PROCESS FOR PREPARING IMPROVED TiO2 BY SILICON HALIDE ADDITION

Abstract: A process for producing substantially anatase-free TiO2 by addition of a silicon halide in a reaction of TiCl4 and an oxygen-containing gas in a plug flow reactor is disclosed. Pigmentary properties such as gloss and CBU are enhanced without loss of durability.

Description: TITLE PROCESS FOR PREPARING IMPROVED TI0 2 BY SILICON HALΓDE ADDΓΠON CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of pending U.S. Patent Application No.08/267,329, filed 28 June 1994. BACKGROUND OF THE INVENTION This invention relates to a process for producing substantially anatase- free Tiθ2 in a vapor phase oxidation of ΗCI4 in a plug flow reactor by addition of a silicon halide at a process temperature from about 1200°C to about 1600°C at one or more points downstream of where an oxygen-containing gas and TiC-4 are initially contacted. The process for producing Ti0 2 pigment by reacting 02-containing gas and ΗCI4 at temperatures ranging from 900 to 1600°C in a vapor phase is known. The resulting hot gaseous suspension of ΗO2 particles and free chlorine are discharged from the reactor and must be quickly cooled below 600°C within about 5 to 100 seconds. This cooling is accomplished in a conduit, i.e., a flue so that undesired ΗO2 particle size growth is prevented and particle agglomeration is minimized. Pigment product properties such as carbon black undertone (CBU) and gloss are a function of primary particle size and particle agglomeration, respectively. If high agglomeration of Tiθ2 results, the ΗO2 must be milled or ground in an energy intensive, expensive process such as rnicronizing to reduce the size of agglomerates in order to achieve the desired pigment properties. Further, two crystal structures of ΗO2 may form: rutile and anatase. The rutile Tiθ2 is preferred for its higher durability and its higher refractive index, both of which are important performance characteristics. Anatase ΗO2 is inherently less durable than rutile ΗO2 and possesses a lower refractive index. The presence of the anatase phase will in particular adversely affect durability. Even low levels of anatase affects durability of a finished ΗO2 pigment. It is well known in the prior art that addition of a silicon compound during the oxidation stage promotes anatase formation. Particle size has been controlled previously by prernixing volatile silicon compound and ΗCI4 prior to reacting it with the oxygen-containing gas. For example, British Patent 689,123 discloses premixing volatile silicon and aluminum compounds with ΗCI4, oxidizing and resulting in ΗO2 pigments with greater than 90% rutile. Therein, the aluminum serves as a rutile promoter, but also causes the formation of coarser particles and the silicon compound serves to decrease particle size. U.S. Patent 3,219,468 provides a process in a dynamic bed reactor and discloses separate addition of silicon halide from all other reactants. Therein, however, the temperature is raised to no more than 1200°C and less than 0.3% Siθ2 is present. When higher concentrations of the silicon halide is used therein, more anatase is formed. The need therefore exists for a process for preparing improved quality of ΗO2 pigment with decreased particle size and decreased agglomeration thus reducing the need for grinding operations while concomitantly eliminating the formation of anatase without loss of durability of the Tiθ2 pigment. The present invention meets that need. SUMMARY OF THE INVENTION In accordance with this invention, there is provided a vapor phase process for producing substantially anatase-free ΗO2 comprising reacting vaporous ΗCI4, an oxygen containing gas and an aluminum halide in a plug flow reactor and introducing a silicon halide at a process temperature in the range from about 1200°C to about 1600°C and the silicon halide is added at one or more points downstream of where the oxygen containing gas and ΗCI4 are initially contacted. It has been found that the process of this invention improves the quality of a ΗO2 pigment with decreased particle size and decreased agglomeration thus reducing the need for grinding operations while virtually eLiminating the formation of anatase. Further, Tiθ2 pigment product properties such as gloss and carbon black undertone are improved without loss of durability, and economic benefits in the ΗO2 manufacturing process are obtained. Surprisingly, the present process produces substantially anatase-free ΗO2 at a higher concentration of silicon halide addition during oxidation. The prior art results in more anatase formation at lower concentrations of a silicon compound, which is known to be an anatase promoter. Another advantage of the present process is the flexibility of modifying the amount of silica added at a subsequent surface treatment step. Reduced silica added during surface treatment results in easier grinding. It has been further discovered that introducing the silicon halide at one or more points downstream of where the oxygen and ΗCI4 are initially contacted virtually eliminates fouling of a reaction zone, i.e., silica adheres to the inside of a reaction vessel and tends to plug the reaction vessel when the silicon halide is introduced with the TiCl4. DETAILED DESCRIPTION The production of ΗO2 by the vapor phase oxidation of ΗCI4 is well known and disclosed in U.S. Patents 2,488,439 and 2,559,638, the teachings of which are incorporated herein by reference. The present invention relates specifically to an improvement in the aforementioned processes. ΗCI4 is evaporated and preheated to temperatures of from about 300 to about 650°C and introduced into a reaction zone of a reaction vessel. Aluminum halide such as AICI3, AlBr3 and AII3, preferably AICI3, in amounts sufficient to provide about 0.5 to about 10% AI2O3, preferably about 0.5 to about 5%, and more preferably about 0.5 to about 2% by weight based on total solids formed in the oxidation reaction is thoroughly mixed with ΗCI4 prior to its introduction into a reaction zone of the reaction vessel. In alternative embodiments, the aluminum halide may be added partially or completely with the silicon halide downstream. The oxygen containing gas is preheated to at least 1200°C and is continuously introduced into the reaction zone through a separate inlet from an inlet for the ΗCI4 feed stream. Water tends to have a rutile promoting effect. It is desirable that the reactants be hydrous. For example, the oxygen containing gas comprises hydrogen in the form of H2O and can range from about 0.01 to 0.3 wt % hydrogen based on ΗO2 produced, preferably 0.02-0.2 wt %. Optionally, the oxygen containing gas can also contain a vaporized alkali metal salt such as inorganic potassium salts, organic potassium salts and the like, particularly preferred are CsCl or KC1, etc. to act as a nucleant. In carrying out the invention, the silicon halide is added downstream from the ΗCI4 stream addition. The exact point of silicon halide addition will depend on the reactor design, flow rate, temperatures, pressures and production rates, but can be determined readily by testing to obtain substantially anatase-free Tiθ2 and the desired affects on agglomeration and particle size. For example, the silicon halide may be added at one or more points downstream from where the TiC-4 and oxygen containing gas are initially contacted. Specifically, the temperature of the reaction mass at the point or points of silicon halide addition will range from about 1200° C to about 1600 °C, preferably about 1400° C to about 1600° C, at a pressure of about 5-100 psig, preferably 15-70 psig and more preferably 40-60 psig. It is believed that higher temperatures and higher pressures also assist in achieving rutile formation. Often, the point or points of addition will not exceed the downstream distance traveled by the reactants or reaction products by about 0.002 to about 2 seconds, preferably about 0.005 to about 0.3 seconds, after the initial contact of the reactants. An alternative example of determining the point of SiC-4 addition is a minimum length of about 3-6 inside diameters of the flue after the TiC-4 and oxygen are initially contacted. Suitable silicon halides include SiCl4, SiBr4 and SLI4, preferably SiC-4. The SiCl4 can be introduced as either a vapor or liquid. In a preferred embodiment, the SiC-4 is added downstream in the conduit or flue where scouring particles or scrubs are added to minimize the buildup of Tiθ2 in the interior of the flue during cooling as described in greater detail in U.S. Patent 2,721,626, the teachings of which are incorporated herein by reference. In this embodiment the SiCl4 can be added alone or at the same point with the scrubs. In liquid SiCl4 addition, the liquid is dispersed finely and vaporizes quickly. The silicon halide added becomes incorporated as silica and/or a silica mixture in the Tiθ2 meaning the silica and/or silica mixture is dispersed in the ΗO2 particle and/or on the surface of Tiθ2 as a surface coating. Often, the silicon halide will be added in an amount sufficient to provide from about 0.1 to about 10% Siθ2, preferably about 0.5 to 5% Siθ2 and more preferably about 0.5 to 3% Siθ2 by weight based on total solids formed in the oxidation reaction, or Tiθ2 (basis). Typically, higher amounts of Siθ2 are desirable to improve the Tiθ2 properties described herein. Feeding SiCl4 downstream after ΗCI4 and O2 are initially contacted assists in rutile formation, controls particle size and limits agglomeration. As a result of mixing of the reactant streams, substantially complete oxidation of ΗCI4, AICI3 and SiC-4 takes place but for conversion limitations imposed by temperature and thermochemical equilibrium. Solid particles of Tiθ2 form. The reaction product containing a suspension of Tiθ2 particles in a mixture of chlorine and residual gases is carried from the reaction zone at temperatures considerably in excess of 1200°C and is subjected to fast cooling in the flue. The cooling can be accomplished by any conventional means as known in the art and described above. The ΗO2 pigment is recovered from the cooled reaction products by conventional separation treatments, including cyclonic or electrostatic separating media, filtration through porous media or the like. The recovered ΗO2 may be subjected to surface treatment, milling, grinding or disintegration treatment to obtain the desired level of agglomeration. It will be appreciated by those skilled in the art that the silica added by the present invention offers the flexibility of reducing the amount of silica added at a subsequent surface treatment step, if desired. Substantially anatase-free is defined herein to refer to Tiθ2 w^h ^ ess than about 0.7% anatase formation, preferably 0.5% or less, and more preferably 0.2% or less anatase formation. Plug flow reactor or pipeline reactor is defined herein to mean a reactor in the form of a conduit having a unidirectional flow at velocities of about 50 feet per second (about 15 m/s) or higher and exhibiting substantially little or no backmixing. Tiθ2 pigment products are tested for Carbon Black Undertone (CBU), a measure of particle size. The higher the CBU, the smaller the particles. A typical CBU for Tiθ2 use d m paint is about 10. CBU is determined by mulling together a suitable liquid, such as light colored oil and standard weights of the sample and a standard carbon black. The mixture is spread with a standard mixture on a panel and the relative blueness of the gray mixtures observed. Fine particles give bluer undertone or higher CBU. CBU values set forth in the Examples are determined by the methods described in greater detail in U.S. Patent 2,488,440, the teachings of which incorporated herein by reference except using a rating or value of 10 rather than 100 as used therein. Particle size distribution of the pigment products is measured by sedimentation analysis, with a Sedigraph® (Micromeritics Instrument Corp., Norcross, GA) after dispersion in aqueous suspension by fixed level sonication. The particle size measurement of the oxidation base and the % > 0.6 microns fraction will indicate the potential for peak gloss in the finished product, a value that cannot be exceeded while applying any reasonable energy level. In addition, less grinding energy is required for improving the quality of the oxidation base. To a clearer understanding of the invention, the following Examples are construed as illustrative and not limitative of the underlying principles of the invention. EXAMPLES COMPARATIVE EXAMPLE 1 SiCl4 was premixed with ΗCI4 feed in an amount sufficient to provide 0.17% by weight Siθ2 based on total solids formed in the oxidation reaction. The TiC-4 feed also contained AICI3 thoroughly premixed in an amount sufficient to provide 1.2% by weight AI2O3 based on total solids formed in the oxidation reaction. The ΗCL4 feed was evaporated and preheated to 460°C and introduced to a reaction zone at a rate corresponding to the production rate of 10 tons/hr of ΗO2 pigment product. Simultaneously, preheated oxygen was continuously introduced through a separate inlet adjacent to the ΗCL4 inlet. Trace amounts of KC1 dissolved in water were added to the oxygen stream as disclosed in British Patent 922,671 and U. S. Patent 3,208,866, the teachings of which are incorporated herein by reference. The temperature of the reaction mass at the point where the SiC-4 was added was about 800-900° C The reactant streams were rapidly mixed. At a location 5 feet (1.5 meters) downstream (or about 0.02-0.04 seconds from the point at which the ΗCI4 and oxygen are initially contacted) a reaction temperature of about 1180°C was estimated for the reaction zone and the pressure was 22 psig. The gaseous suspension of titanium dioxide pigment formed in the reactor was discharged and quickly cooled. The Tiθ2 pigment was separated from the cooled gaseous products by conventional means. The recovered Tiθ2 pigment was then treated by conventional pigment treatment procedures and ground to desired texture. Upon addition of the SiCl4, the CBU of the product was 8 with 3.0% anatase formation as detected by powder X-ray diffraction. COMPARATIVE EXAMPLE 2 The process of Comparative Example 1 was repeated except SiCl4 was premixed with ΗCI4 feed in an amount sufficient to provide 0.5% by weight Siθ2 and 1% by weight AI2O3 based on total solids formed in the oxidation reaction. The ΗCI4 feed was evaporated and preheated to 400 to 430° C and introduced to a reaction zone at a rate corresponding to the production rate of 4 tons/hr of ΗO2 pigment product. Further, at a location 5 feet (1.5 meters) downstream (or about 0.02-0.04 seconds from the point at which the TiC-4 and oxygen are initially contacted) a reaction temperature of about 1400 to 1500°C was estimated for the reaction zone and the pressure was 45 psig. Upon addition of the SiC-4, the CBU of the product improved from 12.7 to 15.2 but with 0.7% anatase formation as detected by powder X-ray diffraction. EXAMPLE 3 The process described in Comparative Example 2 was repeated with the exception that SiCl4 was not added to the ΗCI4 feed. Instead, the SiCl4 was fed to the reactor as a finely dispersed liquid at a location 5 feet (1.5 meters) downstream (or about 0.02-0.04 seconds from the point at which the ΗCI4 and oxygen are initially contacted) at the same point as the scrubs at a rate and at an amount sufficient to provide a loading of 1.0% by weight Siθ2 based on the total solids formed in the oxidation reaction. The TiC temperature was 425°C and the oxygen temperature was 1590°C for both control and test conditions. The estimated temperature of the reaction mass at the point of SiC-4 injection was about 1400 to 1500°C for the reaction zone at a pressure of about 50 psig. The production rate was 4.5 tons/hr. The CBU improved from 9.7 to 12.6 with substantially no measurable anatase formation by X-ray diffraction. The percent of particles >0.6 microns was also found to decrease from 33.5% without the addition of SiCl4 to the process to 22% with the added SiCU. DURABILITY The product of Example 3 was subjected to a subsequent surface treatment step and a resulting finished product was subjected to long term Florida exposure chalk fade tests. The chalk fade results were comparable to chalk fade results of a finished product, i.e., a) 99.8% rutile ΗO2 prepared under comparable conditions described in Example 3 but without SiCl4 addition. This demonstrates there is no loss of durability of the product prepared by the process of the present invention. In a further comparison, an anatase pigment, Kronos 1070 (available from Kronos, Leverkuesen, Germany) was tested for chalk fade and it chalked so readily that a chalk fade value could not be calculated. This demonstrates that rutile ΗO2 has higher durability than anatase Tiθ2- Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following Claims are not to be limited but are to be afforded a scope commensurate with the wording of each element of the Claims and equivalents thereof.

Claims: CLAIMS 1. A vapor phase process for producing substantially anatase-free Tiθ2 comprising reacting vaporous TiC-4, an oxygen containing gas and an aluminum halide in a plug flow reactor and introducing a silicon halide at a process temperature in the range from about 1200°C to about 1600°C and the silicon halide is added at one or more points downstream of where the oxygen containing gas and ΗCI4 are initially contacted. 2. The process of Claim 1 wherein the silicon halide is SiC-4 and is added in an amount sufficient to provide about 0.1 to 10.0% by weight Siθ2- 3. The process of Claim 2 wherein the SiCl4 is added in an amount sufficient to provide about 0.5 to 5.0% by weight Siθ2 and the process temperature is about 1400°C to about 1600°C . 4. The process of Claim 3 wherein the Siθ4 is added in an amount to provide about 0.5 to 3% by weight Siθ2- 5. The process of any of Claim 4 wherein the aluminum halide is premixed with the ΗCI4 and the oxygen containing gas further comprises a vaporized alkali metal salt. 6. The process of Claim 5 wherein the aluminum halide is AICI3 and is added in an amount sufficient to provide about 0.5 to 10% by weight AI2O3. 7. The process of Claim 6 wherein the SiCl4 is added at a point where scrubs are introduced in a cooling conduit. 8. The process of Claim 1 wherein the amount of silica added at a subsequent surface treatment step is reduced. 9. A ΗO2 pigment comprising a substantially anatase free Tiθ2 particle having silica and/or silica mixtures dispersed in the ΗO2 particle and/or on the surface of the Tiθ2 particle made by the process of Claim 1.

Inventors: GONZALES RAUL ALBERTO, GONZALES, RAUL, ALBERTO
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1995
Main classification: C01G23
IPC codes: C01G23