Composite pigments
Composite pigments are provided which comprise a mineral pigment (such as kaolin clay, titanium dioxide, talc, mica or a mixture of two or more of these mineral pigments) and calcium carbonate precipitated in-situ on the surfaces of the particles of the mineral pigment.
1. A process for the manufacture of a high brightness composite pigment which comprises a mineral pigment and calcium carbonate, wherein the process comprises the steps of:
(a) obtaining a slurry of particles of the mineral pigment;
(b) adding slaked lime to the slurry to form a mixture;
(c) passing carbon dioxide gas through the mixture to precipitate calcium carbonate on surfaces of the mineral pigment particles;
(d) stopping passage of the carbon dioxide gas when the mixture has a pH of about 7.0; and
(e) repeating steps (b)-(d) a plurality of times to produce the composite pigment,
wherein the composite pigment has a reduced amount of particles of less than 0.2 microns compared to the starting mineral pigment and an equivalent or increased amount of particles of 1-2 microns compared to the starting mineral pigment.
2. A process as defined by claim 1 wherein the mineral pigment is kaolin clay, titanium dioxide, talc, mica or a mixture of two or more of these mineral pigments.
3. A process as defined by claim 1 wherein the mineral pigment is kaolin clay.
4. A process as defined by claim 3 wherein the kaolin clay is hydrous kaolin clay.
5. A process as defined by claim 3 wherein the kaolin clay is delaminated kaolin clay.
6. A process as defined by claim 3 wherein the kaolin clay is calcined kaolin clay.
7. A process as defined by claim 3 wherein the kaolin clay is a blend of hydrous kaolin clay and calcined kaolin clay.
8. A process as defined by claim 1 wherein the amount of calcium carbonate in the composite pigment is from about 5.0 to about 90.0 weight percent based on the weight of the dry composite pigment.
9. A process as defined by claim 1 wherein the amount of calcium carbonate in the composite pigment is from about 10.0 to about 60.0 weight percent based on the weight of the dry composite pigment.
10. A process as defined by claim 1 wherein the amount of calcium carbonate in the composite pigment is from about 20.0 to about 40.0 weight percent based on the weight of the dry composite pigment.
11. A process as defined by claim 1 wherein the calcium carbonate is uniformly precipitated on the surfaces of the mineral pigment.
12. A process as defined by claim 1 wherein the composite pigment additionally comprises titanium dioxide, talc, mica or a mixture of these materials.