Note: Descriptions are shown in the official language in which they were submitted.
CA 02213895 1997-08-26
C~-25_7 -1-
TITLE
GRANULAR SCRUBS FOR USE IN MANUFACTURING
TITANIUM DIOXIDE PIGMENT
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an improved
process for making titanium dioxide pigment, wherein
granular scrubs comprising a water-soluble salt are used for
cooling a hot gaseous suspension of titanium dioxide
particulate in a cooling conduit.
Description of the Related Art
In producing pigmentary titanium dioxide (TiO2), a
titanium tetrahalide such as titanium tetrachloride (TiCl4)
in the vapor phase is reacted with an oxygen-containing gas
in a reactor at a temperature in the range of about 900~ to
1600~C to produce a hot gaseous suspension of TiO2 solid
particulate and free chlorine. This hot gaseous suspension
must be quickly cooled below 600~C within about 1-60 seconds
following discharge of the suspension from the reactor.
This cooling is accomplished in a conduit, e.g., a flue,
which is externally cooled with flowing water so that
undesired TiO2 particle size growth is prevented and
particle agglomeration is minimized. Particle size and
particle agglomeration are important TiO2 pigment
properties.
The particle size of the TiO2 pigment is measured
in terms of carbon black undertone (CBU). Pigments
containing smaller sized particles have a relatively high
CBU, and finished products (e.g., paints, plastics, etc.)
containing such pigments tend to have a bluish tint.
Pigments with larger sized particles have a relatively low
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CBU and finished prod~cts c~nt2 .~ing such pigments tend to
have a more yellowish tlnt. The particle agglomeration of
the pigment is typically measured in terms of its particle
size distribution (coarse tail). Plgments, wherein a low
weight percentage of the particles (e.g., less than 30%)
have a particle diameter size greater than 0.6 microns, tend
to have low particle agglomeration and finished products
made with such pigments tend to have high gloss. Pigments,
wherein a high weight percentage of the particles have a
particle diameter size greater than 0.6 microns, tend to
have greater particle agglomeration and finished products
made with such pigments tend to have less gloss.
It is known that the production of titanium
dioxide pigment may be improved when the TiC14 and an
oxygen-containing gas are reacted in the presence of a
nucleant. For example, Allen et al., US Patent 5,201,949
discloses a method for making TiO2 pigment, wherein TiCl4
vapor is oxidized in the presence of water vapor and a
nucleant consisting essentially of a cesium substance. The
oxidation occurs at high pressure or at a short residence
time of the reactants in the mixing zone of the reactor
to produce TiO2 pigment having improved CBU and gloss
properties.
Lewis et al., US Patent 3,208,866 discloses a
method for producing TiO2 pigment by the oxidation of TiCl4
vapor in the presence of a metal ion nucleant selected from
the group consisting of sodium, potassium, lithium,
rubidium, cesium, calcium, barium, strontium and cerium.
The method provides TiO2 pigment having improved particle
size uniformity, color, and in-process bulk density.
However, in the manufacturing methods disclosed in
the foregoing patents, the TiO2 particles have a strong
tendency to deposit on the inner walls of the cooling
conduit. The cooled TiO2 particles tend to form adherent
layers on the inner walls and can cause plugging of the
conduit. Further, the TiO2 deposits are poor heat
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S conductors and the internal surfaces of the cooling conduit
can become insulated which inhibits the heat-exchange
properties of the conduit. In the past, some manufacturing
methods which involve oxidizing TiC14 and an oxygen-
containing gas in the presence of a nucleant have used NaC1
granular scrubs in order to remove TiO2 particulate deposits
from the internal surfaces of the cooling conduit. Although
the NaCl scrubs have been somewhat effective in removing the
TiO2 deposits and producing good quality pigment, there is a
need for an improved process.
IS It has now been found that in methods for making
TiO2 pigment, wherein the TiC14 and an oxygen-containing gas
are reacted in the presence of a nucleant, the method may be
improved by adding granular scouring particles (scrubs)
comprising specific water soluble salts (KCl, CsCl, or
mixtures thereof) to the conduit which cools the hot gaseous
suspension of TiO2 particulate. This process produces TiO2
pigment having improved CBU levels and helps to minimize
particle agglomeration. The process can be run at a high
production rate to produce TiO2 pigment having high CBU
levels.
It is also recognized that the art includes
methods for making TiO2 pigment which do not specify
oxidizing TiC14 in the presence of a nucleant but which
describe using a variety of granular scrubs for removing
TiO2 deposits from the internal surfaces of cooling
conduits. For example, Rick, US Patent 2,721,626 discloses
adding relatively dense, hard abrasive particles into the
hot suspension of TiO2 particulate. The granular scrubs
mentioned are sand, sintered particles of titanium dioxide,
mullite or refractory alumina particles. The particle size
of the scrubs is described as being in the range of 100 mesh
to 1/4 inch.
Rahn et al., US Patent 3,475,258 discloses the use
of solid, inorganic water-soluble salts to remove metal
oxide deposits, such as TiO2 particle scale, from the
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internal surface~s of indirect heat exchange apparatus. Such
inorganic salts are removed from the product stream with
other water-soluble impurities during aqueous treatment of
the metal oxide particles. The particle size of the salts
is described as being in the range of 200 mesh to 1/4 inch.
The metal halides of alkali metals, alkaline earth metals,
aluminum, zirconium, and mixtures thereof are described as
being suitable for use as the salts in this process. The
halides, especially chlorides of sodium, potassium and
calcium are described as being economically preferred.
Nerlinger, US Patent 3,511,308 discloses adding a
particulate, anhydrous, water-soluble salt having a melting
point above about 700~C and a hardness on the Mohr scale not
greater than about 5 to the hot gaseous suspension of TiO2
solids. After the suspension has been cooled, the salt is
separated from the TiO2 solids by dissolving the salt and
recovering the solids. The particle slze of the salts is
described as being in the range from 100 mesh to 4 mesh.
Sodium chloride is preferred due to its availability in pure
form.
The present invention provides an improved
process for making TiO2 pigment, wherein the TiCl4 and an
oxygen-containing gas are reacted in the presence of a
nucleant. The improvement involves adding scouring
particles comprising specific water soluble salts (KCl,
CsCl, or mixtures thereof) to the cooling conduit.
SUk~RY OF THE INVENTION
The present invention relates to an improved
process for producing titanium dioxide pigment, comprising
the steps of:
a) mixing a titanium tetrahalide and aluminum
halide and oxidizing the mixture in the vapor
phase in the presence of a nucleant to form a
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gaseous suspension of titanium dioxide
particulate;
b) passing the suspension of titanium dioxide
particulate to a cooling conduit; the
improvement comprising introducing into the
cooling conduit scouring particles comprising
a water soluble salt selected from the group
consisting of KCl, CsCl, and mixtures thereof.
Preferably, the titanium tetrahalide is titanium
tetrachloride and the aluminum halide is aluminum chloride.
The nucleant is preferably a compound comprising an element
selected from the group consisting of sodium, potassium,
lithium, rubidium, cesium, calcium, barium, strontium, and
cerium.
The oxidation of the titanium
tetrachloride/aluminum chloride mixture may occur by mixing
and reacting the mixture and an oxygen containing gas in the
presence of water vapor in a reactor having a reaction zone
at a temperature of at least 800~C and at a pressure of at
least 10 pounds per square inch gage. The oxidation of the
titanium tetrachloride/aluminum chloride mixture may also
occur by mixing and reacting the mixture and an oxygen
containing gas in the presence of water vapor in a reactor
having a reaction zone and mixing zone, wherein the
temperature in the reaction zone is at least 800~C and the
residence time of the reactants in the mixing zone is about
1 to 25 milliseconds.
Preferably, the scouring particles comprise KCl,
and the nucleant is KCl or CsCl. Alternatively, CsCl may be
used as the scouring particles and the nucleant may be KCl
or CsCl. Preferably, the diameter size of the KCl and/or
CsCl scouring particles is in the range of about 60 mesh to
about 0.5 inches.
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--6--
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides an improved process
for producing titanium dioxide (TiO2) pigments, comprising
the following steps:
1) mixing a titanium tetrahalide and aluminum
halide to form a mixture;
2) oxidizing the mixture in the vapor phase in
the presence of a nucleant to form a gaseous
suspension containing titanium dioxide
particulate;
3) passing the suspension of TiO2 to a cooling
conduit; and
4) introducing scouring particles comprising a
water-soluble salt selected from the group
consisting of KCl, CsCl, and mixtures thereof
into the cooling conduit.
The production of TiO2 pigment by vapor phase
oxidation of a tetrahalide, particularly TiC14, in the
presence of a nucleant is known and disclosed in Lewis et
25 al., US Patent 3,208,866 and Allen et al., US Patent
5,201,949, the disclosures of which are incorporated herein
by reference. The present invention relates specifically to
an improvement in the aforementioned processes.
In the production of TiO2 pigment by the vapor
phase oxidation of titanium tetrahalide, various titanium
tetrahalides such as titanium tetrachloride, titanium
tetrabromide, and/or titanium tetraiodide may be used, but
it is preferable to use TiC14. First, TiC14 is evaporated
and preheated to temperatures of from about 300 to about
650~C and introduced into a reaction zone of a reaction
vessel. Aluminum halides such as AlC13, AlBr3 and/or AlI3
in amounts sufficient to provide about 0.5 to about 10~
A12O3, preferably about 0.5 to about 5%, and more preferably
about 0.5 to about 2% by weight based on total solids formed
in the oxidation reaction are thoroughly mixed with the
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S TiCl4 prior to its introduction into the reaction zone of
the reaction vessel. Preferably, AlCl3 is used in the
process of this invention. However, it is also recognized
that other co-oxidants and rutile promoters may be added at
this point or further downstream in the process.
The oxygen containing gas is preheated to at least
1200~C and is continuously introduced into the reaction zone
through a separate inlet from an inlet for the TiCl4 feed
stream. By "reaction zone", it is meant the length of the
reactor in which substantial reaction of the reactants takes
place. The reaction ~f ~2 and TiC14 in the vapor phase is
extremely fast and is followed by a brief period of particle
growth. The oxygen containing gas which is introduced into
the reaction zone contains a nucleant. By "nucleant", it is
meant any substance which can reduce the particle size of
the pigment such as metals, oxides, salts, or other
compounds of sodium, potassium, lithium, rubidium, cesium,
calcium, barium, strontium, or mixtures thereof. The salts,
CsCl and KCl, are preferred for use in this invention.
The pressure for carrying out the oxidation
reaction is preferably at least 10 pounds per square inch
gage (psig). More preferably, the pressure will be at least
20 psig. The upper pressure limit can be the practical
upper limits of the process, e.g., 200 psig. The residence
time of the reactants in the mixing zone of the reactor
should be at least 1 millisecond, preferably at least 3
milliseconds. The maximum residence time should be about 25
milliseconds. Typically, the residence time is in the range
of about 1-25 milliseconds. By "mixing zone", it is meant
the length of the reactor in which substantial mixing of the
reactants takes place. The reaction temperature should be
at least 800~C and preferably in the range of about 800~ to
1800~C. Preferably, the reaction occurs in the presence of
water vapor.
The hot gaseous suspension of TiO2 particulate is
then rapidly cooled in order to prevent undesirable particle
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growth. In accordance with this invention, cooling of the
hot gaseous suspension may be performed by methods known in
the art. These methods typically involve passing the hot
gaseous suspension through a cooling conduit having
relati~ely cool walls in comparison to the gaseous
suspension. The walls of the conduit are typically cooled
by passing cool fluid externally over the walls. For
example, the conduit may be immersed in cool water. Various
forms of conduits or flues which are preferably cooled by
water externally, may be used in the process of this
invention. Examples include, but are not limited to,
conventional round pipes and conduits which are described in
greater detail in US Patents 2,721,626; 3,511,308;
4,462,979; 4,569,387; and 4,937,064 (finned flue). The
benefits provided by the process of this invention may be
especially apparent as the diameter of the conduit is
increased. As the hot TiO2 particles come in contact with
the relatively cooler surfaces of the inner walls, the
particles deposit on the walls and cool to form adherent
layers. These deposits and scale reduce the cooling rate of
the reaction mass thereby affecting the quality of the
pigment formed.
The present invention provides an improved
process, wherein granular scouring particles comprising
certain water-soluble salts are introduced into conduit, to
remove the TiO2 deposits and substantially improve the
quality of pigment formed.
The granular scouring particles employed in this
improved process comprise a water-soluble salt selected from
the group consisting of KCl, CsCl, and mixtures thereof.
Surprisingly, it has been found that when KCl or CsCl salts
are used, the resulting TiO2 pigment has significantly
higher CBU values than TiO2 pigment produced by a process
which uses NaCl as the scouring particles.
In this invention, the scouring particles
preferably have a diameter (size distribution) in the range
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from about 60 mesh (0.0098 inches or 0.250 mm) to about 0.5
inches (12.7 ~m) Preferably at least 80~ of the particles
will be of a size 10 mesh (0.0787 inches or 2.00 mm) or
larger. More preferably at least 90% of the particles will
be of a size 10 mesh or larger. Particle size distribution
of the scouring particles is very important since use of
proper size is essential in providing scrubbing action. If
the particle size is too small, this will result in the
scouring particles melting and not providing the scrubbing
action. If the particle size is too large, this may cause
feeding problems and insufficient surface area to provide
the scrubbing required.
The amount of scouring particles used is variable
and will depend upon the particular needs. Typically, the
addition of an amount of scouring particles ranging from
about 0.5 to 20 wt.% scouring particles, preferably from
about 3 to 10 wt.%, based on total TiO2 suspended solids
will be found adequate to effect the desired removal of
accumulated pigment deposits and will allow a relatively
high, uniform rate of heat removal from the product stream.
It will be appreciated by those skilled in the art that
enough scouring particles must be added to bring the
reaction mass at the end of the conduit to a temperature
compatible with downstream process equipment such as
cyclones, filters, screw conveyers, etc. Such temperatures
are in the range of about 100~ to about 600~C.
The scouring particles can be added to the conduit
by any suitable means. For example, the scouring particles
can be added intermittently or continuously by gravity from
a hopper (or bin) through a solids metering valve to the
flue. Continuous feeding to the TiO2 suspension under
treatment is preferred. The scouring particles can be added
at any convenient point in the system but are most
conveniently added at the front end of the conduit as the
product stream discharges from the reactor. Further, the
scouring particles can be added at a multiple of addition
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points and especially at those points adjacent to which a
relatively severe build-up occurs due to the configuration
of the ap2aratus, such as at return or other forms of bends
employed in the system.
The water-soluble scouring particles can be
removed from the TiO2 pigment during conventional subsequent
wet-treatment steps.
The improved process of this invention provides
many advantages. In general, the process is effective in
cooling hot suspensions of particulate TiO2 in corrosive,
chlorine-containing gases by a rapid and efficient heat-
exchange technique. This process minimizes the deleterious
effect which build-up of the cooled solids on the surfaces
would otherwise produce while further improving pigment
properties. The process of this invention results in
improved TiO2 pigment properties by control of the primary
particle size and level of particle agglomeration. The KCl
and/or CsCl scouring particles disperse more efficiently and
provide a means to effectively cool down the suspension of
particulate at the front end of the conduit where the
pigmentary properties are primarily set. These scouring
particles are also effective in cooling down the particulate
in the remainder of the conduit where the pigment is brought
down to lower temperatures for handling.
If it is desirable to improve the quality of the
pigment being produced, then the improved process may be run
at the given set of conditions (pressure, reaction
temperature, etc.), and the quality of the pigment, as
measured in terms of the CBU, will be improved. If it is
desirable to increase the production rate of the pigment,
then the process conditions (pressure, reaction temperature,
etc.) can be appropriately adjusted, and the improved
process may be run at these high production conditions
without a loss in quality of pigment.
While not wishing to be bound by any theory, it is
believed that when granular scrubs are added to cooling
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5 conduits containing hot gaseous suspensions of TiO2, they
effectively scour the internal surfaces of the conduit. It
is further believed that the specific process of this
invention is effective in producing TiO2 pigment having high
CBU levels due to the interaction of the nucleant and the
10 specific granular scrubs (KCl and/or CsCl).
The present invention is further illustrated by
the following examples, but these examples should not be
construed as limiting the scope of the invention.
15 Test Methods
Carbon Black Undertone
The carbon black undertone (CBU) of a TiO2 pigment
20 sample is measured according to the methods described in US
Patents 2,488,439 and 2,488,940, the disclosures of which
are hereby incorporated by reference, using a benchmark
value of lO rather than lO0 as used in the patents. The CBU
ls measured by mulling together a suitable liquid, such as
25 light colored oil, the TiO2 pigment sample, and carbon
black. The mixture is spread on a panel and the relative
blueness of the gray mixture is observed. Pigments
containing smaller sized particles have a relatively high
CBU and a bluer undertone. Pigments with larger sized
30 particles have a relatively low CBU and have a more
yellowish undertone.
EXAMPLES
35 Examples 1-2
Comparati~re Example A-B
Employing a vapor phase oxidation reactor as
described in US Patents 2,488,439; 2,488,440; 2,559,638;
2,833,627; 3,208,866; 3,505,091; and 5,201,949, a series of
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trials were run using KCl scouring particles of this
invention and NaCl scouring particles as comparison.
Process and additive conditions (rate, pressure,
temperatures, purges, and concentration of additives such as
AlC13 rutile promoter, KCl or CsCl nucleants, etc.) were
held constant throughout each trial and were as follows:
TiCl4 - 400~C
~2 - 10-15% excess at 1550~C
Reaction Temperature - 1550~C
lS Pressure - 50 psig
Production Rate of TiO2-Al2O3 - 4.5 tons per hour
Feed Rate of Scouring Particles - 5-10 wt.% based on TiO2
being produced.
The hot, gaseous suspension of TiO2 pigment formed
in the reactor was separated from the gas stream and quickly
cooled in a conduit to a temperature of about 180~ to 200~C.
The scouring particles were introduced into the gaseous
suspension at the front end of the conduit as the suspension
was discharged from the reactor. A substantial 100%
conversion of TiCl4 and AlCl3 to their respective oxides was
obtained to produce a pigment containing about 99% rutile
TiO2 and about 1% Al2O3. The recovered base TiO2 pigment
was then tested for carbon black undertone (CBU) with the
results reported below in Table 1.
The particle size distribution of the sodium
chloride scouring particles in Comparative Examples A and B
was such that at least 90% of the particles had a diameter
size greater than 8 mesh (2.36 mm or 0.0937 inches) and at
least 98% of the particles had a diameter size greater than
12 mesh (1.70 mm or 0.0661 inches). The particle size
distribution for the potassium chloride scouring particles
in Example 1 was such that about 88% of the particles had a
diameter size greater than 10 mesh (2.00 mm or 0.0787
inches). The particle size distribution for the potassium
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chloride scouring particles in Example 2 was such that about
72~ of the particles had a diameter size greater than 10
mesh.
TABLE 1
Metal Nucleant Scouring
Example (ppm) Particles CBU
Comparative A KCl (190) NaCl 14.2
1 KCl (190) KCl 15.7
Comparative B CsCl (200) NaCl 15.2
2 CsCl (200) KCl 17.3
As can be seen from Table 1, CBU improves when KCl
scouring particles are used versus NaCl scouring particles.
It is expected that using CsCl scouring particles or
mixtures of KCl and CsCl scouring particles would provide
similar improvements. The highest CBU values were obtained
with CsCl as the nucleant (200 ppm) and KCl as the scouring
particles.
