 99% was obtained. Therefore, it could be concluded that maintaining the initial pH value of the hydrolysis system below 3 could effectively prevent the small amount of other impurities precipitating. It was a feasible way to improve the purity of the TiO2 product. 4. Conclusion In this study, nano-TiO2 with different polymorphs was prepared from Panzhihua ilmenite using sulfuric acid-decomposition by liquid phase method. The main conclusions were as follows: (1) These four factors including H2SO4 molar volume, reaction time, initial pre-heating temperature and H2SO4 concentration have a significant effect on the decomposition rate of ilmenite. The decomposition rate increased with the increasing of sulfuric acid molar volume from 0.28 mol to 0.32 mol, while little change appeared from 0.32 mol to 0.36 mol. It reached 93.32% using the H2SO4 molar volume of 0.32 mol. The decomposition rate increased from 30 min to 120 min, and kept stably later than 120 min. The decomposition rate increased with an increase in the initial preheating temperature from 100 °C to 160 °C and reached 95.21% at 160 °C. The decomposition rate increased with the increasing of H2SO4 concentration from 11.5 mol/L to 13.5 mol/L. However, a sharp decrease was observed when the concentration was greater than 14.5 mol/L. When ilmenite reacted with 13.5 mol/L H2SO4 solution at 160 °C for 120 min, it
262
Z. Li et al. / Powder Technology 287 (2016) 256–263
Fig. 16. Morphology images of the as-prepared TiO2: anatase (a, b and c); anatase–rutile mixture (d, e and f); rutile (g, h and i).
required 0.32 mol H2SO4, and the decomposition rate could be achieved at 95.21%. These would be considered as the optimal experimental conditions. (2) Fe3+ in the titanium solution was reduced to Fe2+ via adding an appropriate amount of iron powder late into the reaction, then Fe2 + was removed after cooling and crystallization. The result
showed that it required 0.5 g iron powder to reduce Fe3 + for 20 g ilmenite. (3) The initial pH value of the hydrolysis system below 3 was beneficial for improving the purity of H2TiO3 during hydrolysis. (4) The spherical anatase with a particle size of about 40 nm was obtained after being calcined at 600 °C for 4 h. The anatase was homogeneous with a crystallinity of 93.89% and a purity of 99.938%. A mixture of anatase and rutile with a particle size of about 60 nm was obtained by calcination at 850 °C. The sample was spherical with agglomeration. The rutile with a crystallinity of 96.82% was obtained by calcination at 950 °C, exhibiting nonuniform particles with irregular morphology. The particles were heavily aggregated. Based on the above results, we have proposed an environmentfriendly, economical and simple method to prepare nano-TiO2 from ilmenite, which was promising for the large industrialized production in wastewater treatment, cosmetics and coatings and other areas of chemical industry. Acknowledgments The authors gratefully acknowledge support from the Fundamental Research Funds for the China Central Universities (2013ZY1348).
Fig. 17. EDS pattern of anatase.
References Table 4 Major chemical composition of the as-prepared anatase. Elements
Ti
Fe
Si
Al
Mg
Ca
As-prepared anatase (wt.%)
99.938
0
0
0.018
0.003
0.041
Mn 0
[1] H. Rafik, I. Mahmoud, T. Mohamed, B. Abdenacer, TiO2 films photocatalytic activity improvements by swift heavy ions irradiation, Radiat. Phys. Chem. 101 (2014) 1–7. [2] A. Molea, V. Popescu, N.A. Rowson, A.M. Dinescu, Influence of pH on the formulation of TiO2 nano-crystalline powders with high photocatalytic activity, Powder Technol. 253 (2014) 22–28.
Z. Li et al. / Powder Technology 287 (2016) 256–263 [3] A. Molea, V. Popescu, N.A. Rowson, Effects of I-doping content on the structural, optical and photocatalytic activity of TiO2 nanocrystalline, Powder Technol. 230 (2012) 203–211. [4] H.H. Tseng, W.W. Lee, M.C. Wei, B.S. Huang, M.C. Hsieh, P.Y. Cheng, Synthesis of TiO2/SBA-15 photocatalyst for the azo dye decolorization through the polyol method, Chem. Eng. J. 210 (2012) 529–538. [5] F. Li, H. Li, L.X. Guan, M.M. Yao, Nanocrystalline CO2+/F-codoped TiO2–SiO2 composite films for environmental applications, Chem. Eng. J. 252 (2014) 1–10. [6] T. Tao, A.M. Glushenkov, Q.Y. Chen, H.P. Hu, D. Zhou, H.Z. Zhang, B. Markus, S. Liu, R. Amal, Y. Chen, Porous TiO2 with a controllable bimodal pore size distribution from natural ilmenite, CrystEngComm 13 (2011) 1322. [7] X.H. Xiong, Z.X. Wang, F.X. Wu, X.H. Li, H.J. Guo, Preparation of TiO2 from ilmenite using sulfuric acid decomposition of the titania residue combined with separation of Fe3+ with EDTA during hydrolysis, Adv. Powder Technol. 24 (2013) 60–67. [8] J. Yu, Y. Chen, A.M. Glushenkov, Titanium oxide nanorods extracted from ilmenite sands, Cryst. Growth Des. 9 (2009) 1240–1244. [9] Q. Wang, L. Jiang, F. Ni, T. Wang, The separation and concentration of titanium (IV) from sulfuric acid solution with trioctylamine, Ind. Eng. Chem. Res. 51 (2012) 3430–3435. [10] S.C. Zhang, M.J. Nicol, Kinetics of the decomposition of ilmenite in sulfuric acid solutions under reducing conditions, Hydrometallurgy 103 (2010) 196–204.
263
[11] I. Szilagyi, E. Konigsberger, P.M. May, Characterization of chemical speciation of titanyl sulfate solutions for production of titanium dioxide precipitates, Inorg. Chem. 48 (2009) 2200–2204. [12] C.X. Tian, S.H. Huang, Y. Yang, Anatase TiO2 white pigment production from unenriched industrial titanyl sulfate solution via short sulfate process, Dyes Pigments 96 (2013) 609–613. [13] K.M. Khalil, R.M. Elkhatib, T.T. Ali, H.A. Mahmoud, A.A. Elsamahy, Titania nanoparticles by acidic peptization of xerogel formed by hydrolysis of titanium(IV) isopropoxide under atmospheric humidity conditions, Powder Technol. 24 (2013) 156–162. [14] Z.C. Xu, Q. Li, S.A. Gao, J.K. Shang, As(III) removal by hydrous titanium dioxide prepared from one-step hydrolysis of aqueous TiCl4 solution, Water Res. 44 (2010) 5713–5721. [15] C. Li, B. Liang, H. Song, J.Q. Xu, X.Q. Wang, Preparation of porous rutile titania from ilmenite by mechanical activation and subsequent sulfuric acid leaching, Microporous Mesoporous Mater. 115 (2008) 293–300. [16] T. Tao, Q.Y. Chen, H.P. Hu, Z.L. Yin, Y. Chen, TiO2 nanoparticles prepared by hydrochloric acid leaching of mechanically activated and carbothermic reduced ilmenite, Trans. Nonferrous Metals Soc. China 22 (2012) 1232–1238.