Titanium dioxide (“TiO2”) has many known uses in industry. For example, TiO2 is used as a pigment for many paints. It is also used in other applications such as paper making, plastics applications and coating compositions.
Methods for producing TiO2 are also known in the art. TiO2 is generally produced by hydrolyzing an aqueous solution of titanium sulfate and calcining the hydrolyzate at about 750–1000° C., or oxidizing titanium tetrachloride (“TiCl4”) at elevated temperatures followed by cooling at temperatures below about 600° C. The TiO2 thus prepared is dry or wet ground and then dry milled. One method of dry milling is accomplished by utilizing a micronizer (also sometimes referred to as a fluid energy mill). Utilizing a micronizer typically involves introducing dry TiO2 pigment and a fluid, typically steam, into the micronizer. The output of the micronizer typically contains from about 20 percent of TiO2 to about 50 percent of TiO2, with substantially all of the remaining output composition comprising steam.
In most pigment manufacturing operations the TiO2 in the output from the micronizer is separated from the steam, cooled and packaged as a dry particulate. This finished TiO2 pigment can then later be formed into an aqueous slurry for the production of paints, coatings, and the like. In one process for preparing such aqueous slurries of TiO2, the finished TiO2 pigment is added to a water-dispersant package in a batch mixer having a high shear agitator. The slurry is then stirred until the degree of dispersion required is achieved in the slurry. Typically the dried TiO2 pigment is shipped to a customer and the slurry preparation is performed by the customer. However, it is becoming more and more common for the producer of the TiO2 pigment to produce and sell the pigment in a slurry form, and higher solids (i.e., more concentrated) slurries containing from 60 to 80 percent solids and higher, more particularly comprising at least about 60 weight percent of titanium dioxide, preferably at least about 65 and more preferably at least about 76 weight percent of titanium dioxide, are in increasing favor.
To separate the TiO2 in the micronizer output from the steam in the micronizer output, the output of the micronizer is typically introduced into a cyclonic gas-solids separator (or cyclone), wherein solid TiO2 product falls to the bottom of the cyclone and is removed in the underflow while the steam typically exits from the top in the overflow. The solid TiO2 product is then sent to a silo for packaging and shipping or to be used to produce a slurry as described above. The steam that exits from the cyclone is sometimes referred to as micronizer tailings. These micronizer tailings can be processed to produce TiO2 slurries using known methods such as the one taught in U.S. Pat. No. 4,427,451 issued to Baloga.