Note: Descriptions are shown in the official language in which they were submitted.
2~ 9~5
TITANIUM DIOXIDE, A PROCESS FOR ITS PRODUCTION AND ITS
USE
__._ _
This inven~ion relat~s to Litanium diox;d~, to a
proceC~ for its produc~ion and to i~s us~ a~ an
opacifi~r in vitrQous enam~la and fuQion ~nam~ls.
BACKGROUND OF THE INVENTION
Titanium dioxide is produced as a pigment either by
reaction of TiC14 with 2 or by hydrolysis of an aqueous
solution of titanyl sulfate containing sulfuric acid and
subsequent calcination.
In the case of the sulfate process, the titanium
dioxide hydrate suspension is calcined ~or 5 to 20 hours in
a rotary kiln. The deposit which accumulates during
hydrolysis consists of fine particles having a specific BET
surface of 50 to 500 m2/g which undergo extensive agglomera-
tion during calcination.
The agglomerates formed as a result of calcination are
in the form of 0.1 to 1.0 mm diameter particles which are
made up of 0.2 to 0.4 ~m primary particles. This material
is unsuitable as ~linker or even as a ground pigment for
incorporation in vitreous enamels or fusion enamels because
it agglomerates when mixed in dry form with the other
constituents of the enamel frit, resulting in incomplete
and inhomogeneous dissolution of the titanium dioxide
during melting of the frit.
DE-AS 1 207 363 describes a process for the production
of a TiO2 product for incorporation in vitreous enamels by
calcination of precipitated titanium dioxide hydrate. In
this proce~s, a precipitated titanium dioxide hydrate of
coar~e particle size, which consists of S to 20 ~m primary
particles, is mixed with a precipitated titanium dioxide
hydrate of fine particle size, which consists of primary
particles mostly below 1 ~m in size, as obtained in the
production of Tio2 p~gments, and the resulting mixture is
calcined. ;-The disadvantage of this process is that it is
extremely expensive on account of the two different
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titanium dioxide hydrates which first have to be produced.
In addition, the disadvantage of Tio2 obtained in this way
is that it is distinguished by a high apparent density of
1 to 2 kg/l and has a low specific surface of distinctly
below 1 m2/g, resulting in inadequate reactivity during
melting in the frit.
US 2,721,787 describes free-flowing, dust-free
aggregates consisting of calcined anatase ~primary particle
size 0.2 to 0.3 ~m, specific surface 4.5 to 9 m2/g) and 0.1
to 3% of an inorganic binder. These aggregates are pro-
duced by spray-drying aqueous 61urries of hydroclassified
titanium dioxide pigments emanating from normal pigment
production at a temperature in the range from 150 to 400C.
The disadvantage of this process is that calcined titanium
dioxide pigments first have to be produced and are then
taken up in water to form a slurry which is then spray-
dried to obtain the claimed aggregates.
DE-AS 1 202 259 describes a process for the production
of Tio2 agglomerates for enamel frits in which a suitable
titanium dioxide (crystallite size 0.2-0.7 ~m) is
granulated with addition of a hydrolyzable metal or silicon
halide.
According to Japanese patent application 85/220884,
fine, spherical Tio2 particles are mixed with a Tio2 sol,
spray-dried at 50C and calcined at temperatures above
S00C. The Tio2 801 iS obtained from Tio2 hydrate contain-
ing sulfuric acid which is neutralized with ammonia, fil-
tered, washed and peptized with concentrated sulfuric acid.
The product thus produced has particle sizes of from 0.1 to
50 ~m and is used in cosmetic products, catalysts and
ceramics. It is not suitable for incorporation in enamels.
Where calcination is carried out below 800~C, relatively
large quantities of sulfate remain behind and give rise
during firing of the enamel to rough, dull enamel surface.
At higher calcination temperatures, the powder particles
are too hard so that they cannot be completely digested in
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the frit.
GB-P 1,176,912 deseribes a finely divided a~atase
having a sulfate content of 0.45 to 1.2%, based on Tio2~ a
BET surface of 35 to 55 m2/g and a primary particle siæe of
0.025 to 0.05 ~m. This anatase is produced by calcination
of a Tio2 hydrate eontainin~ sulfuric acid at 72S to 760C
and subsequent grinding. The produet is used in photo-
graphy and in copying papers. It is not suitable for
incorporation in enamels on account of its high sulfur
eontent, whieh leads to rough, dull surfaces, and on
aecount of its partiele fineness which causes elumping in
the frit raw material mixture and, hence, incomplste dis-
solution and sludge formation during melting of the frit.
Aeeording to DE-OS 1 592 428, titanium dioxide hydrate
or preferably sm~ll titanium dioxide partieles are ehemie-
ally treated in a liguid to promote agglomeration, the par-
ticles are agglomerated to mierospheres 1 to 500 ~m in
diameter and the agglomerated mierospheres are dried and
ealeined. The ehemical treat~ent eomprises reaetion with
a mineral aeid, an organie aeid and a base. ~he apparent
density of the Tio2 powders obtained, whieh are suitable for
use as eatalyst supports, is between O.8 and 1.0 g/l. ~he
products are unsuitable for ineorporation in enamel beeause
the individual granulate partieles become very elose-paeked
and hard as a result of the chemieal treatment in conjunc-
tion with the subsequent agglomeration, leading to poor and
ineomplete melting in the enamel frit.
BRIEF DESCRIPTION OF THE INVENTION
Accordingly, thQ problem addres~ed by the invention
was to provide a titanium dioxide suitable for use as an
opacifier for vitreou~ enamels nnd fu~ion enamels.
It has now been found that the titsnium dioxide
according to the in~ention ~stiafies the requiremQnts
stated above. This titanium dioxide h~s a Tio2 content of
> 87% by weight, a sulfste content of lesE than 0.45X by
weight and preferably le~s than 0.3X by weight, based on
TiO2, an alkali oxide content of less than 0.5X by weight
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and pr~ferably less th~n 0.2% by w~ight, bas~d on TiO2, a
residual water conten~ of le~s than 12X by w~ight, based
on Tio2, a BET surface of 30 to 500 n~/g, an apparent den-
sity of 0.2 bD 0.8 kg/l, a primary particl~ ize of 0.005 to
0,1 ~m ~nd a m~an aggr~gated grain size of 30 to 500 ~m and
i~ present in amorphou~ form ~nd/or a~ ~rystalline
~nst.a~e .
DETAILED DESCRIPTION OF THE INVENTION
The Tio2 contQnt of the granulate~ i~ >87% by weight,
but can be increa~ed by hea~ treatment. Their ~ulfat~
content i~ le~s than 0.45X by weigh~ and preferably less
than 0.3X by we;ght, based on Tio2. Higher s~lfate contents
a~n~Rly affect recrys~llization of the Tio2 during firing
of the enam~l and l~ad ~o irregular, dull ~namel finishes.
The total ~A~tent of alkali oxides, particularly sodium
~nd potassium oxids, i~ below O.5X by weight and preferably
b~low 0.2X by weight, based on Tio2. Lary~r conten~ result
in reduction~ in the opacity of ~he enamel. A recidual
wat~r contQnt of up to 12X by weight, ba~d on Tio2, do~s
not affect the properties of the titanium dioxide with
resp~ct to ~he enamel.
Tha large BET surfac~ of 30 to 500 nF/g tBET 3urfac~
determin~d in aecordance wi~h Ger~an Indus~rial Standard,
DIN 66 131, Section~6, Oc~ober, 1973; 5-point method,
me~surin~ gas: ni~rogen, sdsoption at the t~mperature of
~oiling ni~rogen, one molecule of ni~rogen being as~umed
~o occupy sn ar~a of 0,162 nn~, pratr~atment: h~ting for
1 hour in a s~ream of nitrog~n at 13~ C) and th~ small
primary particlo ~ize of 0.005 to 0.1 ~m re~ul~ in high
reac~ivity and h~nce in excQllent digestion bahavior of the
Tio2 in the enamel m~lt.
The 6pherical aggregates 30 to 500 ~m in size show
satisfactory flow behavior, do not stick to the other com-
~5 ponent~ of the frit during mixing and show high stability,i.e. they do not disintegrate into fine grain during the
mixing process, which would result in clumping and incom-
plete melting.
The aggregates are uniformly distributed in the frit
during mixing and then melt completely and uniformly.
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In the quenched frit, the particles form near zones
rich in titanium dioxide which act as seed cells for
recrystallization of the titanium dioxide during the
initial stages of firing of the enamel. As firing of the
enamel continues, anatase is almost exclusively formed in
a suitable crystallite size, in isometric form and in
uniform distribution, so that high reflectivity is obtained
for excellent opacifying power.
In another embodi~en~, ~he ~i~anium dioxide prefer~b1y
con~ins oxid~, phospha~e~ and/or ~ilica~9~ of the metals
Mg, Ca, Sr, Ba, Sn, Sb, Bi, Y, Zr, Zn, V, Nb, W andlor Mo
andlor oxides of ~hQ e1Qments P and/or Si in a quan~i~y of
from trace amount up ~o 10% by weight and preferab1y in a
quantity of no more than 7X by weight, bas~d on ti~anium
dioxide. For example, an addition of P205 impro~e~ the re-
crystalliza~ion of Tio2 and, hencs, the opscity of the
enamel whil~ an addition of W03 improYes the c~lor of the
enamel. The origin~1 ~patial separation of the additive~
from the Tio2 grain is surprising1y of considerable im-
portsnce to the subsequ~nt rocry~t~11iz~tion of Tio2~
the introduction of the same additive~ in the same quantity
directly into the frit would ha~e a ~r wsaker Qffsc~
The titanium dioxide may additionally contain binders
such as, for example, waterglass, carboxymethyl celluloses
and/or polyvinyl alcohols and/or deflocculating agents such
as, for example, polyphosphates and/or polyacrylates in
quantities of from 0.1 to 5% by weight, based on Tio2.
The present invention also relates to a suitable pro-
cess for the production of the titanium dioxide according
to the invention in which precipitated acid-containing
titanium dioxide hydrate is adjusted to pH 7-13 with a
base, optionally heated for O.l to lO hours to a temper-
ature between 40-C and the reflux temperature, filtered and
washed, the excess base is optionally removed, the product
is spray-dried, optionally with addition of auxiliaries,
and is then optionally heat-treated at 200 to 800C and
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preferably at 400 to 600C.
The object of treating the acid-containing Tio2 hydrate
with a base is to convert the adhering acid completely into
the corresponding salt which is subsequently washed out.
Preliminary heating of the base-treated Tio2 hydrate for 0.1
to 10 hours to a temperature between 40C and the reflux
temperature promotes removal of the salt.
In one particular embodiment, the salt-containing Tio2
hydrate may be subjected to a pressure treatment before
washing. This may be done, for example, by carrying out
the heat treatment in a closed pressure vessel. It is also
possible, for example, to use the high-pressure expansion
technique in suitable apparatus.
The measures described above lead to an improvement in
filtration and in the washing out of foreign salts which in
turn results in an improvement in the properties of the
titanium dioxide thus treated with respect to the enamel.
After filtration and washing, the excess base is
preferably larqely removed again. Where sodium hydroxide
and/or potassium hydroxide are used, this is done by pH
adjustment to 7-2, for example wi~h hydrochloric, nitric
and/or phosphoric acid, and removal of the sodium and/or
potassium salt formed by re-filtration and washing.
Measures for improving filtration and washing, such as heat
or pressure treatment, may also be applied in this step.
In another preferred embodiment, ammonia is used as
the base. The ammonium salt formed may also be removed by
filtration and washing after pH adjustment to 7-2 with an
acid. However, it may also be removed with advantage by
heat treatment after spray drying at 400 to 800-C, elimin-
ating the need for the second filtration cycle.
In one preferred embodiment, auxiliaries may be added
in a suitable form to the washed, substantially salt-free
titanium dioxide hydrate before spray drying, being present
in the end product largely as oxides, phosphates and/or
sili~ates of the elements Mg, Ca, Sr, Ba, Sn, Sb, Bi, Y,
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Zi, Zn, V, Nb, W and/or Mo and/or as oxides of the elements
P and/or Si in a guantity of up to 10% by weight and pref-
erably in a quantity of up to 7% by weight, based on
titanium dioxide.
The auxiliaries are salts of the above-mentioned
metals, such as carbonates, nitra~es, chlorides, fluorides,
silicates, phosphates or oxides.
Binders such as, for example, waterglass, carboxy-
methyl celluloses or polyvinyl alcohols and/or defloccu-
lating agents such as, for example polyphosphates, or poly-
acrylates may be added to the Tio2 hydrate before calcina-
tion in quantities of from 0.1 to 5% by weight.
The spray drying process takes places at a waste air
temperature above 90DC and preferably above 140C. To this
end, auxiliaries are optionally added to the substantially
salt-free Tio2 suspension which is then introduced into the
drying tower with a solids content of 2 25% by weight and
preferably > 35% by weight either through a one-component
or multi-component nozzle or through a centrifugal atom-
izing disc. ~oth a high solids content of the feed suspen-
sion and a high drying temperature in the spray dryer have
a favorable effect on granule stability and hence on the
processibility of the Tioz with respect to the enamel.
The Tio2 content and grain stability may optionally be
increased by subsequent conditioning of the spray-dried
powder at 200 to 800-C and preferably at 400 to 600C.
Precipitated titanium dioxide hydrate containing
sulfuric acid with a BET surface of 50 to 500 m2/g, as
obtained ~or example in pigment production by the sulfate
process, is preferably used as starting material for the
production process according to the invention.
The titanium dioxide according to the invention may be
used as an opacifier in the production of vitreous enamels
or fusion enamels.
Exa~ples 1 and 2 below are intended to illustrate the
invention without limiting it in any way.
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EXAMPLE 1
An aqueous titanium dioxide hydrate suspension having
a Tio2 content of 25.5% by weight, a sulfate content of 7.5%
by weight, based on Tio2~ and a BET surface of 280 m2/g is
adjusted to pH 8 with sodium hydroxide, heated for 1 hour
to 90C, filtered and washed free from salts, re-suspended
in H2O, adjusted to pH 4 with sulfuric acid, re-filtered and
washed free from salts. The filter cake, w~ich has a
solids content o~ 40% by weight, is re-suspended. 0.5% by
weight P2Os, based on Tio2~ is added to the suspension which
is then dried in a spray dryer with centrifugal disc atom-
ization at a waste air temperature of 170C.
The product obtained has a Tio2 content of 92.0% by
weight, a sulfate content of 0.2% by weight, based on Tio2~
an alkali oxide content of 0.1% by weight, based on Tio2, a
chloride content of 0.4% by weight, based on Tio2~ a P2O5
content of 0.5% by weight, based on TiO2, an H2O content of
6.7% by weight, based on Tio2~ a BET surface of 280 m2/g, a
primary particle size of 0.01 ~m, an apparent density of
0.45 kg/l and a mean aggregated grain size of 80 ~m and is
present as amorphous and crystalline anatase.
The product is non-tacky and free-flowing and shows
excellent ~ixing and digestion behavior in a boron-titanium
white enamel frit. High reflectivity, high opacifying
power and a favorable color of the enamel finish are
obtained after application and firing.
EXAMPLE 2
A titani~m dioxide hydrate suspension as described in
Example 1 is adjusted to pH 8 with ammonia, heated for hour
to 60-C, filtered and washed free from salts. The residue
is taken up in water to form a 35% by weight suspension
which is then dried in a spray dryer with nozzle atomiza-
ation at a waste air temperature of 140~C and conditioned
for 30 minutes at 500C. The product obtained has a TiO~
content of 97% by weight, a sulfate content of 0.25% by
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weight, based on Tio2~ an H20 content of 1.7% by weight,
based on TiO2, a BET surface of 115 m2/g, an apparent
density of 0.6 kg/l, a primary particle size of 0.03 ~m and
a mean aggregated grain siz~ of 185 ~m and is predominantly
present as crystalline anatase.
The product is non-tacky and free-flowing and shows
excellent mixing and digestion behavior in a titanium
boride white enamel frit. High reflectivity, high
opacifying power and a favorable color of the enamel finish
are obtained after application and firing.
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