501
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 501-504
Vapor-Phase Treatment of Titanium Dioxide with Metal Chlorides. 2. Kinetics of the Reaction between Titanium Dioxide and Aluminum Chloride E. Santacesarla and S. Carrl* Dipartimento di Chimica Fisica Applicata, Poiitecnico di Miiano, P.zz8 L. ob Vinci 32, 20 133 Milano, Italy
R. C. Pace and C. Scott1 SIBIT S.p.A., sOCiet8 Itallan8 Biossklo di Titmio, Spinetta Marengo. Alessandrla, Italy
The kinetics of the reaction between titanium dioxide, industrially prepared by the sulfate process, and aluminum chloride have been studied by using a fixed-bed and a fluidized-bed reactor. The experimental results c a n be interpreted with a shrinking core kinetic model applied to the elementary particles of titania that constitute the aggregrates used in the experiments. The reaction rate is zeroth order with respect to the gaseous reagent. The kinetic parameters, obtained from the data collected with the two employed reactors, are in agreement. During the reaction, oxychlorides are formed, especially at low temperatures, but it seems without affecting the overall kinetic behavior. This finding suggests that oxychlorides are intermediate species in the reaction. The coatings obtained are compact and uniform, and the reaction c a n be prolonged to a high degree of conversion. The reaction can be considered as a corrosion of the rutile which is replaced by alumina.
Introduction In the preceding paper, the reactions between titanium dioxide and vaporized chlorides as A12C4,SiC14,and ZrC14 have been studied by Santacesaria et al. (1982). The possibility of obtaining very compact and coherent coatings of titania pigments has been reported. In particular, the reaction with A12C4gives place to a coating that strongly reduced the photoactivity of the pigment. The coating does not hinder the occurrence of the reaction until a very high degree of titania conversion. In the mentioned previous work, and in agreement with Schiifer et al. (1950, 1958), it was observed that, a t low temperature, the formation of oxychlorides is preeminent, according to the reaction A12C16+ TiOz 2A10C1+ T i c & (1)
-
while at temperatures higher than 600 "C the oxide becomes the main product 2AlzC16 3TiO2 2A1203 + 3TiC1, (2)
+
At present, no data exist in the literature on the kinetics of these reactions that are industrially very promising, as it appears in several patents: Pekulas (1948), Hughes (1958), Scotti et al. (1980a,b). In the present paper the results of the kipetic study on the overall transformation of TiOz to TiC1, by reaction with A12C&by using both a fixed- and a fluidized-bed reactor are given. Some interpretations on the reaction mechanism are offered.
Experimental Section Apparatus, Reagents, and Operating Conditions. The titanium dioxide was industrial rutile, prepared by the sulfate process, in sieved granules having a diameter of about 100 pm and a specific surface area of 5.5 m2/g. The granules are constituted by poorly porous fine particles with a mean diameter of about 0.2 pm. The TiOz, before use, was dried at the reaction temperature by feeding dry nitrogen for 2 or 3 h. The fixed-bed reactor was constituted by a tube of quartz, 40 cm long, with a diameter of 1cm. The tube was 0196-4321 /82/ 1221-0501$0 1.25/0
divided into two parts by a quartz frit. In the first part granules of aluminum were arranged, while the second chamber contained 6 g of TiOz. The length of the TiOz bed was 7.2 cm. Two independent tubular ovens heated the bed of aluminum granules and of TiOz, as in the scheme of Figure 1. When chlorine diluted with nitrogen was fed through the aluminum bed kept at 550 "C, the following reaction quantitatively occurred 3C12 2A1- A12C16 (3) The flow rates and the concentrations of A12C&in nitrogen could be changed, when necessary, by adjusting the chlorine flow rate. Runs have been made about 400,500, and 600 "C with a constant flow rate of the inert gas, equal to 3.96 NL/h. In the runs performed at 500 "C the concentration of A12C16has been changed in a wide range. The fluidized bed reactor was constituted by a quartz tube having a diameter of 4.5 cm and a length of about 40 cm. A quartz frit at the bottom of the tube allowed a good distribution of the fluidizing gas. A12C&was produced by the same system described for the fixed bed, but the apparatus contained more aluminum granules and was designed in such a way to be used also as a preheater for the fluidizing nitrogen stream. At the outlet of the reactor, three traps, cooled with liquid nitrogen, condensed the reaction product TiC1, and the unreacted A1,Cb. Similar traps were also used for the fixed-bed reactor. The overall scheme of the plant with the fluidized-bed reactor is reported in Figure 2. The collected TiC14was dissovled with 1M sulfuric acid, diluted to a known volume, and treated with hydrogen peroxide. The obtained yellow solution was analyzed spectrophotometrically, as described by Sandell (1950). The presence of AlC1, did not interfere. The analysis of TiC1, produced was made at sect ral times during 3 run by rapidly changing the traps. At the end of a run, ;he solid was washed by a stream of inert gas for eliminating physisorbed A12Cls and TiC14. Then, the solid was discharged from the reactor and analyzed to
+
0 1982 American Chemical Society
502
Ind. Eng. Chem. Prod. Res. Dev.. Vol. 21, No. 3, 1982 4
8
0
.E
-
Figure I. Scheme of the plant with the fixed bed reactor: (A) aluminum granule reactor; (B) TiO, reactor; (C) avens; (D) heating ribbon; (E) frozen traps; (F) Dewar for liquid nitrogen; (G)P,O, trap; (TC) thermocouples.
4
c
0
0
"
,
*
.
n
_
.
0
504'C
390ar
3
4
LlmP
lhaurri
Figure 3. Change of TiOl conversion during the time in the fixed bed reactor for runs performed at 390, 504, and 600 "C. The concentrations of AlCI, are reported in Table 111. Each curve corresponds to a set of runs, while each run is represented by a different symbol. Unreactod TI O2 p a r t i c l e
R e a c t e d 110, p a r t i c l e
Figure 4. Picture of the gaesolid reaction between rutile aggregates and AI&. Table I. List of the Operative Conditions for the Runs Reported in Figure 3a run
T,"C
Figure 2. Scheme of the plant with the fluidized bed reactor: (A) aluminum granule reactor; (B) preheatar for nitrogen stream: (C) ovens; (D) fluidized bed reactor; (E) frozen traps; (F) Dewar for liquid nitrogen; (G)P,O, traps; (H) heating ribbon.
determine aluminum and hhlorides. The aluminum dissolved by using hydrofluoric acid was analyzed by atomic adsorption. Chlorides were dissolved with nitric acid and analyzed by the classical Volhard method. With the fluidized bed, runs have also been made at about 400,500, and 600 "C. Chlorine flow rates were ruled in such a way to ohtain the desired concentration of A1C13 Chlorine was diluted first with a nitrogen stream of about 10 NL/h and then sent to the aluminum bed. Another nitrogen stream of about 75 NL/h was preheated in the same apparatus and mixed with the one containing AI&& at the exit of the aluminum reactor before entering the fluidized-bed reactor. Results (A) Runs Performed with Fixed Bed Reactor. The conversion of titania, defined as z = TiOz (reacted)/TiOp (initial) (4) can be determined at several times by evaluating the amounts of TiCl, produced. In Figure 3 the conversions obtained at 504 "C as a function of time are reported for different initial concentrations of AlC1,. As can be easily observed, the concentration of AlC1, does not affect the conversion. This fact can be interpreted by assuming that the reaction rate is zeroth order with respect to the volatile reagent. This results can be justiiied by assuming that the gaseous AlC1, is strongly adsorbed on the titania surface, which is saturated with the reagent.
time,s 10073 1xx29
8622 12044 504 504 390
a
7
600
8
600
3 202 8 340 11673 1800 6913 10126 3 153 6780 3308 10877 13643 3389 8569
PAL,Cl,,
total con"
,020 0.031 0.026 0.037
0.008 0.014 0.016
atm
0.069 0.071 0.021 0.037 0.037 0.006 0.022 0.022 0.018 0.020 0.032 0.046 0.045 0.053 0.049
0.011 0.011 0.016 0.018 0.009 0.015 0.016 0.006 0.013 0.015 0.0025 0.0028 0.034 0.075 0.098 0.037 0.070 ~~
~
The compositions of the coatings obtained at the end
of the runs are: ratio Al/Cl (0.8 a t 390 "C, 0.35 at 506 "C, and 0.02 at 600 "C).
The runs performed at 390 and 600 "C are also reported in Figure 3. In Table I the operative conditions for the runs reported in Figure 3 are summarized together with the amounts of aluminum and chlorides found at the end of the reaction on the discharged titania samples. From these data it arises that the amount of chlorine in the coating decreases with increasing temperature. This is in agreement with the statement reported by Santacesaria et al. (1982) and Schiifer et al. (1950, 1958) that at low temperature the formation of oxychlorides is favored. The values of the time reported in the figures or in the tables are corrected for the dead volume of the apparatus.
Ind. Eng. Chem. Prod.Res. Dev., Vol. 21, No. 3, 1982 503 Conversion A'
F(X)x103
I
e-
30t 30 -
6-
20 4-
500'C 400'C
Ot
2 1
, 2
39OOc 3
time (hours)
0
4
Figure 5. Application of the shrinking core kinetic model to the conversion data obtained by the fiied-bed reactor.
2
1
3
time
Figure 6. Conversions obtained, at several times for the runs performed in the fluidized bed reactor at 400, 500, and 600 "C. Table 111. List of the Operative Conditions for the Runs of Figure 5a
Table 11. The Kinetic Parameters Obtained by the ExDerimentd Data of the Fixed Bed Reactor
T, "C
k,, mol/cm2 s
run
T. "C
time. s
PAI,CI,, atm
TiO, conv
390 504 600 E, kcal/mol frequency factor
1.54 x 10-14 1.64 x 1 0 4 3 1.07 X 23.2 c 1.4 6.0 x 10-7
1
600
2
500
3
400
4017 7 624 13104 3598 7 026 5399 10618
0.0146 0.0181 0.0272 0.0149 0.0188 0.0151 0.0180
0.0257 0.0518 0.0847 0.0048 0.0081 0.0011 0.0018
As previously mentioned, the titania granules are formed by the collection of smaller particles (as shown in Figure 4): as a consequence diffusion of the reagents must occur through the pores of the granules before the attack of the smaller particles could take place. The influence of internal diffusion on the overall reaction rate can be estimated by evaluating the Thiele modulus. For a zerothorder reaction it is given by
by assuming C,equal to the bulk concentration. This very small value evidences a negligible influence of internal diffusion. By assuming a shrinking-core kinetic model with a zeroth-order reaction according to Szekely et al. (1976) and Wen (1968), the material balance of the solid leads to the expression
The integration of the expression 6 yields
k,
F ( x ) = 1 - (1- x y 3 = -t
RpP,
4
(hours)
(7)
If the values of F(x) for the performed runs are plotted as a function of time, the trends reported in Figure 5 are obtained. They reveal that the points can be interpolated with straight lines which do not go through the origin. This finding indicates that the proposed kinetic model fits the experimental data but it does not describe some features of the initial kinetic behavior of the system under examination. This is probably due to an apparent delay in reaching a homogeneous distribution of the remperature inside the bed in the starting conditions. From the slope of the straight lines of Figure 4 the kinetic constant k, has been evaluated. The obtained values at the different temperatures are collected in Table 11.
a The compositions of the coatings obtained at the end of the runs are the same as seen in Table I.
(B) Runs Performed with the Fluidized-Bed Reactor. In Figure 6 the conversions obtained at 400,500, and 600 "C are reported as a function of time. The operative conditions are summarized in Table 111, together with the amounts of aluminum and chloride present in the solid discharged from the reactor at the end of each run. Again, the chlorides decrease with increasing reaction temperature. The statement of the model of a fluidized-bed reactor is generally difficult since this apparatus behaves as a polyphase system, in an intermediate situation between a perfectly mixed and plug-flow reactor. This effect can be accounted for by means of an effective axial dispersion coefficient, which is difficult to predict, however. Actually, if the order of the implied chemical reaction is equal to zero, it is easy to verify that the two mentioned models coincide. In fact, for a zeroth-order reaction the mixing regime of the reactant mixture does not affect the rate of conversion, and the only variable which affects the reactor behavior is the contact time. Therefore, the material balance of the solid is still expressed by eq 6, which has then been used to fit the experimental data of Figure 5. The mass balance of the gas phase can be easily made by considering the stoichiometric equation. The values of F ( x ) are plotted as a function of time in Figure 7. The obtained points can be interpolated with straight lines: in this case the lines go through the origin, and the effects observed in fixed bed are not present. Furthermore, the data line up better than those obtained in the fixed-bed reactor. These findings are probably a consequence of the better control of the temperature in the fluidized bed. Another important point is that the zeroth order with respect to the gaseous reagent is confirmed by these runs performed in the fluidized bed. In fact, despite the concentrations of AlC1, being lowered an order of magnitude with respect to fixed bed, the reaction rates remain unchanged. In Table IV the kinetic param-
504
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 Ink,
F(Xlx103
-2d
?
--
0 0
2
1
500°c 40OOc
3
10
Figure 7. Application of the shrinking core kinetic model to the data from the runs performed in the fluidized-bed reactor. Table IV. The Kinetic Parameters Obtained by Fitting t h e Experimental Data of t h e Fluidized Bed Reactor
T. "C 400 500 600 E, kcalimol frequency factor
11
12
13
14
15
lo'
~
4 time [hours)
k.. mol/s cm' 2.81 x 10-14 2.02 x 1 0 - 1 ~ 1.17 X 10.'' 21.73 i 0.73 2.92 X 10.'
eters obtained for the runs reported in Figure 6 are reported. An Arrhenius type plot for both the kinetic constants of Tables I1 and IV are given in Figure 8. From this figure it arises that the kinetic constant for the shrinking-core model in both cases are quite close, and the same activation energy has been found. Discussion and Conclusion From the reported experimental runs, it is apparent that the reaction between TiOz and AlzClGcan be considered as a corrosion of the rutile that is replaced by alumina. The shrinking-core kinetic model is valid if applied to the elementary fine particles (0.2 p ) that constitute the aggregates. No mass transfer resistance is encountered by the gaseous reagent in passing through the macropores of the aggregates. Therefore, each elementary particle is uniformly coated by alumina. This fact is proved by the observation that by milling the aggregates, the obtained powder has a very low photoreactivity. Moreover, in the preceding article, the compactness of the coating has been shown by electron microscopy and by measuring the specific surface area that is not increased by the coating. The alumina coating is poorly protective with respect to further chemical attack, even at very high conversions. This fact suggests that the diffusion of AlCl, and TiC14, through the film of coating, is poorly hindered. As has been shown, the formation of oxychlorides in the coating decreases with increases in temperature. It is difficult to separate the kinetics of formation of oxychlorides from those of alumina formation. However, the observation that the zeroth-order, for the overall Ti02, transformation is valid at all the experienced temperatures supports the hypothesis that the reaction mechanism,
Figure 8. Arrhenius plots of the kinetics constants obtained both with the fixed- and the fluidized-bed reactor.
which implies the intermediate formation of oxychlorides, remains unchanged. Finally, it is known that interstitial titanium exists in the rutile lattice and that it can diffuse along the c direction of the crystal as described by Goodenough (1971). If the AlCl, reacts with this interstitial titanium atom it could penetrate in the bulk. This phenomenon, which has been demonstrated in a previous article by the employment of the XPS technique, strongly contributes in reducing the photoreactivity of the coated pigments, but has no relevance from the kinetic point of view. The high electron acceptor character of the aluminum, in fact, suggests that oxygens or hydroxyls, present on the surface of titania, be first subjected to the chemical attack by AlC1,.
Nomenclature C, = concentration of the gaseous reagent at the particle surface, mol/cm3 De = effective diffusion coefficient, cm2/s E = activation energy, kcal/mol k , = kinetic constant, mol/s cm2 k , = kinetic constant, mol/s cm3 R, = mean particles radius, cm t = time, s T = temperature, O C x = conversion as TiOz (reacted)/TiO$ (initial) ps = molar density of titania, mol/cm Literature Cited Goodenough, J. B. "Progress in Solid State Chemistry"; Pergamon Press: New York, 1971; Voi. 5, p 344. Hughes, W. British Patent 867479, 1958. Pekukas, A. U S . Patent 2 4 4 1 225, 1948. Sandeii, E. B. "Colorimetric Determination of Traces of Metals": Interscience Publishers: New York, 1959; p 870. Santacesaria,E.; CarrB, S.; Pace, R. C.; Scotti, C. Ind. f n g . Chem. Prod. Res. Dev. 1982, preceding article in this issue. Schafer, H.; Groser, C., Bayer, L. 2.Anorg. Allg. Chem. 1950, 8 7 , 263. Schafer, H.; Witting, F. E.; Wiiborn, W. 2.Anorg. Allg. Chem. 1958, 95, 297.
Scottl, C.; Pace, R. C.; CarrB. S.;Santacesaria, E. Italian Patent 1980.
Scotti, C.; Pace, R. C.: Car&
S.; Santacesaria, E.
Italian Patent
19156A/80, 19157A/80,
1980.
Szekely, J.; Evans, J. W.; Young Sohn, H. "GaslSolid Reactions": Academic Press: London, 1976, Chapter 3, p 65. Wen, C. Y. Ind. Eng. Chem. 1088, 9 , 60. Received for reuiew November 17, 1981 Accepted April 2, 1982
