Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-02-14
2002-05-14
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
Reexamination Certificate
active
06387347
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes in multi-stage gas phase oxidation reactors for controlling particle sizes and particle size distribution of titanium dioxide.
BACKGROUND
Titanium dioxide, TiO
2
, is well known as the premier pigment for a variety of applications, including paints, coatings, plastics and other such products. The optical properties of TiO
2
pigments, in particular light scattering characteristics, are strongly dependent on pigment particle size and particle size distribution. Optimum pigmentary performance (opacifying and thus aesthetic properties of the pigment) is typically achieved when the mean TiO
2
particle size is between about 0.2 to about 0.3 &mgr;m and the particle size distribution is typically a log normal or geometric distribution having a standard deviation of less than about 1.50.
Two additional pigment-related optical properties are undertone and gloss. These properties are also functions of particle size and distribution. For example, gloss, a particularly important property of certain paint systems using TiO
2
pigments, is adversely affected by particulates larger than about 0.5 &mgr;m. Additionally, the reflective, or undertone values, of small particles impart a desirable bluish tone in paint systems.
Significant effort has gone into learning how pigments are formed and how to control their manufacture. The prior art is replete with methods purported to solve problems associated with particle size control while maintaining acceptable production rates.
A common process for manufacturing useful rutile titanium dioxide is by the so-called “Chloride Process” which involves a reaction between titanium tetrachloride (TiCl
4
) vapor and an oxidizing gas, such as air, oxygen or mixtures of such oxidants, to produce particles of TiO
2
. For example, Equation (I) describes the overall reaction scheme:
TiCl
4
+O
2
→TiO
2
+2Cl
2
(I)
While not being bound to any one particular theory, in the Chloride Process, TiO
2
particles are believed to form by a sequence of three basic steps:
(1) formation of TiO
2
nuclei;
(2) nuclei growth to form crystals or single, primary particles; and
(3) Single or primary particle agglomeration to form larger clusters of particles wherein the primary particles are held together through various degrees of sintering.
It is well known that the oxidation reaction (I) has a high activation energy. Thus, a significant amount of energy is required to preheat the reactants to initiate the reaction converting TiCl
4
and an oxidant into pigmentary TiO
2
and to obtain stable reactant conversions. Further, since reaction (I) is also highly exothermic, there is a large increase in the temperature of the efflux stream containing TiO
2
from the reactor. Therefore, when substantially all of an initial fraction of a TiCl
4
/oxidant charge reacts, there is a large heat release causing a significant rise in reactor/reaction mass temperature which increases the rate of the reaction and causes the TiO
2
particle growth rate in the reaction mixture to increase. Further, in processes such as the Chloride Process where high throughputs of reactants at high pressures are used, the ability to satisfactorily cool reaction products is reduced or minimized by the temperature increase. Thus, the ability to limit and control the growth of the TiO
2
particles to the optimum size range of from about 0.2 to 0.3 &mgr;m, while maintaining an acceptable particle size distribution, is lost as higher temperatures result from higher throughputs.
The high temperature conversion of reactants into products tends to lead to large agglomerates of sintered primary TiO
2
particles, which are generally detrimental in pigmentary applications. Strongly agglomerated particles, where the agglomerate sizes are greater than about 0.5 &mgr;m, typically result when high temperatures and high throughputs are encountered. In order to obtain particle sizes and particle size distributions useful as commercial pigment, energy intensive milling is required.
One variation of producing titanium dioxide via the Chloride Process relies on introducing reactants, titanium tetrachloride and oxidants, through multiple inlets into a reactor. These types of reactors are generally referred to as “multi-stage oxidation reactors” and they are typically designed to allow the heat of reaction generated in one reactor stage to be used to heat the reactants in a subsequent stage thereby improving the economy of operation. Patents describing variations on the multi-stage process of producing pigmentary rutile TiO
2
include: U.S. Pat. No. 3,463,610; U.S. Pat. Nos. 4,803,056; 5,599,519; U.S. Pat. No. 5,840,112; and GB Patent 2,037,266B.
U.S. Pat. No. 3,463,610 discloses a process for the production of particulate TiO
2
facilitated by the introduction of particulate matter to seed the formation of pigments. The process discloses introducing reactants into the reaction zone through at least two inlets that are spaced long a reaction zone. It is specifically desirable that substantially all of he reactants (or reactant) introduced into the gas stream in the reaction one react completely before that portion of the gas stream reaches the next inlet into the reaction zone where another introduction of reactant(s) is made. Means or steps for ensuring substantially complete reaction of the reactants in each reactor stage are disclosed.
U.S. Pat. No. 4,803,056 describes a multi-stage reactor for increasing production capacity and utilizing the heat of reaction in a multi-stage process for producing TiO
2
. The patent compares the output of a single stage reactor to that of a similar sized two-stage reactor. The multi-stage reactor relies on splitting the total flow of TiCl
4
and subsequently completely reacting it with excess oxygen. The excess heat in the first reactor stage is then used to heat the remainder of the TiCl
4
added in the final reactor stage.
U.S. Pat. No. 5,599,519 describes a multi-stage process for producing titanium dioxide which avoids the need for auxiliary heating of the reactants using known means such as hydrocarbon combustion or electric discharge. The patent discloses that complete reaction of titanium tetrachloride in each stage should be achieved prior to adding any additional TiCl
4
in a subsequent reactor stage.
An objective of each of these patents is to react substantially all of the TiCl
4
added in each stage of the multi-stage reactor prior to passing the reacted mass into a subsequent reactor stage where any additional reactants may be added.
U.S. Pat. No. 5,840,112 additionally discloses a multi-stage process for producing TiO
2
where oxygen is separately introduced in significantly less than stoichiometric quantities into each stage of the reactor and at differing temperatures to control the reaction. This patent also describes secondary additions of TiCl
4
, but does not disclose limited lo conversion of TiCl
4
in the presence of excess oxygen, nor the control of TiCl
4
conversion within the reactor in order to affect the particle size distribution of titania produced.
Thus, based on the prior art, there still exists a need for improved methods of producing titania pigments which do not yield excessive quantities of undesirably sintered primary particles which typically result from a substantially complete conversion of reactants in a particular stage of a multi-stage reactor.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a process for producing TiO
2
having fewer, smaller, or more easily milled titania agglomerates, while potentially increasing the production rate of pigment by controlling the extent or degree of conversion of titanium tetrachloride into titanium dioxide within a first or intermediate reactor stage.
A second objective is to provide a stable yet flexible process for producing TiO
2
in a multi-stage gas phase reactor wherein there are multivariate controls over the reaction profile within the reactor. A third objective of the invention is to provide
Akhtar Kamal
Beach Anthony J.
Deberry James Clark
Pomponi Mark Douglas
Robinson Michael
Bos Steven
Kalow David A.
Kalow & Springut LLP
Millennium Inorganic Chemicals, Inc.
Schmidt William D.
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