Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 496-500
406
Vapor-Phase Treatment of Titanium Dioxide with Metal Chlorides. 1. The Reactions of Coating Performed by AI& SiCI4, and ZrCI, in the Vapor Phase E. Santacesarla and S. Carri" Dipartimento di Chimica Fisica Applicata, Polltecnico di Milano, P.zza L. da Vincl 32, 20133 Milano, Italy
R. C. Pace and C. Scott1 SIBIT S.p.A., Societa Itaiiana Biossido di Titanio, Spinetta Marengo, Alessandria. Italy
A procedure of coating titania pigments by reacting solid titanium dioxide with some metal chlorides, in the vapor phase, is described. Aluminum, silicon, and zirconium chlorides have been used as reagents. The obtained coatings are very compact and coherent. Particularly, the alumina coating is very promising from the industrial point of view, having the properties of strongly reducing the photoreacthri of the pigment and giving high levels of titanium conversion. A tentative explanation is given about the high photostability of titania pigments coated with alumina, obtained using the described treatment.
Introduction The reduction of photoreactivity and the improvement of durability of titania pigments can be pursued through a surface after-treatment that is usually performed through the precipitation, from a solution, of aluminum hydroxide or mixed aluminum and silicon hydroxide, in strictly controlled conditions. The pigment is then filtered and calcined. It has been reported by Parfitt and Sing (1976) that the more compact and coherent the obtained coatings, the greater are the reduction of the photoreactivity of the pigment and improvement of the weathering resistance of the pigmented film. It is also known, as reported by Blechta and Lavicka (1967) and by Poisson et al. (1964), that several producers of titania pigments add zinc oxide in order to reduce the photoreactivity still more, but this substance can have negative effects, for example, in affecting the stability of pigment suspensions. It is difficult, therefore, to obtain compact and coherent coatings on titania pigments by the above mentioned "wet treatment" and to avoid the disadvantages of using zinc oxide. Moreover, the "wet treatment" requires that large volumes of suspension be stirred and filtered with large energy consumption. In a recent patent of Scotti et al. (1980a) there is described the possibility of obtaining compact and uniform coatings through the reaction of titanium dioxide with Al,Cl,, SiCl,, or ZrC1, in the vapor phase and in the presence of nitrogen or air as carrier gas. The reaction between TiOz and A&C&has been considered also in other previous patents of Pekukas (1948) and Hughes (1958). However, very few scientific publications have been devoted to this subject (Schiifer et al., 1950, 1958). In the first section of the present paper the thermodynamics of the following reactions will be examined 2A12C16+ 3Ti0, 2Al,03 + 3TiC1, (1)
-
above mentioned reactions, will be experienced, and some features of industrial interest, for the obtained coatings, will be examined. As will be seen, reactions others than (1)-(3) occur, especially at low temperatures, with the formation of oxychlorides. These compounds are unstable and very reactive and can be easily eliminated by washing with steam or liquid water. The presence of oxychlorides on the surface of TiOz gives rise to a high acidity, and this fact can be profited by bonding inorganic or organic molecules, as described by Scotti et al. (1980b). However, if the temperature of the reactions is kept higher than 600-700 "C, the formation of oxychlorides is lowered to negligible levels. The coatings obtained by the "dry treatment" are very compact and uniform if compared with those obtained by the "wet treatment". Moreover, the "dry" alumina coating strongly reduces the photoreactivity of the rutile powders and eliminates the need of the addition of zinc oxide. The kinetics of this very promising reaction between vaporized Al,Cl, and TiO, will be described in detail in a following paper. A feasible mechanism of the reduction of the photoreactivity of the "dry coated" pigment will conclude the present paper. Thermodynamics of the Coating Reactions Let us consider, first of all, the reaction between TiO, and A12C&. It is known (see, for example, Kirk-Othmer, 1968) that A1C13 exists as A12C1,. Therefore, in the thermodynamic calculations we will consider the dimer as reagent. The reactions occurring between A12C&and TiOz have already been studied by Schafer et al. (1950, 1958). At low temperatures (200-400 "C), under low pressures (2-5 atm), the following reaction occurs Al,Cl, + TiO, 2A10C1 + TiC1, (4)
-
(3)
If the temperature is raised, reaction 1 becomes preeminent. The aluminum oxychloride at high temperature is unstable and decomposes according to the reaction 6A10C1- 2A1203+ AlzC16 (5)
Then, the operative conditions, necessary to perform the
as reported by Schafer et al. (1950, 1958). Moreover, the
SiC1,
+ TiO,
ZrC1,
+ TiO,
-
+ TiC1, ZrO, + Tiel, SiO,
(2)
0196-4321/82/1221-0496$01.25/0
0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 3, 1982 497
AGO [ k cal/moles)
0
(3)
-20 25'
3 0'
,
3 5'
40 '
45'
'
Y
j
l
50'
Figure 2. The X-ray spectrum showing the existence of orthorhombic aluminum oxychloride. The peaks related to AlOCl are marked,while the others are referred to the tetragonal structure of ruti1e.
-40
I
-80
-loo1
I
300
I
I
700
500
I I 900 T ( O K ) 1100
Figure 1. The standard free energy changes for the reactions occurring in the coating of titania by using chlorides such aa A12C1,, SiCl,, and ZrC1, in the vapor phase. The reactions are numbered as in the text.
oxychloride is very sensitive to the moisture, leading to hydrolysis. In Figure 1, the values of the standard free energy changes, as a function of temperature, for reactions 1-4 are reported. All the data have the same source, the JANAF' Thermochemical Tables (1971). As can kobserved, all the mentioned reactions are largely thermodynamically favored. Al,03is thermodynamically very stable but apparently the oxychloride formation is kinetically favored. In Figure 1 are also reported the standard free energy changes for the reactions
--
A12C&+ 3/202 A1203+ 3c12 SiCl, ZrCl,
+ O2 + O2
SiOz + C1, ZrO,
+ C12
(6) (7) (8)
that could occur when air is used as carrier gas. Also these reactions are thermodynamically favored.
Experimental Section Techniques. The preparation of the aluminum oxychloride, from TiOz and A12C&,according to reaction 4 has been performed in a sealed Pyrex vessel. Three grams of Ti02 was put in a thick glass tube together with 5 g of AlC13. Then the glass tube was heated under vacuum and sealed by melting the opening. The vessel was then placed in a furnace, kept a t about 300 OC. All the other reactions have been performed, on the contrary, in dynamic conditions by feeding the chlorides in the vapor phase, diluted with nitrogen or air in a fluidized bed of titanium dioxide kept at 400-600 O C . The fluidized bed reactor, in quartz, had a diameter of 4.5 cm
and was loaded with 60 g of commercial untreated Ti02, prepared by SIBIT Co. by the sulfate process and having a surface area of 5.5 m2/g. Ti02, in the form rutile, was a rotary kiln discharge, subjected to a sieving process in order to obtain a size of particle aggregates of about 100 pm. Primary particle size was 0.2 pm. In order to fluidize the bed, the flow rate of the carrier gas was kept at about 90 NL/h. The feeding of chlorides was obtained through vaporization, in the case of Sic&through sublimation in the case of ZrCl, and by reacting chlorine with aluminum for AlC13. This last reaction is very fast, at about 550 "C, and it is sufficient to feed chlorine on aluminum grit, kept at the above mentioned temperature, for quantitatively obtaining AICIB. In all cases, if reactions 1-3 occur, Tic&is formed. This compound is volatile and can be easily trapped, at the outlet of the fluidized bed reactor, by freezing with liquid nitrogen. Analysis of the trapped TiCl, can be performed spectrophotometrically by treatment with sulfuric acid solution and hydrogen peroxide. Details on the analytical method are described by Sandell (1950). The other reaction products, as oxides or oxychlorides,are formed on the TiO, surface. Their analysis has been performed by atomic absorption after withdrawing a small amount of the reacted TiOz and solubilizing the coating by a chemical attack with hydrofluoric acid. Chlorides present on the TiOz surface were analyzed with the classical Volhard method, after the treatment of the reacted powder with a boiling nitric acid solution.
Results The runs for obtaining AlOC1, have been performed in sealed vessels at 300 "C, for 170 h. The TiOz reacted, after this time, was about 65% of the total amount. The reaction resulted, in our case, slower than that described by Schafer et al. (1950,1958) which obtained the total conversion in 24 h. Probably, the difference in the kinetic behavior is due to the different morphological characteristics of the starting material. However, the possibility of completing the reaction has been confirmed obtaining AlOCl quantitatively, as suggested by Schhfer et al. (1950, 1958). The presence of AlOC1, in the reacted sample, has been confirmed by the X-ray spectrum of the powder, which shows the orthorhombic structure of this compound, together with the tetragonal structure of rutile, as can be seen in Figure 2. When the reaction between A12CI,and TiO, is carried out in dynamic conditions, the presence of chlorides in the obtained coatings decreases with increasing temperature. This behavior is represented in Figure 3, in which the obtained ratios C1/A1 are reported as a function of the temperature. As can be observed, for the reaction of TiOz with A12CI,,the presence of chlorides in the coating is very small when the temperature is higher than 600-700 O C .
498
Ind. Eng. Chem. prod. Res. Dev.. Vol. 21. No. 3. 1982
01
:\
0
1
1
1
A c i d (Irtnqhl
Figure 3. The evolution of the stoichiometricratio between CI and AI with the temperature for the reaction of TiO, with AI&.
1 U.
0
1
1
1
*.
1
0
1
L
6
1
0
1
~
6 I.
Figure 4. The surfaceacidity, measured with the Benesi method, on several TiOzsamples. A is referred to an uncoated TiO, sample. B is related to an Al103-coated sample obtained with the “dry treatment”;the alumina is 1.48%of the total weight. C is the histognun for S O t coated pigment, the silica is 1.2%of the total weight. D is the plot obtained for Alto, coated pigment by using the “wet treatment“;the amount of Also, is 3.0%.
Table I. The Conversions Percent Obtained as a Function of T i e for the Reactions of TiO, Rutile with AI,CI., SiCI,, and ZrCI, in the Vapor Phase‘ volatile reagents time, min AI,CI, SiCI, ZrCI. 20 60 63 70 75 120 133 180 227 AI,O,
9.35
1.30 1.83
1.15
2.57 1.71 1.88 5.18 2.10 8.48
final compn of powders, % CI SiO, CI ZrO, 0.2
1.62
0.22
2.64
CI 0.22
The composition of the powders has been controlled also at the end of the run, by chemical analysis, and it is reported as weight percent. The data reported in Figure 3 for the lower temperatures (300-400 “C) can be interpreted by assuming a s preeminent the formation of aluminum oxychloride, but the formation of titanium oxychlorides and also the presence of small amounts of AlCI,, strongly adsorbed on to the surface, cannot be excluded. In order to compare the reactivity of TiO, with AI&, ZrCI,, and SiCI,, the temperature of 600 “C has been assumed as reference. In Table I the conversions expressed as x = (moles of reacted TiO,)/(moles of initial TiO,) (9)
obtained a t several times in the fluidized bed a t 600 ‘C for the different chlorides are reported. In the same table, the results of the chemical analysis of the discharged powders a t the last time indicated in the table are also reported. The chlorides present in the coating can be almost completely eliminated by treating with steam or washing with water. The reactivities do not change if dry air is used as fluidizing gas instead of nitrogen. By considering the reaction of TiO, with AICI, a t 600 “C the presence of air leads also to the occurrence of the
Figure 5. The electron microscopy view of an uncoated TiOl rutile sample. The magnification is 48000X. reaction 6, with the formation of chlorine. However, a t the mentioned temperature, only 15% of the coated aluminum oxide is formed through this reaction. The contribution of this reaction has been found to increase with increasing temperature. The reacted TiO, has been submitted to several tests for defining the nature of the coating and evaluating the properties of the obtained materials as pigments. The values of pH developed in 100 cm’ of distilled water contacted with 10 g of TiO,, before and after the washing with steam, are reported in Table 11. The same test has been made also for uncoated TiO, and the results are reported in the same table. In Table I1 the values of the surface area measured after the treatments and for comparison the surface area of a TiO, sample coated by hydrolytic precipitation are also reported. On the samples treated with Al,CI, and SiCI, measurements of surface acidities, after washing with water, have been performed. The obtained results are given in Figure 4, together with the measurement of the surface acidity of untreated TiO,. These measurements have been made by using the method suggested by Benesi (1956, 1957). Some samples containing high levels of alumina in the coating (4-18% by weight) have been examined by electron microscopy. The coating is clearly visible only for the highest values of alumina contents, as can be observed by comparing Figures 5 , 6 , and 7 which refer respectively to
Table 11. pH Generated by Contacting 100 cm’ of Distilled Water with 10 g of the Pigment, before and after Washing with Water“ coating AI,O,, coating AI,O,
