Process for the manufacture of rutile titanium dioxide powders
This invention pertains to processes for producing ultra-fine
rutile titanium dioxide powders.
Titanium dioxide is capable of crystallizing into three different
forms, namely anatase, rutile and brookite. Methods of making
ultra-fine titanium dioxide powders include gas phase synthesis,
colloidal precipitation and mechanical grinding. Important
challenges to face are control of the crystal phase, the particle
size and distribution, the degree of agglomeration and
aggregation of the particles, and the degree of doping.
During gas phase synthesis, the particle size and distribution of
Ti02 can be controlled by rapid quench methods as explained in US-
5935293 and US-5851507, or by using low flame temperatures and
short residence times as shown in US-5698177 and by Akhtar et
al.. Dopants in Vapor-phase synthesis of titania, J. Am. Ceram.
Soc. 75[12], 34C8-16, 1992. EP-1514846 describes a method for
eliminating over-sized particles in the vapour phase synthesis of
metal oxide-containing particles, comprising reacting oxygen with
one of more vapour streams comprising a titanium halide, a
silicon halide, and a compound selected from the group consisting
of phosphorus, germanium, boron, tin, niobium, chromium, silver,
gold, palladium, aluminium, and mixtures thereof.
Other methods are related to control of the crystal structure of
TiO2, either anatase or rutile. The most obvious method of making
rutile TiO2 powder is by calcination of anatase at temperatures of
at least 900 °C. However, such high temperatures will cause an
undesired sacrifice in particle size and particle size
distribution because of sintering and aggregation of the
particles. Alternatively, rutile-promoting additives can be
added. In GB-1031647, US-2559638 and US-3214284, aluminium
trichloride is added to titanium tetrachloride as rutile

promoting additive, thereby achieving a rutile yield of more than
99 %. This same result is reported to occur when zirconium or
zinc compounds are added. US-5536487 describes the manufacture of
titanium dioxide by the chloride process, into which aluminium
trichloride and one or more particle size control compounds are
added. In EP-1138632, it is found that the photocatalytic
activity of titanium dioxides in optionally acidified aqueous
suspension can be either increased or reduced by doping with
oxides of (noble) metals or metalloids. Moreover, a change in the
amount of doping component leads to a change in the rate of
photocatalytic degradation. However, undoped rutile TiO2 powders
can not be produced by these methods.
Rutile TiO2 powders are used in UV blocking compositions, e.g. in
paints, plastics, coatings, pigments and sunscreens. Titanium
dioxide absorbs UV light efficiently, but it also tends to
catalyse the formation of super-oxide and hydroxyl radicals,
which may initiate unwanted oxidation reactions. Such photo-
oxidations may explain the ability of illuminated titanium
dioxide to degrade organic matter. As titanium dioxide, when
present in sunscreens, may enter human cells, the ability of
illuminated titanium dioxide to cause DNA damage has also been a
matter of investigations. EP-1079796 provides UV screening
compositions that address the above problem, by incorporating
manganese or chromium in a rutile titanium dioxide host lattice.
Unfortunately, neither manganese nor chromium are effective
rutile-promoting additives.
The present invention provides a method of producing ultra-fine
titanium dioxide powders. A rutile yield above 50 %, above 90, or
even above 99 % can be obtained. The reaction scheme allows
producing powders with or without doping elements. Typically,,
particles with an average primary particle size between 1 and 100
nm can be made by the process of this invention. Accordingly, a
new process for the production of rutile TiO2 powder is divulged,

comprising the steps of providing a hot gas stream and of
introducing therein, firstly:
- a titanium-bearing first reactant; and
- a carbon- and/or nitrogen-bearing second reactant;
the temperature of said gas stream being chosen so as to vaporize
said first and second reactants, the reactants being selected so
as to form, at the prevalent temperature, titanium carbide,
titanium nitride or a mixture thereof, as a nano-sized precursor;
and, thereafter:
- a volatile oxygen-bearing reactant selected so as to react with
the nano-sized precursor, converting it to nano-sized titanium
dioxide powder having a rutile content of at least 50 %.
This latter reaction is assumed to be heterogeneous, the carbide
and/or nitride particles from the first step being converted to
oxide by the volatile (i.e. gaseous at the prevalent temperature)
reactant in the second step, while remaining in the solid state.
It is believed that, due to the cubic crystal structure of the
carbide and/or nitride precursor, the formation of rutile is
strongly promoted compared to the formation of anatase during
subsequent oxidation.
The initial gas stream temperature is chosen so as to preferably
result, after introduction of the reactants, in a gas stream
above 1000 K. This is to ensure adequate reaction kinetics for
the formation of the carbide and/or nitride precursor. However,
if oxygen and carbon are present, originating from the gas stream
itself or from the first and second reactants, a Carbon/O2 ratio
of at least 0.5, and a temperature above 1800 K are preferred.
Otherwise, undesired anatase TiO2 may form irreversibly in this
step. By Carbon/O2 ratio is meant: the molar ratio of the total
amount of carbon to the total amount of oxygen (expressed as O2)
in all species present in the gas stream in the first process
step.

Once the precursor is formed, and if the gas temperature is
higher than about 2000 K, it is advisable to quench the gas
stream to below 2000 K. This is to avoid grain-growth of the
carbide/nitride particles, and also to avoid melting the TiO2 when
it is synthesised in the second process step. Such a particle
meltdown might indeed interfere with the rutile-promoting effect
of the precursor.
It is also advisable to quench the gas stream after the step of
introducing an oxygen-bearing reactant. A temperature down to 600
K is preferred, to avoid grain-growth of rutile in the second
step.
The hot gas stream may advantageously be generated by means of
either one of a gas burner, a hot-wall reactor, an RF plasma, and
a DC arc plasma.
Preferred titanium-bearing first reactants comprise either one or
more of titanium chloride, oxide, sulphate, and organo-metallic
compounds.
Preferred carbon- and/or nitrogen-bearing second reactants
comprise either one or more of carbon, carbonate, carbon
monoxide, carbon dioxide, hydro-carbons, nitrogen, amines, and
nitrous oxide.
Preferred oxygen-bearing reactants comprise air, oxygen, carbon
dioxide, and nitric oxide.
It can be advantageous to introduce both the first and the second
reactants simultaneously in the hot gas stream.
In a particularly advantageous embodiment, the titanium-bearing
first reactant and the carbon- and/or nitrogen-bearing second
reactant are actually a single compound that will vaporize and

form the needed nano-sized precursor upon heating. Such a single
compound is e.g. titanium isopropoxide.
The oxygen-bearing reactant could advantageously be air.
Another embodiment of the process concerns the synthesis of
metal-doped rutile. To this end, an additional metal-bearing
compound, which has to be volatile at the prevalent reaction
temperature of the first step, is introduced in the hot gas
stream, together with the firstly introduced reactants. This
metal-bearing compound preferably contains manganese, more
preferably as an organic compound. In this case, the amount of
the manganese-bearing compound is preferably adjusted so as to
obtain a doping level of between 0.01 and 30 wt% in the rutile.
The invention is further illustrated by the following examples.
Table 1 summarizes the results obtained when using an inductively
coupled plasma torch as a hot gas generator.
Example 1
A 25 kW radio frequency (RF) inductively coupled plasma (ICP) is
used, using an argon/nitrogen plasma with 12 NmVh argon and 3
Nm3/h nitrogen gas. Titanium isopropoxide is injected in the
plasma at a rate of 1 1/h, resulting in a prevalent (i.e. in the
reaction zone) temperature above 2000 K in this first process
step, wherein the titanium isopropoxide is totally vaporized,
whereupon it readily nucleates, forming a cubic TiC nano-powder.
A nitrogen flow of 5 NmVh is used as quench gas immediately
downstream of the reaction zone. This lowers the temperature of
the gas to below 2000 K. Further downstream, 10 Nm3/h of air is
blown into the gas stream, thereby triggering the second process
step whereby the TiC powder is oxidized to nano-sized rutile Ti02.
Any residual carbon is also oxidised in this step. After
filtering, nano-sized TiO2 powder is obtained, having a rutile
content of 97 + 2 % and a specific surface area of 25 ± 2 m2/g.

This corresponds with a mean primary particle size of about 60
nm.
Example 2
The apparatus according to Example 1 is operated in similar
conditions. However, along with titanium isopropoxide/ manganese
iso-octoate is injected in the plasma, at a total injection rate
of 1 1/h. After filtering, nano-sized manganese doped TiO2 powder
is obtained, having a rutile content of 97 ± 2 %, a specific
surface area of 25 + 2 m2/g, and a Mn content of 0.67 + 0.02 %.
Example 3
A 250 kW direct current (DC) plasma torch is used, with nitrogen
as plasma gas. The gasses exit the plasma at a rate of 150 Nm3Vh.
A mixture of titanium isopropoxide and manganese iso-octoate is
injected downstream of the plasma, at a rate of 25 kg/h. In this
step, the reactants are vaporised, resulting in a prevalent gas
temperature of 2200 K, and nucleate as a Mn-doped TiC powder.
Subsequently, a nitrogen gas flow of 160 NrnVh is applied in
order to reduce the gas temperature. Further downstream, air is
blown at a flow rate of 6000 NrnVh, thereby oxidizing the TiC
into nano-sized rutile TiO2. After filtering, doped nano-powder is
obtained, with a rutile content of 97 ± 2 %, a Mn content of 0.67
+ 0.02 %, and a specific surface area of 18 ± 2 m2/g, which
corresponds with a mean primary particle size of about 80 nm.
Example 4
This example is similar to Example 3. However, the mixture
titanium isopropoxide and manganese iso-octoate reactants is
injected at a much higher rate, namely at 100 kg/h. this results
in a prevalent temperature of 1100 K only. Subsequently, a
nitrogen gas flow of 200 NrnVh is applied in order to reduce the
gas temperature. Further downstream, air is blown at a flow rate
of 15000 Nm3/h. After filtering, doped nano-powder is obtained,
with a rutile content of 99 ± 1 %, a Mn content of 0.67 ± 0.02 %,

and a specific surface area of 25 ± 2 m2/g. An FEG-SEM micrograph
of the particles is shown in Fig. 1, illustrating doped nano-
sized TiO2 powder with an average primary particle size below 100
nm. This example shows that a relatively low reaction temperature
in the first process step, down to about 1000 K, does not impair
the rutile yield.
Example 5
A 50 kW RF ICP is used, using a plasma with 12 Nm3/h argon and 3
Nm3/h nitrogen gas. Titanium isopropoxide is injected in the
plasma at rate of 500 ml/h, resulting in a prevalent temperature
above 3000 K. As described above, titanium isopropoxide is
totally vaporized, whereupon Tie is formed. Subsequently, a
nitrogen flow of 5 Nm3/h is used as quench gas. In this case, the
obtained powder is not further oxidised. After filtering, nano-
sized TiC powder is obtained, having a specific surface area of
40 ± 4 m2/g. This result appears to validate the mechanism
described in this invention, namely the formation of an
intermediate carbide (in this example).
Example 6
This example is similar to Example 1. However, titanium
isopropoxide is injected in the plasma at a rate of 500 ml/h
only. After filtering, nano-sized TiO2 powder is obtained, having
a rutile content of 97 ± 2 % and a specific surface area of 22 ±
2 m2/g. No critical dependency of the reactant flow rate is thus
observed.
Examples 7-9 and comparative Example 10
These examples are similar to Example 1. However, variable oxygen
flow rates of respectively 0.2, 0.5, 1 and 3 Nm3/h are injected
in the plasma, together with argon or argon/nitrogen. All
obtained powders reveal specific surface in the range of 20 to 25
m2/g. At Carbon/O2 ratios of respectively 2.2, 0.9, 0.45 and 0.15,
the rutile yield is, respectively, 95, 90, 50 and 35 %. At

Carbon/O2 ratios above 0.5, adequate rutile contents, i.e. of more
than 50 %, are obtained. However, in the latter case (comparative
Example 10, Carbon/O2 of 0.15), most carbon gets oxidised in the
plasma, leaving an insufficient amount of free carbon for the
synthesis of the required Tie intermediate. Consequently, too low
a rutile yield is obtained.
Example 11
A 50 kW RF ICP according to Example 5 is used. However, the
plasma gas is substituted for argon/ammonia, with 12 Nm3/h argon
and 3 NmVh ammonia. Titanium chloride is injected in the plasma,
at rate of 1 1/h, resulting in an average temperature above 2000
K. By analogy with the above examples, the titanium chloride is
totally vaporized, whereupon it readily nucleates, forming a
cubic TiN nano-powder. Subsequently, a nitrogen flow of 5 Nm3/h
is used as a quench gas. Further downstream, 10 Nm3/h of air is
blown, thereby oxidizing the powder and producing nano-sized
rutile TiO2. After filtering, nano-sized TiO2 powder is obtained,
having a rutile content of 95 + 5 % and a specific surface area
of 25 ± 4 m2/g.

Claims
1. A process for the production of a nano-sized rutile powder
comprising the steps of providing a hot gas stream and of
introducing therein, firstly:
- a titanium-bearing first reactant; and
- a carbon- and/or nitrogen-bearing second reactant;
the temperature of said gas stream being chosen so as to
vaporize said first and second reactants, these reactants being
selected so as to form, at the prevalent temperature, titanium
carbide, titanium nitride or a mixture thereof, as a nano-sized
precursor; and, thereafter:
- a volatile oxygen-bearing reactant selected so as to react
with the nano-sized precursor, converting it to nano-sized
titanium dioxide powder having a rutile content of at least
50%.
2. Process according to claim 1, wherein, before the step of
introducing an oxygen-bearing reactant, the gas stream is
quenched.
3. Process according to any one of claims 1 to 2, wherein,
after the step of introducing an oxygen-bearing reactant, the
hot gas stream is quenched.
4. Process according to any one of claims 1 to 3, whereby the
hot gas stream is generated by means of either one of a gas
burner, a hot-wall reactor, an RF plasma, and a DC arc plasma.
5. Process according to any one of claims 1 to 4, wherein the
titanium-bearing first reactant comprises either one or more of
a titanium chloride, an oxide, a sulphate, and an organo-
metallic titanium compound.

6. Process according to any one of claims 1 to 5, wherein the
carbon- and/or nitrogen-bearing second reactant comprises
either one or more of carbon, carbonate, carbon monoxide,
carbon dioxide, hydro-carbons, nitrogen, amines and nitrous
oxide.
7. Process according to any one of claims 1 to 6, wherein the
oxygen-bearing reactant comprises air, oxygen, carbon dioxide
or nitric oxide.
8. A process according to any one of claims 1 to 7, wherein the
titanium-bearing first reactant and the carbon- and/or
nitrogen-bearing second reactant are introduced simultaneously
in the hot gas stream.
9. A process according to claim 8, wherein the titanium-bearing
first reactant and the carbon- and/or nitrogen-bearing second
reactant are embodied as a single compound.

10. Process according to claims 9, wherein said single compound
is titanium isopropoxide.
11. Process according to any one of claims 1 to 10, wherein the
oxygen-bearing reactant is air.
12. Process according to any one of claims 1 to 11, wherein an
additional volatile metal-bearing compound is introduced in the
hot gas stream, together with the firstly introduced reactants,
thereby forming a metal-doped rutile.
13. Process according to claim 12, wherein said additional
metal-bearing compound contains manganese, preferably as an
organic compound.

14. Process according to claims 13, wherein the amount of said
additional manganese-bearing compound is adjusted so as to
obtain a doping level of between 0.01 and 30 wt% in the rutile.

This invention pertains to a process for producing ultra-fine rutile titanium dioxide powders. This particular compound
is useful as UV-blocker in paints, plastics, coatings, pigments and sunscreens. The new process comprises the steps of
providing a hot gas stream and of introducing therein firstly: - a titanium-bearing first reactant; and - a carbon and/or nitrogen-bearing
second reactant; the temperature of said gas stream being chosen so as to vaporize said first and second reactants, these being
selected so as to form, at the prevalent temperature, titanium carbide, titanium nitride or a mixture thereof, as a nano sized precursor;
and, thereafter: - a volatile oxygen-bearing reactant selected so as to react with the nano sized precursor, converting it to nano-sized
titanium dioxide powder having a rutile content of at least 50%. This reaction scheme allows for the manufacture of powders with
or without doping elements with a primary particle size between 1 and 100 nm.