Patent Application
20080069764
A preferred embodiment of a process of the present invention for making pigmentary titanium dioxide involves oxidizing titanium tetrachloride in the gas phase in the presence of aluminum chloride gases, to produce a crude titanium dioxide product having alumina incorporated into its crystalline lattice (so-called “burned-in alumina”), as part of a chloride process for making a rutile titanium dioxide pigment such as commonly used in paper, in plastics and in coatings of various types. The chloride process for making rutile titanium dioxide pigments need not be described in any detail herein, as the general process is well-known and described in many references and as the details of the chlorination, oxidizing and finishing operations involved in such a process are not affected by the present invention. The incorporation of alumina in the oxidizing step is also well-known as described above, the contribution of the present invention being found in the manner in which the aluminum chloride gases are supplied for being oxidized with the titanium tetrachloride gases in the oxidizer. Namely, the aluminum chloride gases are supplied in the present invention by subliming aluminum chloride solids and then combining aluminum chloride gases from the sublimation step with titanium tetrachloride gases, the combination preferably occurring just prior to the oxidizer and more preferably occurring as close to the entry point of the titanium tetrachloride gases into the oxidizer as possible given equipment constraints, obstructions and the like, so that in turn the corrosive mixture of aluminum chloride and titanium tetrachloride gases is encountered as little as possible in the process equipment.
The aluminum chloride solids in this case will typically be purchased, rather than being generated through the carbo-chlorination of aluminum-bearing ores and the eventual condensation of aluminum chloride solids as practiced in the making of aluminum metal. Usually purchasing the aluminum chloride solids will prove to be more economical on the whole, however, it is by no means excluded that the solids might be generated and conceivably stored onsite to be used when needed.
In any event, the aluminum chloride solids are combined with one or more inert, thermally conductive solids in a vessel, and one or more inert gases are supplied to the vessel at flowrates at least sufficient with the flow of aluminum chloride sublimation gases to maintain the combined aluminum chloride solids and inert, thermally conductive solids in a fluidized condition. Heat is supplied through heating the vessel and/or through heating the inert gases for causing aluminum chloride solids in the fluid bed to sublime, and the heat transfer to the aluminum chloride solids for such purpose is aided and accomplished at least in part by means of the inert, preferably highly thermally conductive solids employed in the fluid bed with the aluminum chloride solids. To avoid the capital issues associated with the use of aluminum chloride generators such as described by Hartmann et al., in which all of substantially all of the titanium tetrachloride gases in question have been passed through the generators, preferably in our sublimer the inert gas flowrates will not greatly exceed the minimum required flowrate for keeping the inert and aluminum chloride solids fluidized—preferably being not more than 200 percent of that required with the flow of sublimed aluminum chloride gases to achieve fluidization of the combined hot, inert solids and aluminum chloride solids.
Suitable inert gases may be, for example, nitrogen or carbon dioxide—both of which, of course, are found in the gas products stream from the oxidizer in the normal course of operations. In contrast to the known aluminum chloride generators involving large flows of titanium tetrachloride gases, the inert gas flow requirement for the sublimer of our invention can be as little as 1 to 2 percent by volume of the total oxidizer flow; consequently, the inventive sublimer can advantageously—for both capital and space reasons—be sized much smaller for the same production of aluminum chloride gases, as compared to a conventional aluminum chloride generator. In addition, the corrosion issues associated with the mixing of the aluminum chloride sublimation gases and titanium tetrachloride gases can be minimized as the gases are combined just prior to the oxidizer.
The inert, thermally conductive solids are present to aid in heat transfer from heated vessel walls and/or from hot inert gases supplied to the vessel to the aluminum chloride solids to be sublimed. Preferably too, beyond being inert and thermally conductive in the application, the inert solids will be selected to be of a type of material that could be carried into the oxidizer and into the desired pigmentary titanium dioxide product without unduly complicating conventional downstream processes or compromising pigment quality—by way of nonlimiting example, a scour medium such as alumina or sand which is conventionally removed from the process or perhaps a pelletized or sintered titanium dioxide material that can remain with the product through finishing of the pigment—while at the same time preferably having a thermal conductivity at least about equal to that of silica sand (thermal conductivity of between 3 and 4 btu/hr-ft-deg F. at 600 degrees Fahrenheit). Suitable inert solids are described, for example, in U.S. Pat. No. 6,419,893 to Yuill et al., United States Patent Application Publication No. 2005/0249651 to Flynn et al., U.S. Pat. No. 5,544,817 to Brownbridge et al., U.S. Pat. No. 6,036,999 to Zhao et al, and United States Patent Application Publications Nos. 2004/0187392 and 2004/0239012 to Krause et al.
While the novel aluminum chloride sublimer described and claimed herein is especially useful for providing aluminum chloride gases to be combined with titanium tetrachloride gases for being subsequently oxidized in the oxidizer of a chloride TiO2 process, those skilled in the art will also appreciate that the sublimer can be used in other contexts wherein the sublimation of aluminum chloride solids generally has already been known and practiced (e.g., in aluminum manufacture per the U.S. Pat. No. 4,514,373 to Wyndham or Belgian Patent No. 633119 references mentioned above), but further in either a chloride or sulfate process (for producing pigmentary titanium dioxide) for providing an alumina post-treatment of a crude, chloride- or sulfate-process pigmentary titanium dioxide. Those skilled in the art may readily conceive of still other alterations to, variations of and equivalents to the specific embodiments of the invention described herein but which should properly be considered as within the scope of our inventive contributions. Accordingly, the scope of the present invention should be assessed as that of the appended claims and by equivalents thereto.
