Ind. Eng. Chem. Res. 2010, 49, 7693–7696
7693
Preparation of Rutile Titanium Dioxide White Pigment via Doping and Calcination of Metatitanic Acid Obtained by the NaOH Molten Salt Method Yong Wang, Jie Li, Lina Wang, Tianyan Xue, and Tao Qi* National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, P.R. China, and Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China
Rutile titanium dioxide (TiO2) white pigment has been prepared by doping and calcination of metatitanic acid (H2TiO3) obtained by the NaOH molten salt method. It was found that the properties of the rutile TiO2 sample prepared were improved by adding K2O, P2O5, Al2O3, and rutile nuclei. The X-ray diffraction (XRD) results show the rutile content of the rutile TiO2 sample prepared is 97%, and scanning electron microscopy (SEM) results show that the rutile TiO2 particles prepared have well-shaped morphology and narrow particle size distribution. The purity and color performance of the rutile TiO2 sample prepared approached the commercial TiO2 pigment standards. Introduction Anatase and rutile TiO2 are manufactured in the chemical industry as white pigments. Rutile TiO2 is characterized by the highest thermodynamic stability, a higher packing density of atoms in its structure, and a higher refractive index.1 The pigment properties of rutile TiO2 are better than that of anatase TiO2. At present, the industrial production of TiO2 white pigment generally employs two processes: i.e. sulfate process and chloride process.2 Recently, a novel metallurgical process for TiO2 production from titanium slag has been developed by Xue et al.3 The key point of this process is a liquid phase decomposition of titanium slag in the NaOH molten salt medium at 500 °C. The new process could realize the higher titanium extraction (about 95-98%)3 and the recycling of NaOH to the front end.4 The reaction of titanium slag with the NaOH system yields sodium titanate (Na2TiO3) and could be described as follows: 2Ti3O5 + 12NaOH + O2 ) 6Na2TiO3 + 6H2O
(1)
impurities were removed. The general flowsheet of this process was shown in Figure 1. TiO2 white pigment could be prepared through the calcination of metatitanic acid. However, the pigmentary properties of the TiO2 white pigment products of this new process were inferior to the commercial standards. It is well-known that the properties of rutile TiO2 are affected by the kind and contents of additives present in the calcined titanium dioxide.2,5-10 The alkali salt, such as potassium compounds, is introduced to improve the tinting strength and hiding power and to resistant the sintering of titanium dioxide during the calcination.11 Phosphate plays an important and desirable role in crystal growth regulation.9 Meanwhile, aluminum is added with the view to decrease the photocatalytic activity of the pigment.9 Therefore, K2CO3, H3PO4, and Al2(SO4)3 were selected as the dopants in this work. In order to improve the pigmentary properties of rutile TiO2 white pigment prepared by this new process, the metatitanic acid was controlled doped and calcined. The prepared samples were characterized by X-ray fluorescence (XRF), X-ray dif-
In order to realize the recycling use of the NaOH molten salt medium, the paste of Na2TiO3 is leached by water and the intermediate (which mainly consists of metatitanic acid) is obtained. The reaction of this process could be described as follows. Na2TiO3 + 2H2O ) H2TiO3 + 2NaOH
(2)
The intermediate is dissolved in the H2SO4 solution at 50 °C for 4 h, and the titanium sulfate (TiOSO4) solution is obtained. H2TiO3 + 2H+ ) TiO2+ + 2H2O
(3)
The purified TiOSO4 solution is hydrolyzed thermally to precipitate out metatitanic acid after the clarify step. TiO2+ + 2OH- ) H2TiO3
(4)
After the hydrolysis step, the obtained metatitanic acid was filtered and washed under reducing conditions until the most * To whom correspondence should be addressed. Tel./Fax: +86 10 62631710. E-mail address:
[email protected].
Figure 1. Principal flow sheet of metatitanic acid prepared by NaOH molten salt method.
10.1021/ie1007147 2010 American Chemical Society Published on Web 07/14/2010
7694
Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010
Table 1. Chemical Compositions, Color, and Rutile Content of Prepared Samples dopant addition (wt %)
chemical composition (wt %)
CIE-L*a*b*
sample
K2O
P2O5
Al2O3
rutile nuclei
SiO2
Nb2O5
ZrO2
Fe2O3
TiO2
L*
a*
b*
rutile (wt %)
S1 S2 C1a
0 0.50
0 0.15
0 0.20
0 3.00
0.000 0.000 0.000
0.078 0.078 0.037
0.114 0.114 0.000
0.003 0.003 0.003
99.46 99.05 99.12
98.68 98.00 98.02
-0.33 -0.45 -0.39
0.94 1.52 1.46
6.5 97.0 97.2
a
Sample C1: commercial titanium dioxide pigment produced by the sulfate process.
Table 2. Properties of Metatitanic Acid Prepared by Different Method sample
prepared method
hydrolysis style
M1 M2 M3
NaOH molten salt NaOH molten salt sulfate process
uncontrolled hydrolysis controlled hydrolysis
surface area (BET, m2/g)
total pore volume (cm3/g)
average pore diameter (nm)
13 263 242
0.006 0.272 0.247
1.90 4.14 4.09
fraction (XRD), scanning electron microscopy (SEM), and CIEL*a*b* color measurements. Experimental Section The metatitanic acid used in this work was prepared by the NaOH molten salt reaction.3 All the chemical regents were analytical grade, and deionized water was used throughout. During the experiments, a metatitanic acid (M2) suspension containing 30 wt % of TiO2 and the dopants (K2O, P2O5, Al2O3, and rutile nuclei) were introduced in the form of solution into the metatitanic acid suspension. Potassium was doped as K2CO3, phosphate as H3PO4, and aluminum as Al2(SO4)3. On the basis of the knowledge of sulfate process and the initial experimental results of our laboratory, the amount of dopants addition was selected and listed in Table 1. The rutile nuclei obtained in the sulfate process in an industrial installation. The pulp obtained after thorough mixing (mechanical stirrer, 500 min-1, time 20 min) was filtered, inserted to a laboratory muffle furnace calcined at 950 °C for 2 h, and then quenched to room temperature. After calcination, the TiO2 samples obtained were milled in an agate mortar prior to powder characterization. Color was determined by optical spectroscopy (SC-80, Kangguang Co.Ltd., China) expressing the results as CIE L*, a*, and b* parameters, where L* is a measure of brightness (100 ) white, 0 ) black) while a* and b* measure chroma (-a* ) green, +a* ) red, -b* ) blue, +b* ) yellow). The morphology of the titanium dioxide products was observed with scanning electronic microscopy (SEM, JEOL-JSM-6700F, Japan) to characterize the microstructural changes. Major elements of the titanium dioxide products were analyzed by an X-ray fluorescence spectrometer (XRF, RIGAKU Simultix12, Japan). The BET surface area and porosity analysis were determined by using a nitrogen adsorption apparatus (TriStar II 3020, Micromeritics, USA). X-ray powder diffraction (XRD, X’Pert PRO MPD, PANalytical, Netherlands) patterns were recorded on a diffractometer (using Cu KR radiation) operation at 40 kV/30 mA. Then, the mass fraction of rutile (WR) in the samples was calculated from following equation:7 WR ) 1/(1 + 1.26IA /IR)
manufacture of titanium dioxide of highest pigmentary properties.11 The properties of metatitanic acid prepared by NaOH molten salt method (M1, uncontrolled hydrolysis, and M2, controlled hydrolysis) and sulfate process (M3), respectively, are shown in Table 2. The nitrogen adsorption-desorption isotherms of M2 and M3 are given in Figure 2. According to the Table 2 and Figure 2, the surface area of M2 is slightly larger than that of M3. Total pore volume, average pore size, and the shape of isotherm of the M2 are similar to M3. The results suggest that the properties of metatitanic acid are affected by the hydrolysis process significantly and the metatianic acid
Figure 2. Nitrogen adsorption-desorption isotherms of metatitanic acid prepared by NaOH molten process and sulfate process.
(5)
Where IA and IR are the integrated (101) intensities of anatase and (110) intensities of rutile, respectively. Results and Discussion The titanium dioxide white pigment quality is predetermined by the properties of the metatitanic acid in sulfate process. Rigid control of the hydrolysis step is of primary importance in the
Figure 3. XRD patterns of samples (a) S1 and (b) S2.
Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010
7695
Figure 4. SEM photographs of samples (a) S1 and (b) S2.
with certain properties could be prepared by the controlling hydrolysis in the NaOH molten process. Figure 3 displays the wide-angle XRD pattern of samples S1 (nondoped) and S2 (doped). The XRD pattern of the nondoped sample S1 shows sharp peaks assigned to anatase (JCPDS 01-071-1167) and some small peaks assigned to rutile (JCPDS 01-086-0148). By contrast, the doped sample S2 shows sharp rutile XRD peaks and some small anatase peaks. The difference in the XRD patterns of the samples (nondoped and doped) confirmed the promoting effect of dopants on the anatase-rutile phase transition. However, it was observed that potassium, phosphate, and aluminum have retarding effect on the anatase-rutile phase transformation.8,9 This case could be described to the fact that the rutile nuclei accelerate the phase transformation. This result suggests that the rutile nuclei could enhance the anatase-rutile phase significantly, which are in agreement with the result of Ratajska.12 The average crystallite size of anatase in sample S1 and rutile in sample S2 can be calculated by Scherrer formula on the anatase (101) and rutile (110) diffraction peaks. The calculated average crystallite sizes of sample S1 and S2 are 54 and 81 nm, respectively. Particle morphology and size distribution can strongly effect on the scattering power of pigments.1 The most preferred commercial rutile spherical particles for white pigment are those of a relatively narrow size distribution and the theoretical optimum particle size is between 0.2-0.3 µm in diameter.2 Figure 4 shows the morphology of the two samples S1 and S2. According to the images, the morphology of sample S1 appear irregular and particles are nonuniform, the major diameter larger than 0.3 µm and a few small ones of less 0.1 µm (Figure 4a). By contrast, the particles of the doped sample are more uniform (0.1-0.3 µm in diameter) and exhibit a smooth surface as shown in Figure 4b. This is most likely due to the growth of TiO2 crystals is inhibited by phosphate at the surface.9 Uncoated pigments contain 98% or more titanium dioxide. As can be seen from Table 2, the sample prepared in this work meet the commercial requirement. Minor constituents, either carried over from the titanium slag or added in the preparing process, can be important in determining pigment properties.13 The content of ZrO2 in the samples prepared in this work is higher than that in the sulfate process. This case could be ascribed to the fact that the NaOH molten salt reacted with zircon in the titanium slag. Silica, another major impurity in the molten salt reaction, could increase the anatase-rutile transformation temperature.14 It has been completely removed from the samples in the impurity separation process. Color performance and rutile content of the samples are shown in Table 1. Commercial TiO2 white pigment sample C1
was chosen as the standard, representative of commercial production as well as color performance. As can be seen from Table 1, the L* value of the sample S1 (L* ) 98.68) is higher than that of the sample S2 (L* ) 98.00), and the b* values of the sample S1 (b* ) 0.94) is lower than that of the sample S2 (b* ) 1.52). However, the rutile content of the sample S1 is 6.5 wt % and cannot meet the commercial standard (R g 97.0 wt %). The likely explanation of the increasing of the b* values is that the rutile content of samples S2 (R ) 97.0%) is higher than that of sample S1 (R ) 6.5%), and rutile TiO2 has an absorption edge at the violet end of visible spectrum. That imparts a slight yellow hue to the solid.2 The L* and b* values of sample S2, which was doped with K2O, P2O5, Al2O3, and rutile nuclei, were 98.00 and 1.52, respectively, indicating very close to those of the standard sample C1. Conclusions TiO2 rutile white pigment with good pigmentary properties was prepared via the NaOH molten salt method. The metatitanic acid with certain properties was prepared through the controlled hydrolysis step in the NaOH molten salt method. With the optimization of preparation conditions, the color performance, purity, and impurities content of the prepared sample approached the commercial pigment standards. In addition, the anatase-rutile phase transformation was enhanced by the addition of rutile nuclei, and the rutile content of the prepared sample is 97.0%. Further, the prepared sample has well-shaped morphology, indicating a good light scattering ability. Acknowledgment The authors gratefully acknowledge supports from the National Key Technologies R&D Program (2006BAC02A05), National Basic Research Program of China (973 Program, 2007CB613501), the Knowledge Innovation Program of the Chinese Academy of Sciences (KGCX2-YW-214), and the special funds of “Mountain Tai Scholar” construction project. Literature Cited (1) Buxbaum, G.; Pfaff, G. Industrial Inorganic Pigments; Wiley: Weinheim, 2005. (2) Juergen, H. B.; Andrejs, B.; Robert, E. TiO2 pigment technology: a review. Prog. Org. Coat. 1992, 20, 105. (3) Xue, T. Y.; Wang, L. N.; Qi, T.; Chu, J. L.; Qu, J. K.; Liu, Ch. H. Decomposition kinetics of titanium slag in sodium hydroxide system. Hydrometallurgy 2009, 95, 22.
7696
Ind. Eng. Chem. Res., Vol. 49, No. 16, 2010
(4) Lin, L.; Wang, J. G.; Wang, L. N.; Chu, J. L.; Qi, T.; Xue, T. Y. Separation of impurity from recycled alkaline solution in the clean production process of titanium dioxide. Chin. J. Process Eng. 2008, 8, 866. (5) LeDuc, C. A.; Campbell, J. M.; Rossin, J. A. Effect of lanthana as a stabilizing agent in titanium dioxide support. Ind. Eng. Chem. Res. 1996, 35, 2473. (6) Matteucci, F.; Cruciani, G.; Dondi, M.; Raimondo, M. The role of counterions (Mo, Nb, Sb, W) in Cr-, Mn-, Ni- and V-doped rutile ceramic pigments Part 1. Crystal structure and phase transformations. Ceram. Int. 2006, 32, 385. (7) Gennari, F. C.; Pasquevich, D. M. Kinetics of the anatase-rutile transformation in TiO2 in the presence of Fe2O3. J. Mater. Sci. 1998, 33, 1571. (8) Grzmil, B.; Rabe, M.; Kic, B.; Lubkowski, B. Influence of phosphate, potassium, lithium, and aluminium on the antase-rutile phase transformation. Ind. Eng. Chem. Res. 2007, 46, 1018. (9) Gesenhues, U. Calcination of metatitanic acid to titanium dioxide white pigments. Chem. Eng. Technol. 2001, 24, 685.
(10) Baltazar, P.; Lara, V. H.; Cordoba, G.; Arroyo, R. Kinetics of the amorphous-anatase phase transformation in copper doped titanium oxide. J.Sol-Gel. Technol. 2006, 37, 129. (11) Barksdale, J. Titanium-its Occurrence, Chemistry and Technology; Ronald Press: New York, 1966. (12) Ratajska, H. The effect of certain promoters on TiO2 crystal structure transformation. J. Therm. Anal. 1992, 38, 2109. (13) Karvinen, S. The effects of trace elements on the crystal properties of TiO2. Solid State Sci. 2003, 5, 811. (14) Okada, K.; Yammoto, N.; Kamshima, Y.; Yasumori, A. Effect of silica additive on the anatase-to-rutile phase transition. J. Am. Ceram. Soc. 2001, 84, 1591.
ReceiVed for reView March 23, 2010 ReVised manuscript receiVed June 13, 2010 Accepted July 6, 2010 IE1007147
