MXPA97007145A - A process for elaborating titanium dioxide pigment which has a hydrated oxide coating using a medium of molie - Google Patents

Abstract

This invention provides a process for producing titanium dioxide pigment, wherein an aqueous suspension of TiO2 particles surface treated with hydrous oxide is subjected to a grinding medium. The TiO2 particles are surface treated with a coating containing hydrous oxide such as silica and / or alumina, and the suspension filtered before grinding. The milled suspension is dried and the dried product is milled with hydraulic energy to produce TiO2 pigment having good particle size distribution. TiO2 pigments could be used to make coating formulations that have high brightness

Description

A PROCESS TO DEVELOP TITANIUM DIOXIDE PIGMENT THAT IT HAS A HYDRATED OXIDE COATING USING A MEDIUM OF GRINDING BACKGROUND DB THE INVENTION The present invention relates to a process for producing titanium dioxide pigment, wherein an aqueous suspension of titanium dioxide particles surface treated with hydrous oxide is subjected to a grinding medium. The milled suspension is dried and the dried product is milled with hydraulic energy to form titanium dioxide pigment having good particle size distribution.

Description of Related Art Titanium dioxide (Ti02) is produced commercially by the well-known processes of "chloride" and "sulfate". In the "chloride process", titanium tetrachloride (TiCl4) is oxidized in the vapor phase with an oxygen-containing gas, to produce a hot gaseous suspension of TiO, particulate solid. This particulate of crude Ti02 undergoes subsequent treatment to produce a finished TiOa pigment product. In Story et al., US Pat. No. 5,332,433, a typical process for producing Ti03 pigment is designed. The process REF: 25607 includes the following steps: (1) dispersion of the crude Ti02 in an aqueous medium to form a suspension; (2) Wet milling of the suspension; (3) precipitation of the hydrated oxides (eg, alumina, silica) on the surfaces of the Ti02 particle from wet milling; (4) recovery of Ti02 surface-treated with hydrous oxide from the aqueous medium by means of filtration; (5) washing and filtering Ti02 to remove salts and impurities from it; (6) drying of the washed Ti02 product; and (7) grinding the dried Ti02 pigment product to a desired size using, for example, a hydraulic power mill. Hinley et al., US Pat. No. 3,942,999 also teaches that the Ti02 pigment is wet milled before being coated with one or more hydrated oxides. Berstein et al., US Patent 3,212,911 discloses a process for preparing Ti02 pigment that is more easily dispersible in a liquid or plastic medium that includes mechanical wear (eg, ball, stone, or colloid mill) of Ti02. in the presence of a solution of alkali metal silicates, aluminates, or titanates. Subsequent to the milling step, the titanium oxide can then be treated with additional alkali metal compounds to effect the coating of the particles with hydrous oxides (eg, alumina or titania). As described in Tear et al., US Patent 4,448,609, it is also known that aqueous suspensions of Ti02 pigment can be prepared, wherein the pigment particles have been surface treated with one or more hydrous oxides containing alumina. The Ti02 is removed from the aqueous medium by filtration and the cake resulting from the filtrate is washed with water. The solids concentration of the cake of the washed filtrate is raised to approximately 60% by weight and introduced into the dispersion. { wet filter cake) a dispersant combination containing a dispersing amine and an anionic dispersant. The dispersion is then ground in a suitable milling machine using grinding elements such as ball mills, bead mills, or sand mill. The present invention is directed to a process for making dry Ti02 pigment product that is ground with hydraulic energy to provide a desirable particle size distribution in the final Ti02 pigment product. As described above, in conventional processes for producing such products, the suspensions of uncoated Ti02 particles are wet milled, and the ground suspension of coated Ti02 particles is filtered and dried, and the dried Ti02 pigment is milled with energy. hydraulics. This stage of grinding with hydraulic energy is energy intensive and requires high capital investment.

It would be desirable to have a process to produce dried Ti02 pigment product that could operate at lower energy costs while maintaining or improving product quality. It would also be desirable to improve Ti02 pigment production rates without additional investment. The present invention provides a process that meets the above needs.

BRIEF DESCRIPTION OF THE INVENTION The present invention provides a process for preparing a titanium dioxide pigment, comprising the steps of: a) preparing an aqueous suspension containing particles of Ti02 base, b) applying a coating containing an oxide hydrated to the TiOa base particles to provide a suspension containing coated Ti02 particles, c) filtration of the suspension of Ti02 coated particles to provide a dehydrated suspension, d) grinding media of the dehydrated suspension, e) drying of the ground slurry to provide a dry Ti02 pigment product, and f) grinding with hydraulic energy of the dried Ti02 pigment product. Although not required, an aqueous suspension high in solids content could be used, wherein the concentration of the Ti02 base particles in the aqueous suspension is in the range of about 30 to 85% weight based on the total weight of the suspension. Preferably, the coating comprises a hydrous oxide selected from silica, alumina, zirconia, and mixtures thereof. The hydrous oxide coating could contain silica, wherein the amount of silica is typically in the range of about 0.5 to about 30% weight based on the total weight of the Ti02 pigment. Preferably, the amount of silica is in the range of about 2 to 11% weight. The hydrous oxide coating could contain alumina, wherein the amount of alumina is typically in the range of about 0.5 to about 10% weight based on the total weight of the Ti02 pigment. Preferably, the amount of alumina is in the range of about 0.5 to 5% weight. Preferably, the concentration of Ti02 particles coated in the aqueous suspension is less than 40% weight based on the total weight of the suspension before filtering the suspension. Subsequent to filtration, wherein the suspension is dehydrated, the concentration of the coated Ti02 particles is greater than 30% by weight and less than 60% by weight based on the total weight of the dehydrated suspension. The dehydrated suspension could be milled by feeding the suspension in a grinding chamber containing beads, and the milled suspension could be dried to less than 5% weight of water. The pigment product of dry Ti02 could be ground with hydraulic energy by feeding the suspension in a chamber with air or steam.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a process for producing titanium dioxide pigment wherein a suspension of Ti02 particles surface treated with hydrous oxide is subjected to a grinding medium. The milled suspension is dried and the dried product is milled with hydraulic energy to form Ti02 pigment having good particle size distribution. The Ti02 used to prepare the pigment of the invention may be of the conventional gloss variety, prepared by the chloride or sulphate process. The chloride process is preferred. This process typically comprises oxidation with oxygen or an oxygen-containing gas, a mixture of TiCl4 and A1C13 / the amount of A1C13 will be sufficient to form 0.1-1.5% of A1303 in Ti02. In the process of this invention, an aqueous suspension of Ti02 base particles is first prepared by techniques known in the art. By "base particles of Ti02", it is understood that the particles of Ti02 containing the pigment have not been coated by a surface treatment process. Preferably, these Ti02 base particles have an average main particle size of less than about 0.4 microns. The base particles of Ti02 are dispersed in water to form a suspension. The concentration of the TiOs base particles in the suspension will vary depending on the subsequent coating process. Typically, the suspension has a pigment solids content of TiO2 (Ti02 base particles) greater than 30% weight based on the total weight of the suspension. Preferably, the pigment solids content of Ti03 is in the range of approximately 30 to 85% by weight. The Ti02 particles of the aqueous suspension are then surface treated with a coating containing a hydrous oxide by techniques known in the art to provide a suspension of coated Ti02 particles. Conventional coating processes can be used. Examples of coating processes include US Pat. Re. 27,818; 4,781,761; 4,125,412, the expositions of which are incorporated herein by reference. The present invention is not limited to these processes and many other known coating processes could be used. In general, these coating techniques include the addition of a coating solution to the suspension and the adjustment of the temperature and pH of the suspension such that the hydrous oxide is precipitated on the surface of the particles in the desired percentage by weight. For example, a solution containing sodium silicate could be added to the suspension and the pH of the suspension will increase above 8, and usually above 11. The pH of the suspension is then gradually reduced over a period of several hours by the addition of dilute acid (e.g., HCl) to the suspension. As the acid is added, the Si02 is deposited on the surfaces of the Ti02 particles. Suitable hydrated metal oxides include, for example, oxides of silicon, titanium, zirconium, zinc, magnesium, aluminum, yttrium, antimony, cerium, and tin. Preferably, the coating contains a hydrous oxide selected from the group consisting of silica, alumina, zirconia, and mixtures thereof. For the purposes of this invention, if the coating contains silica, the silica will generally constitute about 0.5 to 30% of the total weight of the Ti02 pigment, and will preferably be in the range of 2 to 11% by weight. If the coating contains alumina, the alumina will generally constitute about 0.5 to 10% of the total weight of the Ti02 pigment, and will preferably be in the range of 0.5 to 5% weight. On some occasions, the alumina could be precipitated on the Ti03 particles as a surface cover by first curing the surface treated particles of the suspension and then adding sodium aluminate to the suspension and adjusting the pH.

Exiting the coating step, the suspension typically has less than 40% solids weight. This suspension is then filtered, washed, and refiltered to provide a dehydrated suspension by means of known techniques. As used herein, by the term "dehydrated suspension", it is meant that the suspension has been filtered. Optionally, during the filtration process, grinding aids such as polyols (e.g., trimethylolpropane), alkanol amines. { p. ex. , triethanolamine and 2-amino-2-methyl-1-propanol), and others can be added to the suspension. After filtration, washing and refiltration, the suspension has a solids content (coated Ti02 particles) greater than 30% weight.

Preferably, the concentration of solids is less than 60% weight. More preferably, the solids concentration is in the range of 30 to 50% weight. After the stages of filtration, washing and refiltration, the suspension of dehydrated Ti02 is subjected to a grinding medium. The grinding medium, for the purposes of the present invention, can be made in any suitable way, for example, ball mill, sand or beads, or by other means. Many types of media or different sizes could be used, with someone of ordinary skill in the art being possible to adjust the medium to produce the desired result based on the size distribution of the initial material particles according to conventional protocols. The Ti02 suspension is fed to a grinding chamber that is packed with beads. The packing is agitated in the grinding chamber by a series of discs attached to a rotating shaft. The movement of the packing is perpendicular to the direction in which the suspension is pumped, and therefore the particles of Ti02 are cut off by the packing. Typically, a plate holds the packing inside the grinding chamber but allows the suspension of the Ti02 suspension from the mill. Optionally, the product of the mill can also be screened. It has surprisingly been found that by incorporating a grinding medium stage after the surface treatment with hydrous oxide rather than before the surface treatment, an improvement in particle size reduction is achieved. The improvement in particle size reduction provides the following advantages in a process for preparing Ti02 pigments: 1) reduction in the energy requirements of the manufacturing processes, 2) increase in the pigment production rate of Ti02, and 3) Improved quality of Ti02 pigment product. After the milling media, the suspension is dried by a suitable means such as, for example, quick drying, spray drying, or an oven. Preferably, the suspension is dried to less than 5 wt.% Water, and more preferably less than 0.7 wt.% Water. Finally, the dry Ti02 pigment product is subjected to grinding with hydraulic energy. In a hydraulic power mill, p. ex. , air or steam, is used in the grinding of the Ti02 pigment. In the hydraulic power grinding process of US Pat. No. 4,427,451, the disclosure of which is incorporated herein by reference, the dried Ti02 pigment is introduced with air or steam into the outer portion of a vortex traveling spirally in the interior to transport the Ti02 at high speed against the housing of the spiral vortex to fracture the agglomerates of Ti02. The dried Ti02 pigments produced by means of the process of this invention have low particle agglomeration. The particle agglomeration of the pigment is typically measured in terms of its particle size distribution (decrease in thickness). The pigments, wherein a low percent by weight of the particles (eg, less than 30%) have a particle diameter size greater than 0.6 microns, tend to have low particle agglomeration and coatings (e.g. ex., paints) made with such pigments tend to have high gloss. The pigments, wherein a high percent by weight of the particles have a particle diameter size greater than 0.6 microns, tend to have greater particle agglomeration and the final products made with such pigments tend to have less brightness. The present invention is further illustrated by means of the following examples, but these examples should not be construed as limiting the scope of the invention.

Test Methods Particle Size Distribution Particle size distributions of the pigment products were measured by sedimentation analysis, with a Sedigraph * (Micromeritics Instrument Corp., Norcross, GA) after dispersion in aqueous suspension setting the sonication level . The measure of particle size and fraction% > 0.6 micrometers will indicate the potential for maximum brightness in the final product, a value that can not be exceeded while applying any reasonable level of energy.

Emulsion Shine Emulsion Shine (TFW-182) was determined by preparing an emulsion paint formulation using a sample of the slurry, tracing the paint down on black PVC panels, drying the panels for 3 hours at a constant temperature, in a constant humidity cabin (CTCH), measuring 60 degrees of brightness using a Hunter brightness meter (available from Hunter Laboratories, Reston, VA), and calculating the brightness in relation to the reflectance values of the standards .

Gloss of Alkyd The brightness of alkyde was determined by dispersing pigment in an alkyd vehicle. The dispersion was ground with sand and reduced with resin for atomized consistency. Aluminum panels were spray painted using automatic spray equipment under controlled conditions to produce uniform surface thickness and characteristics. The paint films were baked. The brightness was determined by measuring 20 degrees of reflectance with a Hunter brightness meter and calculating the brightness in relation to the reflectance values of the standards.

EXAMPLES COMPARATIVE EXAMPLE A The base Ti02 was prepared by means of the chloride process as set forth in U.S. Pat. 2,559,638, and 2,488,439. He Ti02 base was suspended in water and coated with alumina and silica as disclosed in U.S. Pat. 4,781,761. After the coating step, the coated Ti02 contained in the suspension was filtered using 0.25% trimethylolpropane (TMP), spray dried and ground with hydraulic power at a vapor to pigment ratio of 4.2 lbs. Of steam per pound of pigment. in a 27"hydraulic energy vortac mill. Particle size distribution, emulsion gloss, and alkyd gloss were measured by the above Test Methods and the results are presented in Table 1 below.

COMPARATIVE EXAMPLE B The base Ti02 suspension having one gram per liter of about 325-350 was neutralized to a pH of 7.0 and ground. The conditions of the grinding medium were 2100 ft / min speed of the tip of the disc, 85% load in the medium, power consumption of approximately 29-30 k atts. The milled Ti02 slurry was coated following the same process described in Comparative Example A, filtered using 0.25% TMP, dried and ground with hydraulic power at 4.5 lbs steam per pound of pigment. The particle size distribution, emulsion gloss, and alkyd gloss were measured by the above Test Methods and the results are presented in Table 1 below.

EXAMPLE 1 The base Ti02 was prepared and coated following the same process described in Comparative Example A. The suspension of coated Ti02 particles was filtered using 0.25% TMP and washed in the same manner as described in Comparative Example B. The suspension The dried product was then ground in the same manner as described in Comparative Example B. The mill discharge was dried and ground with hydraulic energy at 4.3 lbs. of steam per pound of pigment in the same manner as described in Comparative Example B. The particle size distribution, emulsion gloss, and alkyd gloss were measured by the above Test Methods and the results are presented in Table 1 below.

TABLE 1 E p plo% > 0.6 Mieras Shine of the Shine of Emulsion Alquíido A * 11.2 53 28 B * 8.0 55 36 1 6.5 57 41 Comparative Examples As can be seen in Table 1, the best results, p. ex. , the lower values of% > 0.6 micrometers and the greater of brightness, were reached by the pigment product of Example 1 in which grinding occurred after coating with hydrated oxides, while milling before coating with hydrated oxides (Comparative Example B) improved the properties of pigment not related to the grinding medium (Comparative Example A), it can be seen that further improvement was achieved by incorporating the grinding medium stage after coating with hydrated oxides.

Claims (11)

1. A process for preparing a titanium dioxide pigment, characterized in that it comprises the steps of: a) preparing an aqueous suspension containing Ti02 base particles, b) applying a coating containing a hydrous oxide to the Ti02 base particles for provide a suspension containing coated Ti02 particles, c) filtration of the suspension of coated Ti02 particles to provide a dehydrated suspension, d) grinding with a grinding medium of the dehydrated suspension, e) drying the ground suspension to provide a dry Ti02 pigment product, and f) grinding with hydraulic energy of the dried Ti02 pigment product.

2. The process according to claim 1, characterized in that the concentration of the Ti02 base particles in the aqueous suspension are in the range of about 30 to 85% weight based on the total weight of the suspension.

3. The process according to claim 1, characterized in that the hydrous oxide is selected from the group consisting of silica, alumina, zirconia, and mixtures thereof.

4. The process according to claim 3, characterized in that the hydrous oxide contains silica and the amount of silica is in the range of approximately 0.5 to 30% weight based on the total weight of the Ti02 pigment.

5. The process according to claim 3, characterized in that the hydrous oxide contains alumina and the amount of alumina is in the range of about 0.5 to 10% weight based on the total weight of the Ti02 pigment.

The process according to claim 1, characterized in that the concentration of Ti02 particles coated in the aqueous suspension is less than 40% based on the total weight of the suspension before filtering the suspension.

The process according to claim 1, characterized in that the concentration of Ti02 particles coated in the aqueous suspension is greater than 30% and less than 60% based on the total weight of the dehydrated suspension.

The process according to claim 1, characterized in that the dehydrated suspension is milled by feeding the suspension in a grinding chamber containing packing beads.

9. The process according to claim 1, characterized in that the milled suspension is dried at less than 5% weight of water. The process according to claim 1, characterized in that the dried Ti02 pigment product is milled with hydraulic energy by feeding the suspension in a chamber with air or steam. The process according to claim 3, characterized in that the hydrous oxide contains a mixture of silica and alumina, wherein the silica is in the range of about 2 to 11% weight based on the total weight of the Ti02 pigment and the Alumina is in the range of approximately 0.5 to 5% weight based on the total weight of the Ti02 pigment.