Continuous processes for producing titanium dioxide pigments

Details Continuous processes for producing titanium dioxide pigments Continuous processes for producing titanium dioxide pigments

2001-04-12

2004-02-24

Koslow, C. Melissa (Department: 1755)

Compositions: coating or plastic

Materials or ingredients

Pigment, filler, or aggregate compositions, e.g., stone,...

C106S442000

Reexamination Certificate

active

06695906

ABSTRACT:

FIELD OF INVENTION
This invention relates to improved processes for preparing titanium dioxide pigments by multistage continuous wet treatment characterized by short reaction times at low temperatures. Finished pigments prepared by the present processes are useful in various applications (i.e. paint, plastic coatings, etc.) and possess improved properties such as gloss and/or durability.
BACKGROUND OF THE INVENTION
Titanium dioxide (TiO
2
) pigments are useful in a variety of applications such as paints, plastics and coatings. Generally pigments useful in these applications contain base TiO
2
particles (i.e., anatase or rutile) having layers or coatings of hydrous oxides of alumina, silica, zirconia, phosphates and the like.
Processes used to coat TiO
2
are well known in the pigmentary arts. These coating processes typically involve wet treating TiO
2
particles by precipitating certain hydrous oxides to form the desired coating.
Wet treatment of TiO
2
is performed using either batch processes or continuous multi-stage processes. Some prior art references describe advantages to continuous processes including reduced treatment times for hydrous oxide coating, and high product throughput without the need for substantial capital investment.
It is well known in the art that pH, temperature and wet treatment times performed on TiO
2
must be carefully controlled to produce fully coated pigment particles having desired physical properties (i.e., gloss and/or durability). Of the hydrous oxides, combinations of silica and alumina have proven to possess excellent properties in coatings applications. For example, pigments having dense silica coatings are well known to influence the optical properties and improve the durability of plastic, paint, or coating that it is used.
Continuous multistage processes to coat TiO
2
particles with hydrous silica and alumina are described in the prior art. Typically, these processes involve preparing an aqueous slurry of TiO
2
particles at temperatures of at least 80° C. and subsequently adding soluble silicates and alumina to the slurry.
The pH of the slurry is adjusted to at least about 9.6 to initiate the deposition of the hydrous silica layer. The prior art describes that in order to produce pigments with desirable silica coating, excessively long treatment times (i.e., over one hour or longer) and specific pH ranges are needed. For example, adequate dense silica coating is obtainable when slurry pH is lowered to about 9 through a series of steps designed to control the curing of the silica.
The prior art discloses that long treatment times and slurry pH are important to alumina deposition. For example, after curing the silica over one hour, alumina deposition is initiated by lowering the slurry pH to between about 5.5 and 7.5 by the addition of an alumina compound such as sodium aluminate. The reaction is allowed to proceed until such time as the desired amount of alumina has been deposited. Subsequently, the slurry is neutralized and washed. In a separate finishing step, the resulting pigment filter cake is finished by known means.
It has been discovered that in order to prepare TiO
2
pigments for paints, plastic and coatings having acceptable durability and/or gloss, that excessively long treatment times and higher treatment temperatures, typically greater than about 80° C. are unnecessary. These temperatures consume excessive energy, and are deleterious to the process. Thus, the present invention represents advancements in the art of continuous processes producing TiO
2
pigments having hydrous oxide coatings.
SUMMARY OF THE INVENTION
The present invention provides a continuous process for preparing titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75° C. and at a pH of from about 3.0 to about 9.0; b) adding a silica compound while maintaining the pH of the slurry in the range of from about 2.4 to about 10.5 and the temperature of the slurry below about 75° C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75° C. so as to form the titanium dioxide pigment.
In one embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75° C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by maintaining the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 10.0, in the second stage the pH is maintained at about 9.4, and in the third stage the pH is maintained at about 4.9, while maintaining the temperature of the slurry below about 75° C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75° C. so as to form the titanium dioxide pigment.
In another embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75° C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by adjusting the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.6, in the second stage the pH is maintained at about 8.9, and in the third stage the pH is maintained at about 2.7, while maintaining the temperature of the slurry below about 75° C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75° C. so as to form the titanium dioxide pigment.
In another embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of titanium dioxide base particles at a temperature below about 75° C. and at a pH from about 3.0 to about 8.0; b) adding a silica compound to coat the base titanium dioxide particles by adjusting the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.5, in the second stage the pH is maintained at about 9.2, and in the third stage the pH is maintained at about 6.1, while maintaining the temperature of the slurry below about 75° C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75° C. so as to form the titanium dioxide pigment.
In yet another embodiment, the present invention provides a continuous process for preparing a titanium dioxide pigment comprising: a) preparing an aqueous slurry of unmilled titanium dioxide base particles at a temperature below about 75° C. and at a pH from about 2.0 to about 9.0; b) adding a silica compound to coat the base titanium dioxide particles by maintaining the pH of the slurry in at least three stages, in the first stage the pH is maintained at about 9.4, in the second stage the pH is maintained at about 9.1, and in the third stage the pH is maintained at about 5.0, while maintaining the temperature of the slurry below about 75° C. so as to form silica coated base titanium dioxide particles in the slurry; and c) adding an alumina compound while maintaining the pH of the slurry in the range of from about 5.5 to about 7.5 and the temperature of the slurry below about 75° C. so as to form the titanium dioxide pigment.
In one exemplary embodiment, the present invention provides a continu

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