PROCESS FOR THE PRODUCTION OF COATED TITANIUM DIOXIDE PIGMENTS
A process for the preparation of pigment-grade titanium dioxide is provided that produces substantially anatase-free titanium dioxide with a uniform coating of a metal oxide without producing separate particles of the metal oxide that are not incorporated into the coating. The process comprises mixing a titanium dioxide precursor with a silicon compound to form an admixture and introducing the admixture and oxygen into a reaction zone to produce substantially anatase-free titanium dioxide. The titanium dioxide produced is contacted with a metal oxide precursor homogeneously mixed with a solvent component downstream of the reaction zone to form a uniform coating of the metal oxide on the titanium dioxide particles.
1 . A process for the preparation of substantially anatase-free titanium dioxide particles comprising a uniform homogeneous coating of a metal oxide on the surface of the titanium dioxide particles comprising:
(a) introducing a titanium dioxide precursor and oxygen into a reaction zone of a reactor to produce substantially anatase-free TiO 2 , wherein the reaction zone is at a pressure greater than 5 psig to about 100 psig and the titanium dioxide precursor is mixed with a silicon compound to form an admixture before introducing the titanium dioxide precursor and oxygen into the reaction zone; and
(b) contacting the substantially anatase-free TiO 2 particles with a metal oxide precursor homogeneously mixed in a solvent component downstream of the reaction zone to form coated titanium dioxide particles with a smooth, homogeneous metal oxide coating, wherein separate particles of the metal oxide coating are not produced; and
(c) isolating the coated ultrafine titanium dioxide particles.
2 . The process of claim 1 , wherein the titanium dioxide precursor is titanium tetrachloride.
3 . The process of claim 1 , wherein the silicon compound is silicon tetrachloride.
4 . The process of claim 1 , wherein the substantially anatase-free TiO 2 is at least 99.9% rutile TiO 2 .
5 . The process of claim 1 , wherein the reaction zone pressure is between about 40 psig and about 100 psig.
6 . The process of claim 1 , wherein the reaction zone pressure is about between about 40 psig and about 70 psig.
7 . The process of claim 1 , wherein the reaction zone has a temperature of between about 850° C. to about 1600° C.
8 . The process of claim 1 , wherein the reaction zone is at a temperature of between 1000° C. to about 1300° C.
9 . The process of claim 1 , wherein the reaction zone temperature is about 1200° C.
10 . The process of claim 3 , wherein the amount of silicon tetrachloride mixed with the titanium dioxide precursor produces TiO 2 with between about 0.05% to about 0.5% SiO 2 by weight of the TiO 2 product.
11 . The process of claim 1 , further comprising adding an aluminum halide to the titanium dioxide precursor before introducing the titanium dioxide precursor and oxygen into the reaction zone.
12 . The process of claim 1 , wherein the reaction zone has multiple stages.
13 . The process of claim 1 , wherein the metal oxide coating comprises a metal oxide selected from the group consisting of SiO 2 , Al 2 O 3 , B 2 O 3 , ZrO 2 , GeO 2 , MgO, ZnO and SnO 2 .
14 . The process of claim 13 , wherein the metal oxide is SiO 2 .
15 . The process of claim 1 , wherein the metal oxide precursor is selected from the group consisting of silicon halides, hexaalkyldisiloxanes, tetraalkylorthosilicates and silanes.
16 . The process of claim 15 , wherein the metal oxide precursor is a silicon halide.
17 . The process of claim 16 , wherein the metal oxide precursor is a silicon tetrachloride.
18 . The process of claim 1 , wherein the solvent component is selected from the group consisting of a liquid halide, a halide gas, liquid carbon dioxide, carbon dioxide gas, nitrogen gas and argon gas.
19 . The process of claim 18 , wherein the solvent component is liquid chlorine.
20 . The process of claim 1 , wherein the TiO 2 particles are contacted with the metal oxide precursor at a point downstream of the reaction zone where at least 90% of the TiCl 4 has reacted to form TiO 2 particles.
21 . The process of claim 1 , wherein the TiO 2 particles are contacted with the metal oxide precursor at a point downstream of the reaction zone where at least 95% of the TiCl 4 has reacted to form TiO 2 particles.
22 . The process of claim 14 , wherein the amount of metal oxide precursor added downstream of the reaction zone produces TiO 2 with between about 1% to about 10% SiO 2 by weight of the TiO 2 product.
23 . The process of claim 14 , wherein the amount of metal oxide precursor added downstream of the reaction zone produces TiO 2 with between about 1% to about 5% SiO 2 by weight of the TiO 2 product.
24 . The process of claim 1 , wherein the metal oxide coating is about 1 nm to about 10 nm thick.
25 . The process of claim 1 , wherein the metal oxide coating is about 2 nm to about 6 nm thick.
26 . The process of claim 1 , wherein the particle size of the TiO 2 particles is between about 100 nm to about 300 nm.