text: The present invention relates to methods of controlling the particle size of titanium dioxide produced by oxidizing a titanium halide in the vapor phase.
  The production of rutile titanium dioxide by reacting a gaseous titanium halide such as titanium tetrachloride with oxygen (often referred to as the “chloride process”) is well known. Heated streams of gaseous titanium halide and oxygen (or an oxygen-containing gas) are combined at high flow rates in a reaction zone of a tubular vapor phase oxidation reactor. Aluminum chloride, typically generated on-site, is often added to the titanium halide stream to promote rutilization of the titanium dioxide. A high temperature oxidation reaction takes place whereby particulate solid titanium dioxide and gaseous reaction products are produced. The titanium dioxide and gaseous reaction products are cooled, and the titanium dioxide particles are recovered. The solid titanium dioxide is very useful as a pigment.
  The pressure at which the oxidation reaction is carried out can vary from atmospheric pressure to about 50 psig. It is often desirable to carry out the reaction at a relatively high pressure, i.e., at a pressure that is at least about 15 psig. For example, at a pressure above about 30 psig, the need to recompress recycle chlorine gas is eliminated.
  The mean particle size of the resulting titanium dioxide can be important, for example, when the titanium dioxide is to be used as a pigment. The particle size has a direct impact on the optical properties of the pigment. Certain plastics grade pigments require a relatively low particle size.
  Unfortunately, the particle size of the titanium dioxide can be difficult to control, particularly when the oxidation reaction is carried out at a relatively high pressure. As the reaction pressure increases, the mean particle size of the titanium dioxide tends to increase as well. The increased pressure increases the density of the particles in the vapor phase in the reactor, which leads to a higher number of particle collisions. The higher number of particle collisions results in coalescence and growth of the particles.
  It is known that the particle size of the titanium dioxide in a vapor phase oxidation reactor can be controlled by adding ionizing metals to the reactor. U.S. Pat. No. 3,208,866, assigned to E.I. du Pont de Nemours and Company, teaches that the particle size of the titanium dioxide can be controlled by introducing a metal ion nucleant to the reactor. The metal ion nucleant can be charged to the oxidation reactor either in its elemental state (as a vapor, liquid or solid) or in the form of various inorganic and organic compounds containing the metal. The nucleant is preferably introduced into the reactor by adding it to the oxygen or oxygen-containing gas stream being charged to the reactor. The nucleant prevents the particles from colliding and coalescing.
  In accordance with U.S. Pat. No. 5,204,083, assigned to Kerr-McGee Chemical Company LLC, a metal ion-containing compound is introduced into the reaction zone of the oxidation reactor in at least two separate and discrete increments. The first increment is introduced into the reaction zone at a point therein prior to the onset of the reaction between the titanium halide and the oxidizing gas within the reaction zone. A second increment is introduced into the reaction zone at a point therein subsequent to the reaction of at least about 20 weight percent of the titanium halide and the oxidizing gas. The addition of the metal ion-containing compound in separate and discrete increments allows for improved particle size control and avoids certain processing problems associated with the use of metal ion-containing compounds.
  U.S. Pat. No. 5,536,487, assigned to Kronos, Inc., describes a process for manufacturing titanium dioxide by the chloride process wherein the metal used to make aluminum trichloride and a particle control additive are introduced by way of two separately-controllable addition branches (a main branch and a subsidiary branch) into the aluminum chloride generator. The particle control additive, an alkali metal salt, is introduced by way of the subsidiary branch in a controlled addition provided by a “dilution” mixture of the salt in aluminum powder. Additional agents are included which improve the free-flowing properties of the mixture.
  While the approaches used heretofore for controlling the particle size of the titanium dioxide have been effective, they are limited in certain respects. For example, it can be labor intensive to mix the alkali metal salt, powdered aluminum, and additional agents in accordance with U.S. Pat. No. 5,536,487. Powdered aluminum is pyrophoric and thus hazardous, and the use of multiple mixing devices is cumbersome. Furthermore, the effectiveness of any ionizing agent can be diminished when the pressure at which the oxidization reaction is carried out is increased. It becomes difficult at high operating pressures to achieve titanium dioxide pigment particle sizes suitable for plastics applications. At such pressures, different mechanisms are needed in order to achieve effective particle size control.