Patent Application
20080022900
The process of the present invention concerns improved processes for making titanium dioxide pigments via the well-known chloride process, wherein a titanium-bearing ore and/or slag feed is chlorinated in the presence of a reductant such as a petroleum coke to produce titanium tetrachloride, unreacted feed and coke as well as undesired chlorination products (for example, waste metal chlorides from other metals present in the ore and/or slag feed) are separated from the desired titanium tetrachloride chlorination product, then the titanium tetrachloride is oxidized to provide a crude titanium dioxide product.
Conventionally the crude titanium dioxide product is incorporated with a dispersant into water to form a slurry, and the slurry is wet- or media-milled in the presence of a grinding medium such as, for example, zircon sand. Thereafter, inorganic and/or organic surface treatments are typically employed in various combinations with a filtering, washing and recovery of the titanium dioxide and with fluid energy milling/micronization of the titanium dioxide, to produce ultimately a finished titanium dioxide product which is adapted for a desired end use. Those skilled in the art will be well familiar with the materials, devices and process steps that can be conventionally employed for taking a media-milled, crude titanium dioxide product through to a finished titanium dioxide pigment, and as these subsequent steps have no bearing on the present invention, further description will not be undertaken herein of the finishing of the media-milled, crude titanium dioxide materials produced by the present invention.
The present invention is concerned more particularly with enabling the media milling of high solids slurries with reduced viscosities, or of higher solids slurries with a given characteristic viscosity, according to the first and second aspects mentioned above, respectively.
As has been mentioned, these objectives are obtainable by means of the discovery that a 60 percent solids slurry made from crude chloride-process titanium dioxide, a dispersant and water can be made to have a lower viscosity for being milled in a sand mill, for example, by making up a slurry that contains in excess of 60 percent solids, then adding in the requisite amount of dilution water with further mixing to achieve the 60 percent solids content. In like fashion, a slurry can be made by the same dilution method which has more than 60 percent solids, but which has the same viscosity for media-milling as only a 60 percent solids slurry that is made up directly from the crude chloride-process titanium dioxide, a dispersant and water.
Preferably the high solids slurries made by the present invention comprise at least 50 percent by weight of crude pigment, and more preferably at least about 60 percent by weight of the crude pigment solids. Preferably the initial, higher solids slurry prepared according to the present invention in its first aspect is at least about 2 percent above the targeted final solids content of the slurry for media milling, and is diluted accordingly to the targeted final solids content. Thus, for example, where the slurry to be media-milled is desirably 60 percent by weight of crude titanium dioxide, preferably the initial slurry is prepared to contain at least about 62 percent of the crude titanium dioxide material.
Crude titanium dioxide pigment obtained from the vapor phase oxidation of titanium tetrachloride and containing 0.8% alumina in its crystalline lattice was dispersed in water in the presence of 0.18% by weight (based on pigment) of sodium hexametaphosphate (SHMP) dispersant, along with a sufficient amount of sodium hydroxide to adjust the pH of the dispersion to 9.5 or greater, to achieve an aqueous dispersion with a solids content of 61% by weight. Using a Brookfield Viscometer (Model DV-1, Spindle #5, 100 rpm), the viscosity of the crude pigment slurry made with 61% solids was found to be 906 cP at 21° C.
A slurry was then prepared by the same method to a 63% solids content, and then diluted with mixing with additional water to the same 61% solids content. This slurry had a measured viscosity of only 284 cP at 21° C.
Similarly, other crude pigment slurries of various solids contents were prepared directly and then by overconcentration and dilution according to the present invention, and the viscosities of each such slurry were measured for comparison. The results are presented in Table 1. Samples made by the direct method were stirred for several hours to see if any change was observed in the viscosity, but no change was observed.
The results in Table 1 demonstrate that a very significant reduction in the viscosity of an aqueous, crude chloride process titanium dioxide pigment slurry can be achieved readily and simply according to the present invention in a first aspect before media milling, without reducing the solids content and so reducing throughput. The results in Table 1 further demonstrate that for an equivalent viscosity going into media milling, according to the second aspect of the present invention the solids content of the crude pigment slurry can be desirably increased to improve throughput and productivity, for example, from a solids content of about 55.4% (shown as having a viscosity of 280 centipoise, for a slurry prepared by the conventional direct method) to a solids content of about 61% (measured viscosity of 284 centipoise for a slurry prepared by the method of the present invention).
