Titanium dioxide
Titanium dioxide has remarkable optical properties, with a very high
refractive index close to that of diamond. It is also a very stable compound and
can be heated to over 2000 K before melting. It is these properties that underlie
its most important uses.
Uses of titanium dioxide
Titanium dioxide is the most used white pigment and provides whiteness and opacity
for paints and coatings (including glazes), plastics and paper.
It has many speciality uses. It is resistant to UV radiation and thus does not discolour
over a long period of time and ultra pure and fine crystal titanium dioxide grades are
being increasingly used for sun screens.
Figure 1 Uses of titanium dioxide
Much smaller amounts of titanium dioxide are
used as a semi-conductor and to catalyze the
photodecomposition of water into hydrogen and
oxygen.
It also has a high dielectric constant and a high
resistance and thus it is used to make
capacitors.
The strong bonding between titanium and
oxygen gives great thermal stability, melting at
2100 K. It is thus used as a ceramic material.
It is also used to increase the acid resistance
of vitreous enamels.
It is also being used in novel nanotechnology
applications
Manufacture of titanium dioxide
There are two main processes, the Sulfate Process and the Chloride Process (Table 1)
which use the two principal ores, ilmenite and rutile, respectively. Ilmenite contains 4560% TiO and rutile contains up to 99% TiO . The ores are mined worldwide but most
production is in Australia and South Africa.
Each large producer of titanium dioxide balances its production between the two
processes. Each produces the oxide in the rutile crystal form but the Sulfate Process can
also produce another form of the oxide, anatase titanium dioxide, which is softer and
which is used for a small number of specialist applications.
It is estimated that about 65% of the world's production is based on the Chloride Process.
2
2
Sulfate Process
Chloride Process
long established and simple technology
new technology
uses lower grade, cheaper ores
needs high grade ore
batch process
continuous process
FeSO4 is formed in large amounts leading small amounts of waste formed with
to disposal problem.
toxicity problems: Cl2 and TiCl4
pollution control expensive
recovery and recycling of chlorine
possible
produces low grade anatase and rutile
pigments
only produces rutile pigments
Table 1 Comparison of the two processes for the manufacture of titanium dioxide.
The Sulfate Process
The chemistry of the process involves three main stages:
a) dissolving the ore
b) formation of hydrated titanium dioxide
c) formation of anhydrous titanium dioxide
(a) Dissolving the ore
The ore is usually ilmenite, FeTiO3 It is ground finely and dissolved in sulfuric acid to
form a mixture of sulfates:
Before the titanium dioxide is extracted, the iron ions must be removed from the solution
so that the colour of the final product is not spoiled. The solution is therefore reacted with
recycled iron sources to convert any iron(lll) ions that may be present to iron(ll) ions:
The solution is allowed to stand so that the unreacted solid settles, and the clear liquid is
poured off before being concentrated by evaporation. Cooling then allows light green
crystals of iron(ll) sulfate to form and to be filtered off. These are sold separately. The
remaining solution contains titanyl sulfate, TiOSO4
(b) Formation of hydrated titanium dioxide
The next stage involves the hydrolysis of the titanyl sulfate in solution to give insoluble,
hydrated titanium dioxide:
This is a critical stage and the conditions must be controlled to ensure that the precipitate
is suitable for filtering and roasting.
(c) Formation of anhydrous titanium dioxide
The final stage of the process is the heating of the solid in a furnace, known as a calciner.
This is a rotating cylinder which is typically heated by gas flames. As the cylinder turns,
the titanium dioxide passes along it and its temperature rises from 313 K, as it enters, to
over 1000 K as it leaves:
Heating evaporates the water and decomposes any remaining sulfuric acid in the solid.
After cooling, the product is 'milled' to form crystals of the size needed. Crystals may
also be coated with another substance, such as aluminium oxide or silica, to make the
titanium dioxide mix more easily with liquids or to make the water-based paints made
from it last longer.
This coating is achieved by dispersing the dry product from the calciner in water
containing the dissolved coating chemicals which precipitate from solution onto the
TiO2 crystals. The coating is usually between 3 and 8% by weight in the final dried
pigment. This coating is achieved by changing the temperature and pH of the solution.
Each TiO2 crystal needs to be coated uniformly to maximize the effectiveness of the
coating. The coated TiO2 crystals are filtered from the water and dried before being
packed for dispatch to the final customer.
The Chloride Process
There are two main stages:
a) the conversion of rutile to titanium(IV) chloride
b) the oxidation of titanium(IV) chloride
(a) The conversion of rutile to titanium(IV) chloride
Feedstock containing, titanium dioxide (85% pure) and finely divided coke are
added to chlorine gas in a reactor, where the titanium dioxide undergoes
chlorination reaction at 800 ºC.
The rutile is fed into a heated bed together with a source of carbon, usually coke.
Chlorine is fed into the bed and the reaction takes place to form titanium (IV) chloride in
the vapour form which is removed from the bed. Iron and other metals in the ore are
chlorinated and also leave the bed in the vapour state. The oxygen in the ores is
combined with the carbon to form carbon monoxide and dioxide.
2 TiO2 + 4Cl2 + 3C
SiO2 + 2Cl2 + C
2FeO + 3Cl2 + C
2 TiCl4 + CO2+ 2CO
SiCl4 + CO2
2FeCl3 + CO2
Pressure is maintained at about 1.5 atm and the residence time in the reactor is about 1
hour. The outlet stream from the chlorinator at 800°C is fed to a cooler and cooled to
320°C using water as coolant. Iron chloride condenses and settles down as liquid.
Unreacted solids also settle down and are separated. The vapor stream from the top is
fed to the condenser.
The gas stream from the cooler is cooled to 137ºC where TiCl4 is condensed. The
effluent gas is then sent to a converter, where Carbon monoxide is converted into Carbon
dioxide. The condensed pure TiCl4 liquid at its bubble point is fed to the oxide burner.
Pure TiCl4 liquid is vaporized and burnt with oxygen to form titanium dioxide solid (0.3μm
particle size) and chlorine gas. The reaction takes place at a temperature of 1000ºC.
TiCl4 + O2
TiO2 + 2 Cl2
The stream containing chlorine gas and titanium dioxide solids are separated using a
high efficiency Cyclone Separator. The Chlorine gas is recycled to the Chlorinator. The
product is removed, cooled and surface treated
(The vapour stream is cooled and the metal chlorides other than titanium (IV) chloride are
condensed and solidified. The titanium (IV) chloride vapour, which contains almost pure
titanium (IV) chloride and has a lower boiling point, is then condensed and stored as
liquid. It is then reboiled and distilled to give a purer product to feed to the next stage.)
(b) The oxidation of titanium (IV) chloride
Liquid titanium (IV) chloride is vaporised and burnt in oxygen, together with a
hydrocarbon fuel source (for example, methane) to a high temperature to initiate the
reaction and keep the temperature high enough for the reaction to proceed:
The titanium dioxide is formed (by adding seed crystals) as a fine solid in the gas stream
and is filtered out of the waste gases using cyclones or filters. Once again control of
crystal growth is important to give particles of the correct size for pigments. This is done
by adding nucleating agents to the gas stream (e.g. water or aluminium chloride) and by
cooling the products. The chlorine in titanium(IV) chloride is released and recycled to the
chlorination stage of the process above.
The product contains small amounts of absorbed chlorine gas which are removed.
SURFACE TREATMENT OF PRODUCT:
'Pure' TiO2 base pigments, whether anatase or rutile, provide high opacity and excellent
color but have a relatively reactive surface. The majority of commercial pigments are
therefore coated with an inorganic treatment or coating, and often an organic treatment.
Reasons for surface treating titanium dioxide pigment
Nearly all commercial TiO2 pigments have a surface treatment or coating to:
· Improve wetting and dispersion in different media (water, solvent or polymer)
· Improve compatibility with the binder and dispersion stability
· Improve color stability
· Improve durability
· Increase air in the film in flat latex paints
Depending on the end application, the surface treatment is usually a combination of
alumina, silica and/or zirconia, deposited onto the core TiO2 particles from either the
chloride or sulfate base pigment process, by a wet precipitation process. A final coating
of polyol or other organic chosen to enhance the dispersion of the pigment in a resin
system and/or dry bulk handling characteristics is then added.
Following surface treatment, the pigment is dried, intensively milled and packed into
bags. It may also be supplied in semi and full bulk deliveries or, especially in North
America, converted to high concentration slurry for direct use in water based paint
systems.
The coatings on TiO2 pigments are deposited under very closely controlled conditions of
time, temperature and pH to develop the required characteristics. The final analysis of a
surface coating shows only part of the expertise in making high performance TiO2
pigments. Order of addition and precipitation as well as the processes parameters are
critical to making the optimum product.