MXPA00006224A - Process for producing coated tio2 pigment using cooxidation to provide hydrous oxide coatings - Google Patents

Abstract

The present invention provides a process for producing titanium dioxide (TiO2) pigment having a coating comprising silica and a second oxide selected from the group consisting of oxides of boron, phosphorus, magnesium, niobium, germanium, and mixtures thereof, comprising the steps of (a) reacting titanium tetrachloride (TiCl4) in the vapor phase with an aluminum compound and an oxygen-containing gas in a reactor at a temperature in the range of about 900°C to about 1600°C to provide a gaseous suspension comprising TiO2 particles, (b) contacting the gaseous suspension comprising the TiO2 particles with a silicon halide and an oxide precursor of boron, phosphorus, magnesium, niobium, germanium, and mixtures thereof, and (c) cooling the gaseous suspension to produce TiO2 pigment having a coating comprising silica and a second oxide.

Description

PROCESS TO PRODUCE TITANIUM DIOXIDE PIGMENT COATED USING COOXIDATION, TO PROVIDE HYDRATED OXIDE COATINGS BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a process for preparing Ti02 pigment which involves adding both a first oxide precursor, which is a silicon halide such as SiCl4, and at least one second oxide precursor, to a gas suspension comprising particles of Ti02. The second oxide is selected from precursors of boron oxide, phosphorus, magnesium, niobium, germanium and mixtures thereof.

Description of Related Art The process for producing titanium dioxide pigment (Ti02) by reacting an oxygen containing gas and titanium tetrachloride (TiCl4) at temperatures ranging from 900 ° to 1600 ° C in a vapor phase is known. The hot gaseous suspension resulting from Ti02 particles and free chlorine are discharged from the reactor and must be rapidly cooled in a duct, ie a smoke duct, REF.119620 so that the growth of unwanted Ti02 particle sizes is avoided and particle agglomeration is minimized. Titanium dioxide is widely used as a white pigment. In many commercial applications, such as paints, high durability is required, and durability means the ability of the pigment to withstand the destructive effects of the environment and, most importantly, of sunlight. Since Ti02 is photoactive, and can promote the degradation of paint systems, hydrous oxide coatings are applied to Ti02 particles to improve the. durability. Typically, hydrous oxide coatings on Ti02 particles are prepared by wet chemical methods. This involves precipitation of the hydrous oxide, such as silica, alumina, zirconia, from the solution. Although these processes provide certain durable coatings on the Ti02 particles, they often result in irregular, non-uniform and porous coatings. These processes often require pigment grinding before the "wet coating methods" to decompose the soft aggregates to ensure that all the particles are coated.Another problem with these processes is that they tend to provide coated Ti02 particles which have low brightness. In addition, these processes require a substantial investment in equipment, involve time consuming, often complicated operations and generate volumes of aqueous waste.Therefore, it would be desirable to have a process to make a durable Ti02 pigment having particles with a coating. It would also be desirable to reduce investment costs involved in a coating process, and at the same time minimize waste.A high percentage of the Ti02 particles should be coated by such a process, as any uncoated particles could result in a pigment that had a durability s significantly less. The present invention provides such a process. Gonzales, U.S. Patent 5,562,764 discloses a process for the preparation of Ti02, by which a volatile silicon compound downstream from where an oxygen-containing gas and TiCl4 are initially contacted is added to produce a Ti02 pigment substantially free of anatase, containing silica . The product of Ti02 has a decreased particle size and agglomeration. Kodas et al., WO 96/36441 discloses a process for preparing coated Ti02 pigment comprising oxidation of a titanium compound to form Ti02 particles in a reactor, subsequently introducing at least one metal coating precursor into the reactor and the oxidation of the precursor to form a metal oxide coating on the Ti02 pigment particles. The metal oxide is selected from Si02, A1203, Zr02 and mixtures thereof. Alien and Gergely, in the co-assigned and coassigned US patent application serial number 08 / 764,414, filed December 11, 1996, now assigned, describes a process for preparing rutile Ti02, which comprises the oxidation of TiCl4 in the presence of an aluminum compound to • produce a suspension of Ti02 particles and then contact the particles with a boron compound to provide a Ti02 pigment containing B203. The Ti02 product has a decreased particle size and agglomeration, and less abrasion than conventional rutile Ti02 pigments. Glaeser, U.S. Patent No. 4,214,913 describes a process for preparing rutile TiOz which comprises the oxidation of TiCl 4 in the presence of AlCl 3 and the addition of PC 13 in the oxidation at a point where at least 80% of the TiCl 4 has been converted to TiO 2. The Ti02 product is provided in a state which can be used directly to prepare slurries for use in paper and cardboard applications. It is established that for use in coatings, Ti02 is wet treated. Angerman and Moore, U.S. Patent No. 3,856,929 describes a process for preparing TiO2 from anatase which comprises oxidizing TiCl4 in the presence of phosphorus and silicon halides. The addition of silicon and phosphorus halides in combination is more effective than any of the halides alone in promoting the formation of anatase TiO2. There is a need to provide a substantially rutile Ti02 pigment having durability and gloss. There is also a need to reduce or eliminate problems associated with conventional wet treatment methods used to provide over-coatings of Ti02. The present invention satisfies the above needs.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a process for producing a titanium dioxide pigment comprising the steps of: a) reacting titanium tetrahalide in the vapor phase with an aluminum halide and an oxygen containing gas in a reactor at a temperature in the range of 900 ° C to 1600 ° C to provide a gaseous suspension comprising Ti02 particles; b) contacting the gaseous suspension with at least two oxide precursors, wherein the first oxide precursor is a silicon halide and the second oxide precursor is selected from the group consisting of boron oxide, phosphorus precursors , magnesium, niobium, germanium and mixtures thereof; and c) cooling the gas suspension to provide a pigment containing Ti02 particles having a coating containing silica and a second oxide, wherein the second oxide is selected from the group consisting of boron oxide, phosphorus, magnesium, niobium, germanium and mixtures thereof. The resulting Ti02 pigment is substantially in rutile form. Preferably, the aluminum halide is selected from the group consisting of A1C13, AlBr3, A1I3 and mixtures thereof, and more preferably the aluminum halide is AlCl3. The A1C13 can be added in a sufficient amount to provide about 0.5 to about 10% by weight of A1203 based on the total weight of the pigment. Preferably, the titanium dioxide pigment has a coating comprising 0.1 to 10% by weight of silica, based on the total weight of the pigment, and 0.5 mol% to 50 mol% of a second oxide, based on the weight total coating composition. Preferably, the second oxide is a boron oxide or a phosphorus oxide. When the second oxide is a boron oxide, the oxide precursor is preferably a boron halide which is selected from the group consisting of BC13, BBr3, BI3 and mixtures thereof, and more preferably, the boron halide is BCl3. The boron oxide precursor may be added in an amount sufficient to provide from about 3 mol% to 50 mol% boron, calculated as B203, based on the total weight of the coating composition. Preferably, the boron compound is added in an amount sufficient to provide from about 6 mole% to 20 mole% bromine as B203, based on the total weight of the coating composition. When the second oxide is a phosphorus oxide, the oxide precursor is preferably a phosphorus halide such as phosphorus chloride or oxychloride, but may also be other phosphorus halides such as phosphorus bromide or oxybromide. Especially useful phosphorus halides are selected from the group consisting of phosphorus trichloride, phosphorus dichloride, phosphorus oxychloride, phosphorus pentachloride, and mixtures thereof, and most preferably, the phosphorus halide is PC13. The phosphorus oxide precursor may be added in an amount sufficient to provide 0.5 mol% to about 30 mol% phosphorus calculated as P205 based on the total weight of the coating composition. Preferably, the phosphorus is added in an amount sufficient to provide from 1 mol% to 15 mol% phosphorus as P205 based on the total weight of the coating composition. The silicon halide and the second oxide precursor are added to the gaseous suspension of Ti02 particles preferably in a conduit or smoke channel where the washing or rubbing particles are added to minimize the accumulation of Ti02 inside the channel of smoke during cooling. The cooled Ti02 pigment having a coating comprising silica and a second oxide, wherein the second oxide is selected from the group consisting of oxides of boron, phosphorus, magnesium, niobium, germanium and mixtures thereof, can be recovered and subject to grinding such as grinding media or fluid energy.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a high magnification transmission electron micrograph of a Ti02 pigment comprising Ti02 particles, each having a substantially uniform coating of about 3 nanometers (nm) comprising silica and phosphorus oxide.

Figure 2 is a histogram of the coating thickness distribution obtained from the sample analysis shown in Figure 1, which shows that approximately 90% of the Ti02 particles have a coating comprising silica and phosphorus halide . The histogram is based on an analysis of at least 1000 particles. Figure 3 is a high magnification transmission electron micrograph of a Ti02 pigment comprising Ti02 particles, each having a substantially non-uniform coating of varying thickness, comprising silica. Figure 4 is a histogram of the coating thickness distribution obtained from the analysis of the sample shown in Figure 3, which represents that approximately 50% of the Ti02 particles are uncoated, and the remaining particles have a non-uniform coating of varying thickness, comprising silica. The histogram is based on the analysis of at least 1000 particles.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for producing titanium dioxide pigment (Ti02) having coatings comprising silica and a second oxide which is selected from the group consisting of oxides of boron, phosphorus, magnesium, niobium, germanium and mixtures thereof. same. The resulting Ti02 pigment is in substantially rutile form. The process of the present invention involves reacting a titanium tetrahalide (for example TiCl4) in the vapor phase with an aluminum halide and an oxygen containing gas in a reactor at a temperature in the range of 900 ° C to 1600 °. C to provide a gaseous suspension comprising Ti02 particles, and to contact the gas suspension comprising particles of Ti02 with a first deoxid precursor which is a silicon halide, and a second oxide precursor which is selected from the group it consists of precursors of boron oxide, phosphorus, magnesium, niobium, germanium and mixtures thereof, which will form an oxide under process conditions. As used herein, the term "first oxide precursor" means to differentiate between the first oxide precursor, ie, silicon halide, from the "second oxide precursor", but it should not be taken to mean that the Silicon halide should be added to the gaseous suspension before, ie, upstream of the second oxide precursor. The silicon halide and the second oxide precursor can be added separately in equal or different points, or they can be combined or premixed and added as a mixture to the gaseous suspension of Ti02 particles. This process is described in more detail later. The production of the Ti02 pigment by the vapor phase oxidation of a titanium tetrahalide, particularly TiCl4, is well known and is described in Schaumann, U.S. Pat. 2,488,439 and Krchma et al., U.S. Pat. 2,559,638. In the production of Ti02 pigment by oxidation in the vapor phase of titanium tetrahalides, titanium tetrachloride (TiCl4), titanium tetrabromide (TiBr4) and / or titanium tetraiodide (Til4) can be used but it is preferable to use TiCl4 . First, TiCl4 is evaporated and preheated at temperatures from about 300 ° C to about 650 ° C and is introduced into a reaction zone of a reaction vessel. The aluminum halide, in an amount sufficient to provide about 0.5 to about 10% Al203, preferably about 0.5% to about 5%, and more preferably about 0.5% to about 2% Al203 by weight based on the total weight of the pigment is carefully mixed with TiCl4 before its introduction into the reaction zone of the reaction vessel. Suitable aluminum halides include, for example, A1C13, AlBr3 and / or All3. Preferably, A1C13 is used, as described in U.S. Pat. 2,559,638, in the process of the present invention.

The oxygen-containing gas is preheated to at least 1200 ° C and continuously introduced into the reaction zone through a separate inlet from an inlet for the TiCl 4 feed stream. By "reaction zone" is meant the length of the reactor in which the substantial reaction of the reactants takes place. The reaction of 02 and TiCl4 in the vapor phase is extremely rapid and provides a hot gaseous suspension consisting of TiOz particles and free chlorine. This reaction stage is followed by a brief period of growth of Ti02 particles. Optionally, the gas containing oxygen contains a nucleant. By "nucleant" is meant any substance which can reduce the particle size of the pigment such as metals, oxides, salts or other compounds of sodium, potassium, lithium, rubidium, cesium, calcium, barium, strontium and the like, or mixtures thereof, as described in Lewis et al., US Pat. 3,208,886 and Alien et al., U.S. Pat. 5,201,949. Particularly preferred nucleants are CSC1 and KCl. The hot gaseous suspension comprising the Ti02 particles is then rapidly cooled in order to avoid undesirable growth in particle size. The cooling of the hot gas suspension can be carried out by methods known in the art. These methods typically involve passing the hot gaseous suspension through a cooling duct (smoke channel) having relatively cold walls in comparison with the gaseous suspension. Granular wash particles (rubbing), such as calcined Ti02, NaCl, KCl, sand and mixtures thereof can be added to the smoke channel to reduce the formation of particulate deposits of Ti02 on the inner walls of the smoke channel. This cooling step is described in greater detail in Rick, U.S. Pat. 2,721,626, Nerlinger, U.S. Pat. 3,511,308, Rahn et al., U.S. Pat. 3,475,258 and Diemer et al., U.S. patent application. co-assigned co-assigned serial number 08 / 703,303, filed August 26, 1996. In carrying out the invention, the silicon halide and the second oxide precursor are added downstream from the addition of the TiCl 4 stream. The exact point of the silicon halide and the second oxide precursor will depend on the design of the reactor, the flow rate, temperatures, pressures and production rates. For example, the silicon halide and the second oxide precursor may add at one or more points downstream from where TiCl4 is initially contacted and the oxygen-containing gas. Specifically, the temperature of the reaction mass at the silicon halide point or points and the addition of the second oxide precursor will vary from about 500 ° C to about 1600 ° C, preferably from about 1000 ° C to about 1600 ° C, at a pressure from ambient pressure to approximately 689 kPa (100 psig) preferably at least 138 kPa (20 psig) as described in Santos, US Pat. 3,505,091. It will be understood by those skilled in the art that the temperature profile in the reactor will determine the choice of appropriate addition points for the silicon halide and the second oxide precursor. The silicon halide and the second oxide precursor can be added separately or in combination to the gaseous suspension of Ti02 particles. When added separately, the silicon halide in the second oxide precursor is added from different points downstream from where the TiCl 4 and the oxygen-containing gas initially come into contact. Preferably, when the silicon halide and the second oxide precursor (for example H3B03 or BC13) are separately added, at least one addition of the second oxide precursor before, ie, upstream of, the first addition of the halide of silicon. When the silicon halide and the second oxide precursor (for example PC13) are added in combination, they can be pre-mixed and then added at one or more points downstream from where TiCl4 is initially brought into contact with the oxygen-containing gas, and form the gaseous suspension of Ti02 particles. Alternatively, the silicon halide and the second oxide precursor can be added in combination to the gaseous suspension of Ti02 particles without premixing. For example, the silicon halide and the second oxide precursor can be fed from individual feed systems, and fed through a feed point common to the gaseous suspension of Ti02 particles. Preferably, there is no premixing of the silicon halide and the second oxide precursor when they are added in. combination and are fed through a common feeding point. Suitable silicon halides include SiCl 4 and Sil 4 preferably SiCl 4. SiCl4 can be introduced either as vapor or liquid. Frequently, the silicon halide will be added in an amount sufficient to provide from 0.1 to 10% by weight of silica, preferably 0.5 to 6% by weight of silica, based on the total weight of the Ti02 pigment. The second oxide precursor may be a precursor of boron oxide, phosphorus, magnesium, niobium, germanium and mixtures thereof. By "oxide precursor" it is meant that the second compound will form an oxide under the conditions present in the process. Typically, the second oxide precursor will be a halide or other volatile compound. Preferably, the second oxide precursor is a boron or phosphorus compound.

For example, suitable boron oxide precursors include boron halides (for example BC13, BBr3 and BI3, preferably BC13), volatile boron organic compounds (for example trimethyl borate, boron hydrides) and boron compounds with a low melting point (for example boric acid).

In general, boron compounds which can be converted to a fluid boron oxide at process temperatures are suitable precursors. • Preferably, boron trichloride or boric acid is used. When the second oxide precursor is a boron compound, it will often be added in an amount sufficient to provide from 3 mol% to 50 mol% of boron calculated as B203, based on the total weight of the coating composition, preferably from about 6 mol% to 20 mol% boron as B203, based on the total weight of the coating composition. Suitable phosphorus oxide precursors include phosphorus halides and oxyhalides, preferably chlorides and oxychlorides, but bromides and oxybromides can also be used. Preferably, the phosphorus oxide precursor is phosphorus trichloride, phosphorus oxychloride, phosphorus dichloride or phosphorus pentachloride. The most preferred is phosphorus trichloride When the second oxide precursor is a phosphorus compound, such as a phosphorus halide, it will often be added in a sufficient amount from about 0.5 mole% to about 30 mole% phosphorus calculated as P205, preferably 1 mole% to 15 mole% phosphorus as P2Os, based on the total weight of the coating composition In a preferred embodiment, the SiCl4 and the second oxide precursor are added downstream of the pipe or smoke channel wherein the washing or rubbing particles are aggregated to minimize the accumulation of Ti02 inside the smoke channel, as described in greater detail in Rick, US Patent 2,721,626 In this embodiment, the SiCl 4 and the second oxide precursor , for example BC13 or PC13 / can be added downstream alone, or at the same point where rubbing is introduced into the smoke channel. practice of the present invention, the addition of rubbing is optional. Although it is not desired to join any theory, it is suggested that the second oxide precursor acts as a melting point modifier for silica, where the melting point of the silica is reduced and the silica is able to deposit more uniformly on the silica. the surfaces of the Ti02 particles. In addition, when the second oxide precursor is added separately before the silicon halide, the second oxide can form a coating on the Ti02 particles. Upon the addition of the silicon halide, the second oxide coating can serve as a bonding layer to facilitate a uniform deposition of silica on the Ti02 particles. The silicon halide and the second oxide oxidant precursor are incorporated into the surface of the TiOa particles as a silica coating incorporating the second oxide. As a result of the reactive streams, the substantially complete oxidation of TiCl 4, AlCl 3, SiCl 4 and the second oxide precursor takes place, but for conversion limitations imposed by temperature and thermochemical equilibrium. Solid particles of Ti02 are formed. The reaction product containing a suspension of Ti02 particles in a mixture of chlorine and waste gases is transported from the reaction zone to temperatures considerably exceeding 1200 ° C and subjected to a rapid cooling in the smoke channel as described above. described above, or in another conventional medium. The Ti02 pigment is recovered from the cooled reaction products by conventional separation treatments including cyclonic or electrostatic separation media, filtration through porous media - or the like. The pigment recovered from Ti02 can be subjected to wet treatment, grinding, crushing or disintegration treatment to obtain the desired level of agglomeration. It will be recognized that the addition of silica and the second oxide in accordance with this invention provides a durable Ti02 pigment, and at the same time provides the flexibility to reduce or eliminate the amount of silica added in a subsequent wet treatment step. It is recognized that wet treatment can be used, if desired, to provide a pigment with even greater durability. In general, the process of the present invention provides several advantages including the following: 1) a coated Ti02 pigment, without the need for a wet treatment process with the inherent disadvantages of wet processing processes; 2) a reduction in the coarse fraction of Ti02 particles (ie, a reduction in the weight percentage of Ti02 particles having a diameter size greater than 0.6 micrometers) and a greater pigment with a low pitch black carbon (CBU); 3) a coated Ti02 pigment that has good gloss; 4) a Ti02 pigment in substantially rutile form; and 5) a more uniform coating of Ti02 particles, i.e., consistent coating thicknesses around a given Ti02 particle, with less silica residue, than those available from wet processing processes. The process of the present invention provides a titanium dioxide pigment comprising Ti02 particles having a coating comprising silica and a second oxide, wherein the second oxide is selected from the group consisting of boron oxide, phosphorus, magnesium, niobium, germanium and mixtures thereof. Surprisingly, the process of this invention provides Ti02 pigments having a uniform coating of silica that incorporates the second oxide. By "uniform coating", it is meant that the thickness of the coating is generally consistent around a given particle. It is understood that although the thickness of the coating in a particle may differ from the coating thickness in a different particle by a certain small amount. In addition, by using the second oxide precursor with the silicon halide, a proportion of particles with silicon is coated which would occur in the absence of the second oxide precursor under the same silica loading. Therefore, with the present invention, durability can be obtained with a lower charge of silica. As shown in Figure 2, in a preferred embodiment, the titanium dioxide pigments produced by the process of this invention using a silicon halide and a second oxide precursor, have approximately 90% of the coated particles at a charge of silica of 2.5% by weight, based on the total weight of the pigment. In comparison, as shown in Figure 4, about 50% of the particles are coated in the absence of the second oxide precursor at the same silica loading of 2.5% by weight, based on the total weight of the pigment. The present invention is further illustrated by the following examples using the following test methods, but these examples should not be considered as limiting the scope of the invention.

Test Methods CAC Test A coefficient of catalytic activity (CAC) is a measure of durability based on the ultraviolet reactivity of the Ti02 pigment in a test based on the reduction catalyzed by Ti02 of lead carbonate to the metal. An air-sealed dispersion of non-durable Ti02 and lead carbonate in an organic medium changes from white to almost black by exposure to ultraviolet light. With durable Ti0 pigments, the paste changes to a light gray. A drop of a paste of basic lead carbonate, glycerol, fumed silica and Ti02 pigment is placed between two microscope glass coverslips and exposed to ultraviolet light for about 5 hours. The dark condition of these coverslips is compared with Munsell chips and the CAC values are determined. The CAC value decreases as the durability increases, that is, a lower CAC value means greater durability. The CAC test is described in more detail in J. Braun, "Ti02's Contribution to the Durability and Degradation of Paint Film II. Prediction of Catalytic Activity", vol. 62, Journal of Coating Technology, pp. 37-42 (1990).

Microscopy High-resolution transmission electron microscopy (HR-TEM) is used to determine the coating thickness, the coating uniformity and the fraction of coated particles. It should be noted that HR-TEM is a projection. Two-dimensional three-dimensional particles. The coating extension and coating on all side surfaces (including the top and bottom) are examined by tilting the sample. A minimum population of 1000 particles is measured from micrographs of a sample. Standard procedures are used for histograms.

EXAMPLES Comparative Example A Base Ti02 produced by the chloride process is fed to a rotary calciner and calcined at 600-1000 ° C for about 8 hours. 39 grams of calcined Ti02 are charged in a fluidized bed reactor. The temperature of the reactor is increased to 15 ° C / minute to 930 ° C. Oxygen is introduced into the reactor to provide fluidization of Ti02 at a rate of 2.8 liters / minute. 30 mol% of SiCl 4 is introduced into argon through the upper part of the reactor through a submerged gas inlet pipe into the fluidized bed, at a rate of 0.5 liters / minute, for 2 minutes. The Ti02 is cured in the reactor for 8 minutes. The reactor is turned off and the product in the reactor is cooled to room temperature. The silica content of the product is 4% by weight.

Example 1 28 grams of calcined Ti02, prepared as described in comparative example A, are charged to a fluidized bed reactor. The temperature is increased and in the fluidized bed as described in example 1. 1.1 grams of boric acid (H3B03) are added. ) to the upper part of the bed for a period of about 5 minutes. SiCl4 is added to the bed as described in Example 1 and the Ti02 is cured in the reactor for 8 minutes. The product is recovered as in example 1. The silica content of the product is 1.9% by weight; the boron content is 6.9 mole%. The products of comparative examples A and example 1 are tested for durability using the CAC method described above.

TABLE 1 Durability Test Example CAC% by weight of SiO? moles% of B ^ ppm B A 0.26 4.0 0.0 0 0.11 1.9 6.9 500 It can be seen from table 1 that the example incorporating both B203 and Si02 shows an improved durability, as measured by the CAC test in relation to comparative example A. 2 A gaseous suspension of uncoated pigmented Ti02 particles, prepared by the chloride process, is fed, using a screw feeder to a quartz tube, at 1000 ° C and a flow rate of 0.50 grams / minute. A vapor mixture of SiCl4 and PC13 is fed with air, countercurrent to the flow of Ti02 to provide a Ti02 pigment having a uniform coating of 3 nm consisting of 2.56% by weight of SiO2, based on the total weight of the pigment and 1.9 mol% of P2Os (0.12% by weight) based on the total coating composition. As shown in Figure 1, each of the Ti02 particles has a substantially uniform coating comprising silica and phosphorus oxide. As shown in the histogram of Figure 2, the percentage of Ti02 particles in Figure 1 which are coated is approximately 90% based on the measurements taken of approximately 1000 Ti02 particles from many areas.

Comparative Example B The feed of TiCl4 is pre-mixed carefully with AlCl3 in an amount sufficient to provide 1% by weight of Al203 based on the total weight of the pigment. The TiCl 4 feed is evaporated and preheated to 425 ° C and introduced to a reaction zone at a speed corresponding to the production rate of 4.5 tons / h of Ti02 pigment product. Simultaneously, preheated oxygen is continuously introduced through a separate inlet adjacent to the TiCl4 inlet. Trace amounts of KCl dissolved in water are added to the oxygen stream as described in British Patent 922,671 and in U.S. Pat. 3,208,866. SiCl4 is fed to the reactor as a finely dispersed liquid at a position of 1.5 meters (5 feet) downstream (or approximately 0.02-0.04 seconds from the point where it is initially contacted in TiCl4 and oxygen) at the same point in which rubs, at a speed and in a quantity sufficient to provide a load of 2.5% by weight of SiO2 based on the total weight of pigment. The temperature of TiCl4 is 425 ° C and the temperature of oxygen is 1595 ° C. The estimated temperature of the reaction mass at the injection site of SiCl 4 is about 1400 to 1500 ° C for the reaction zone at a pressure of about 345 kPa (50 psig). As shown in figure 3, each of the Ti02 particles has a substantially non-uniform coating with a thickness ranging from 0 to 10 nm. As shown in the histogram of Figure 4, the percentage of the Ti02 particles in Figure 3 which are coated, is about 50% based on the measurements taken of approximately 1000 Ti02 particles from many areas. The silica particles are also present as waste.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers.

Claims (11)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A process for producing a titanium dioxide pigment, characterized in that it comprises the steps of: a) reacting titanium tetrachloride in the vapor phase with an aluminum halide and an oxygen-containing gas in a reactor at a temperature of the range of 900 ° C to 1600 ° C to provide a gaseous suspension comprising Ti02 particles; b) contacting the gas suspension with at least two oxide precursors, wherein the first oxide precursor is a silicon halide and the second oxide precursor is selected from the group consisting of precursors of boron oxide, phosphorus , magnesium, niobium, germanium and mixtures thereof; and c) cooling the gas suspension to provide a pigment containing Ti02 particles having a coating containing silica and a second oxide, wherein the second oxide is selected from the group consisting of boron oxide, phosphorus, magnesium, niobium, germanium and mixtures thereof.

2. The process according to claim 1, characterized in that the aluminum halide is A1C1 ,.

3. The process according to claim 1, characterized in that the second oxide precursor is a boron compound and the pigment produced by the process comprises particles of Ti02 having a coating comprising silica and boron oxide,.

4. The process according to claim 3, characterized in that the coating comprises approximately 0.1 to 10% by weight of silica, based on the total weight of the pigment, and from approximately 3 to 50 mole% of boron oxide, in base on the total coating composition.

5. The process of consistency with claim 3, characterized in that the boron compound is a boron halide which is selected from the group consisting of BC13, BBr3, BI3 and mixtures thereof.

6. The process according to claim 1, characterized in that the second oxide precursor is a phosphorus compound and the pigment produced by the process comprises Ti02 particles having a coating comprising silica and phosphorus oxide.

7. The process according to claim 6, characterized in that the coating comprises about 0.1 to 10% by weight of silica, based on the total weight of the pigment, and from about 0.5 to 30 mol% of phosphorus oxide, based on the total coating composition.

8. The process according to claim 6, characterized in that the phosphorus compound is a phosphorus halide which is selected from the group consisting of PCl3, P0C13, PHC12, PCl5 and mixtures thereof.-

9. The process according to claim 1, characterized in that the pigment has a substantially rutile crystalline structure.

10. The process according to claim 1, characterized in that the second oxide precursor is added separately from, and before the first addition of the silicon halide.

11. The process according to claim 10, characterized in that the second oxide precursor is a precursor of boron oxide.