The invention provides a process for removing halide
compounds adhering to finely divided, pyrogenically
prepared metal oxide particles.
It is known to prepare metal oxide particles by flame
hydrolysis or by flame oxidation. Metal oxide particles
prepared by these processes are usually referred to as
pyrogenically prepared metal oxide particles. In general
metal halides, especially chlorides, are used us starting
materials therefor. They are converted under the reaction
conditions into the metal oxides and hydrohalic: acids,
usually hydrochloric acid. While the majority of the
hydrohalic acid leaves the reaction process in the form of
waste gas, some remains adhered to the metal oxide
particles or is bonded directly thereto. In a
deacidification step, it is possible by means of steam to
remove the adherent hydrohalic acid from the metal oxide
particles or to substitute halogen atoms bonded directly to
the metal oxide by OH or 0H2.
DE 1150955 claims a process in which the deacidification is
carried out in a fluidised bed at temperatures of from
450°C to 800°C in the presence of steam. It is possible in.
this process to feed metal oxide particles and steam co-
currently or counter-currently, preference being given to
co-current feeding. The high temperatures required for the
deacidification are a disadvantage of this process.
GB-A-1197271 claims a process for the purification of
finely divided metal oxide particles, in which metal oxice
particles and steam or steam, and air are passed counter-
currently through a column in such a manner that a
fluidised bed does not form. It has thus been possible to
lower the required deacidification temperatures to from 400
to 600°C. It has been found, however, that even these

temperatures can still have an adverse effect on the metal
oxide particles.
EP-B-70934O claims a process for the purification of a
pyrogenic silison dioxide powder. In this process, the
required temperatures for deacidification are only from 250
to 350°C. In the process, metal oxide particles; and steam
are fed co-currently through an upright column from bottom
to top. The speed is in the range of from 1 to 10 cm/s in
order to allow a fluidised bed Co form. The purified
silicon dioxide powder is removed at the head of the
column. It is a disadvantage that the process must be
carried out in such a manner that a fluidised bed is
present, which is associated with an increased outlay ir.
terms of control. Furthermore, there is a constant risk
with the co-current procedure, in which purified silioor.
dioxide powder and hydrochloric acid are removed at the
head of the cclumn, that the purified silicon dioxide may
become contaminated with the hydrochloric acid.
The object of the invention is to provide a process for
removing halice residues from metal oxide particles, which
process avoids the disadvantages cf the prior art. In
particular, the process is to be gentle and economical.
The invention provides a process for removing halide
compounds adhering to finely divided metal oxide particles
by means of steam, the metal oxide particles being formed
by reaction oil halide-containing starting materials by
hydrolyis or oxidising gases, wherein
the fonely divided metal oxide particles containing
residues of halide compounds are applied, together
with reaction gases, to the upper part of an
upright column and migrate downwards by means of
gravity,
the steam, optionally mixed with air, is applied at
the bottom end of the column,

the finely divided metal oxide particles containing
residues of halide compounds and the steam are fed
counter -currently,
the metal oxide particles freed of halide residues
are removed at the base of the column,
- steam end halide residues are removed at the head
of the column,
which process is characterised in that
the column is heated in such a manner that the
temperature difference Tbottom- Ttop between the
lower part and the upper port of the column is a:.
least: 20°c and a maximum temperature of 500°C
prevails in the column, and
the metal oxide particles have a residence time In
the column of from 1 second to 30 minutes.
Halide compounds within the scope of the invention are
generally hydrogen halides, especially hydrochloric acid.
The halide compounds also include those in which a halide
atom or halide ion is bonded to metal oxide particles
covalently or ionically or by physisorption.
Halide-containing starting materials are generally the
corresponding metal chlorides, such as titanium
tetrachloride, silicon tetrachloride or aluminium chloride.
However, they may also be organometallic compounds, such as
chloroalkylsilanes.
Within the sccpe of the invention, metal oxide particles
are understood as being those which can be obtained from
halide-contairing starting materials by flame hydrolysis or
flame oxidaticn. Metal oxide particles are also understood
as being metalloid oxide particles. They are: silicon
dioxide, aluminium oxide, titanium dioxide, cerium oxide
zinc oxide, zirconium oxide, tin oxide, bismuth oxide, as
well as mixed oxides of the above-mentioned compounds.
Metal oxide particles also include doped oxide particles
as are described in DE-A-19650500. Metal oxide particles

are also understood as being metal oxide particles obtained
by flame hydrolysis and enclosed in a shell, for example
titanium dioxide particles encased in silicon dioxide, as
described in DS 10260718.4, filing date 23.12.2002. Of the
above-mentioned, oxides, silicon dioxide, aluminium oxide
and titanium dioxide are of the greatest importance.
The particles are in finely divided form. This is
understood as meaning that they are in the form of
aggregates of primary particles and usually have a BET
surface area of from 5 to 600 m2/g.
Reaction gases are the reaction products of the gases and
vapours used that are formed in the preparation of the
metal oxide particles by flame oxidation or flame
hydrolysis. They may be hydrogen halides, steam, carbon
dioxide, as well as unreacted gases.
The process according to the invention can preferably be
carried out in such a manner that the temperature
difference Tbottom - Ttop is from 20°C to 150°C, where
particular preference may be given to the range from 50°C
to 100°C.
The temperature Tbottom is determined at a measuring point
located from 10 to 15 %, based on the overall height of the
reactor, above the bottom end of the reactor.
The temperature Ttop is determined at a measurine, point
located from 10 to 15 %, based on the overall height of the
reactor, beneeth the upper end of the reactor.
The process according to the invention can preferably also
be carried out in such a manner that the maximum
temperature is from 150°C to 500°C. A range from 350°C to
450°C is generally particularly preferred.
The residence time can preferably be from 5 seconds to
5 minutes, anc. the temperature of the particle stream

entering the column can preferably be from about 100°C to
250°C.
The amount of steam that is introduced is preferably froir
0.0025 to 0.2 5 kg of steam per kg of metal oxid particles
per hour, the range from 0.02 5 to 0.1 kg of steam per kg of
metal oxide particles per hour being particularly
preferred. A steam temperature of from 100°C to 500°C is
preferably chosen, where the range from 12O°C to 200°C may
be particularly preferred.
If air is introduced into the column together with the
steam, it has proved advantageous to choose an amount of
air of from 0.005 to 0.2 m3 of air per kg of metal oxide
particles per hour, the range from 0.01 to 0.1 m3 of air
per kg of metal. oxide particles per hour being particularly
advantageous.
The process can be carried out in such a manner that the
silicon dioxide powder to be purified and the steam,
optionally together with air, form a fluidised bed. More
advantageously however, the process can be carried out so
that a fluidised bed does not form. In this case, the
outlay in terms; of control is reduced and the desired
degree of purification is achieved even at low temperatures
and with relatively short residence times- This procedure
also avoids the discharge of silicon dioxide powder with
steam and air, as is possible with the fluidised-bed
procedure. After the metal oxide particles have beer,
removed at the base of the column, they may, if desired, be
passed through at least one further column in which the
maximum temperature does not exceed 5000C. This measure
enables the content of adherent halide compounds to be
reduced further.
It is possible for the metal oxide particles and the steam
and, optionally, air to be fed co-currently or counter-
currently therein.

It may be advantageous for the second and subsequent
columns to have a temperature difference Tbottom - Ttop
between the lower part and the upper part of the columns; of
at least 5°C.
Figure 1 illustrates the process in diagrammatic form, in
the figure: 1 - admission of the metal oxide particles; 2
admission of steam and, optionally, air; 3 = exit of the
metal oxide particles, 4 = exit of gases.
Examples
Example 1 (according to the invention) : A particle stream
of 100 kg/h of silicon dioxide powder (BET surface area
200 m2/g) having a pH of 1.6, a chloride content of
0.1 wt.% and an initial temperature of 190°C i.3 introduced
in the upper part of an upright cclumn. 5 kg/h of steam
having a temperature of 12 0°C and 4.5 Nm3/h of air are
introduced at the base of the column. The column is heated,
by means of an, internal heating means, to a temperature Ttop
in the upper region of the column of 350°C and a
temperature Tbottom in the lower region of the column of
425°C. After leaving the column (residence time:
10 seconds), the silicon dioxide powder exhibits a pH ol
4.2, a chloride content of 0.0018 wt.% and a thickening of
3110 mPas.
Example 2_(conparative example): analogous to Example 1,
but with a temperature Tbottom of 66 0°C and Ttop of 670°C.
Example 3 (conparative example): A particle stream of
100 kg/h of silicon dioxide powder (BET surface area
200 m2/g, pH 1.6, chloride content 0.1 wt.%, initial
temperature l9O°C) and 5 kg/h of steam and 4.5 Nm3/h of air
are introduced co-currently at the base of an upright
column. The column is heated, by means of an internal
heating mear.s, to a temperature TtoP in the upper region of

the column of 350°C and a temperature Tbottom in, the lower
region of the column of 42 5°C. After leaving the column
(residence tine: 1C seconds), the silicon dioxide powder
exhibits a pH of 4.0, a chloride content of 0.09 wt.% and a
thickening of 2850 mPas.
Example 4 (according to the invention) : analogous to
Example 1, using aluminium oxide powder (BET surface area
99 m2/g, pH :..7, chloride content 0.6 wt.%, initial
temperature 185°C) instead of silicon dioxide powder, anl
6 kg/h of steam having a temperature of 160°C and 5 Nm3/h
of air (residence time: 150 seconds).
Example 5 (according to the invention): analogous to
Example 1, using 200 kg/h of titanium dioxide powder (BET
surface area 46 m2/g, pH 1.7, chloride content 0.6 wt.%,
initial temperature 172°C) instead of 100 kg/h of silicon
dioxide powder , and 12 kg/h of steam having a. temperature
of 180°C and ID Nm3/h of air (residence time: 85 seconds)
Tbottom, was 400°:.
Example 6 (according- to the invention) : In the bottom part
of an upright column there is arranged a controllable flap
for the accumulation of the silicon dioxide powder. A
particle stream of 100 kg/h of silicon dioxide powder (BET
surface area 200 m2/g) having a pH of 1.6, a chloride
content of 0.2 wt.% and an initial temperature of 190°C :.s
introduced in the upper part of the column. 5 kg/h of staam
having a temperature of 1200C and 4.5 Nm3/h of air are
•introduced at the base of the column. The column is heated,
by means of al. internal heating means, to a temperature Ttop
in the upper legion of the column of 350°c and a
temperature Tbottom in the lower region of the column of
425°C. After leaving the column (residence time:
10 minutes), the silicon dioxide powder has a pH of 4.3, a
chloride content of 0.0010 wt.% and a thickening of
3070 mPas.


Examples 1, 4 and 5 show that adherent halides can be
removed efficiently by means of the process according to
the invent i on.
A comparison of Examples 1 and 2 shows that, although
equally efficient purification of nalide residues is
possible in Example 2 owing to the higher temperature, the
higher temperature adversely affects the thickening effect.
Accordingly, the powder obtained in Example 1 exhibits a
thickening effect of 3110 inPas, the powder of Example 2
only 2750 mPas. Example 3 exhibits poorer removal of halide
residues compered with Example 1, and the powder exhibits a
poorer thickering effect.
The thickening effect is determined according to the
following method: 7.5 g of silicon dioxide powder are
introduced at a temperature of 22°C into 142.5 g of a
solution of ar unsaturated polyester resin in styrene
having a viscosity of 1300 +/- 100 mPas, and dispersion is
carried out by means of a dissolver at 3 000 min-1. An

example of a suitable unsaturated polyester resin is
Ludopal® P6', BASF. A further 90 g of the unsaturated
polyester resin in styrene are added to 60 g o:: the
dispersion, and. the dispersing operation is repeated. The
thickening effect is the viscosity value in mPas of the
dispersion at 250C, measured using a rotary viscometer at a
shear rate of 2.7 s-1.

WE CLAIM:
I. Process for removing halide compounds adhering to
finely divided metal oxide particles by means of steam, the metal
oxide particles being formed by reaction of halide-containing
starting materials by hydrolysis or oxidising gases, wherein
— the finely divided metal oxide particles containing
residues of halide compounds are applied) together with
reaction gases, to the upper part of an upright column
and migrate downwards by means of gravity,
the steam, optionally mixed with air, is applied at
the bottom end of the column,
— the finely divided metal oxide particles containing
residues of halide compounds and the steam are fed
counter—currently, and
— the metal oxide particles freed of halide residues are
removed at the base of the column,
— steam and halide residues are removed at the head of
the column,
which process is characterized in that

the column is heated in such a manner that the
temperature difference Tbottom - Ttop between the
lower part and the upper part of the column is at least
20 0C and a maximum temperature of 5OO 0C prevails in
the column, and
the metal oxide particles have a residence time in the
column of from 1 second to 3O minutes.
Process as claimed in claim 1, wherein the temperature
difference Tbottom - Ttop is from 2O 0C to 150 0C.
Process as claimed in claim 1 or 2, wherein the maximum
temperature in the column is from 150 to 5OO 0C.
Process as claimed in claims 1 to 3, wherein the
residence time is from 5 seconds to 5 minutes.
Process as claimed in claims 1 to 4, wherein the metal
oxide particles in the stream entering the column have
a temperature of from about 1OO °C to 5OO °C.

Process as claimed in claims 1 to 5, wherein the
amount of steam that is introduced is from 0.0025 to
O.25 kg of steam per hour per kg of metal oxide
particles.
Process as claimed in claims 1 to 6, wherein the amount
of air admixed with the steam is from 0.005 to 0.2 m
of air per kg of metal oxide particles per hour.
Process as claimed in claims 1 to 7, wherein after the
metal oxide particles have been removed at the base of
the column, they are passed through at lest one further
column in which the maximum temperature does not exceed
5OO °C.
Process as claimed in claim in claim 8, wherein the
metal oxide particles and the steam are fed co-
currently or counter—currently in the further columns.

Process as claimed in claim 8 or 9, wherein the second
and subsequent columns have a temperature difference
T — T between the lower part and the upper
bottom top
part of the columns of at least 5 °C.

The invention relates to a process for removing halide
compounds adhering to finely divided metal oxide particles by
means of steam wherein the metal oxide particles are applied to
the upper part of an upright column and migrate downwards by
means of gravity, the steam is applied at the bottom end of the
column, the metal oxide particles and the steam are fed counter-
currently, the metal oxide particles freed of halide residues are
removed at the base of the base of the column, steam and halide
residues are removed at the head of the column, wherein the
column is heated in such a manner that the temperature difference
Tbottom Ttop between the lower part and the upper part of the
column is at least 20 aC and a maximum temperature of 500 0C
prevails in the column, and the metal oxide particles have a
residence time in the column of from 1 to 30 minutes.