Tin Oxides

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TIN OXIDE
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Tin Oxide":'An Abunchce of Uses
Tin(1V) oxide, SrtO,, occurs naturally widely available in Europe and tin
as Cassiterite, the principal mineral oxide has maintained its use in
source of tin, However, as a com- ceramic grazes and, to a lesser
pound in its own right, tin oxide is extent, in vitreous enamels.
an important industrial chemical
Tin oxide is highly insoluble in
with a w i d e range of applications. In glazes and this gives it a high opaciline with the generally accepted low fying power when used at levels of
toxicity of inorganic tin c~mpctund~, 4 - 8% Although its long held
the EDSO value for SnO, (in rats) is prominence has been challenged by
greater than 10,000 mg/kg and its cheaper opacifiers, anhydrous Sn02
safe nature and ease of handling is still used for high quality artware,
have undoubtedly contrib -_ledto its where the highest: reflectance,
widespread usage.
purest colours, greatest strength
In actual fact, tin oxide exists in and abrasion resistance are
several distinct forms; each prepared required. Since light reflection
by a different route and each exhibit- depends on the particle size distribing different properties to the other ution in the glaze, the milling proceforms. Anhydrous Sn02, manufac- dure for npacified glazes is critical
tured by the thermid oxidation of tin and the highest reflectance is prometal, contains 79% tin by weight, duced with tin oxide particles o€0.2
and probably accounts for 90% or - 0+3microns diameter.
However, the largest use of sno,
more of the total consumption of tin
oxide, which is estimated at around in the ceramics industry has resulted
3,500 - 4,000 tonnes per annum, The from its ability to accommodate certwo hydrous €orrns of tin oxide, a- tain metal colorant ions in its Iattice
(or ortho-) stannic acid, prepared by structure. Three series of commercial
acid hydrolysis of sodium hydroxys- pigments based c~ntin oxide have
tannate or alkaline hydrolysis of found extensive usage in ceramic
tin(1V) chloride, and p- (01: meta-) tiles and pottery. Tin - vanadium yelstannic acid, prepared by the action lows (containing 2 - 5% vanadium)
of concentrated nitric acid on tin and tin - antimony blue-greys (conmetal, are gel-type materials of taining 3 - 8% antimony) are preindefinite composition with typical pared by t h e m 1 readion of tin oxide
tin contents of 65 - 70%, depending (either in its anhydrous form or, paron precise preparation conditions.
ticularly for Sn/Sb pipents, as pstannic acid), with either ammonium
Ceramic glazes and pigments
vanadate or antimony trioxide
The production of decorated potm
tery is one of the oldest crafts
known to man and Babylonian and
Assyrian wall tiles dating from the
9th century BC were decorated with
an opaque white glaze based on tin
oxide, making this almost certainly
the earliest application of a tin compound. During the 19th century,
mass-produced pottery,,. be<
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respectively, at a temperature of 1200
- 1300°C. A third series, the tin c h r ~ h m
pinks, are prepared by firing a mixture of tin oxide, potassium
dichromate, calcium carbonate and
silica at 1150°C. The colour can vary
from a dark maroon to a light pink
depending on the exact composition
and the host lattice is believed to be a
complex oxide, Malayaite (CaSnSi05),
rather than SnO, itself. Each of these
tin-based pigments has outstanding
thermal stabiIity and can be used
under glazes without discoloration at
high glaze firing temperatures.
It is worth mentioning that the
ancient pigment "Purple of
Cassius", which still finds application as an on-glaze decoration for
high quality tableware, is also based
on tin oxide, although it is actually
produced by the aqueous reduction
of gold(II1) chloride by tin(1I) chloride. The product comprises a colloidal dispersion of metallic gold on
hydrous Sn03 with a typical g d d
content of around 20% by weight.
Although no. new tin colours
have been developed in recent years,
the possible use of tin oxide as an
encapsulant for less thennally stable
pigments (or for those which may
leach out toxic components during
use), ha5 been studied by ITRI and
others. In this connection, new
developments in tin oxide sol-gel
technology may provide a means to
effective encapsulation pmcesses.
Glass industry
Tin oxide possesses an unusual combination of properties for a refractory
oxide being a good electrical conductor (especially at high temperatures),
extremely resistant to molten glass,
and virtually non-colouring. As
such, it is ideally suited as an ekc-
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Tin oxide gas =ensor$
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TIN International Vol. 72, No. 7
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TIN OXIDE
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trode material for the electrical melting of grass. Above 800°C, molten
glass becomes electrically conduc-
tive and hrther heating can be
achieved by passing a high current
through electrodes which are
inserted into the glass melt.
Although conventional glass uses
molybdenum or graphite electrodes,
these are incompatible with lead
crystal glass because of their tendency to reduce lead oxide to metallic lead, resulting in an envirmmental hazard from lead fumes and a
discolouring effect on the glass.
Compressed SnQ electrodes,
typically containing I - 2% of CuO
and Sb,O, to enhance sintering and
conductivity respectively, are now
widely used in the manufacture of
lead crystal glass. The electrodes are
usually in the form of cylinders or
block and weigh between 5 - 50kg
each. A single furnace may contain in
excess of 100 such electrodes with an
on-line life of about one to two years,
A major application of tin chemicals in the glass industry invohes
the deposition of thin tin oxide films
cm to the surface of glass to impart
strength, m atch-wesist a w , decorative effects, electrical conductivity or
thermal insulation properties.
However, this use is not strictly
speaking an application of tin oxide,
since the films are produced by
spraying another tin compound,
usually tin(1V) chloride, butyltin
trichloride or dimethyltin dichloride,
cm to the hot glass surface where it
pyrolyses to form a layer nf SnO,.
Tin oxide films will not therefore be
discussed in this article, although it
is worth mentioning that recent
research has included the deposition
of films using sol-gel methods.
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Catalysts
Tin(IV) oxide is one of the components of a number of binary, ternary,
ox WFTI quaternary, metal oxide sys-
tems which find extensive use as heterogeneous catalysts in industrial
processes. Hence, combinations of
Sn02 with oxides of vanadium,
chromium, molybdenum, antimony
bismuth, phosphorus, copper, manganese, tungsten, platinum or
palladium, are designed to provide
catalysts which exhibit a wide range
of activity and selectivity, Although
accounting for only a small tonnage
consumption of tin, these catalysts
are of major importance in the
chemical. industry, being utilised in
the oxidation, ammoxida tion, oxidative dehydrogenation, isomerisation,
dehydration and methylation of
many organic compounds.
In addition, work carried out at
ITRI during the 1970s showed that
tin oxidebased catalysts are highly
effective for the low temperahe oxidation of carbon monoxide.
Although SrtQ itself has some activity in this respect, it is precious metal
- tin oxids systems which have
attracted most attention. One particular area of interest is the
removal of toxic gases from motor
vehicle exhaust emissions, where
the known effects of either palladium or platinum can be improved
if these metals are dispersed on a tin
oxide support, A further advantage
is that, unlike other low temperature CO oxidation catalysts which
are 'poisoned' by the presence of
water vapour, the catalytic activity
of the tin oxide-based systems
appears to be enhanced by water
vapour. However, commercial
usage of Pd/Pt - SnO, catalysts has
to date been limited to carbon dioxide gas lasers (where the catalyst
regenerates CO2 from CO and oxygen in order to maintain the power
output uf the laser) and in certain
air purification applications, such as
safety masks and in submarines.
Electruccmductive applications
The catalytic and electroconductive
properties of tin oxide are together
exploited in its well-established use
in gas sensors for fire alarm and pollution monitoring systems. These
devices, first developed in the earlv
1970s in Japan, usually comprise a
solid pellet or sintered layer uf SnO,
an a ceramic tube. The sensor c m
detect the presence of a combustible
or reducing gas by a change in electrical resis tame when such species
react with the oxygen absorbed cm
the SnO, surface. The decrease in
resistance is a measure of the gas
concentration and such sensors are
available fur the detection of carbon
monoxide, hydrocarbons, alcohol,
ammonia, hydrogen and. many other
Keeling &Walker Limited
Manufacturers of Tin Oxides for use in all applications
WHIELDON ROAD, STOKE-ON-TRENT ST4 4JA
TeI: (01782) 744136 Fax: (01782) 744126 E-mail: keeling.walker@btinternet.com
TIN International Vol. 72, No. 7
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gases. Tin oxide has remained at the
forefront of this tmhology €or mi-?
than two decades.
A more recent devdopmt t
relates to the use of conductive powders based an t-m oxide. Traditional
methods for imparting anti-static or
electroconductive properties to p l y merk materials have generally
involved the incorporation of relat i d y high loadings of either carbon
bIack or metallic powders. Since
these additives severely limit the
choice of colour for the end material
to essentially dark shades, a market
has developed for non-dissdourhg
conductive powders. Two distinct
types of product have been commercially available since the mid-1980s;
antimony-doped tin oxide (which is
highly conductive but blue-grey in
colour and relatively expensive) and
antimony-doped tin oxide coated on
to the surface of a lower cost inert
core particle. The latter type is available from several companies, with
core substrates including titanium
dioxide, mica, silica and potassium
titanate. Both types of product are
finding application in areas including elecbostsrtic discharge protection
(packaging, carpets, laminated plastic panels), conductive paints, didect-ncfilm and paper, and colour toners
in reprographic processes. An electromnductive toner powder based
on fluorine-doped tin oxide has been
developed during the 1990s.
Ion exchange agents
In its hydrous forms, tin oxide
exhibits ion exchange properties.
Unlike conventional organic resins,
SnO, gels are stable at: relatively high
temperatures and are resistant to
strong acids and nuclear radiation.
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This has made these materials suitable for chromatographic columns
used for the separation of radionuclides which are required in medical
techniques such as Positron
Errcission Tomography-In particular,
a-stannic acid has been used €or the
separation of rubidium-82 from
strontium-82, and of ruthenium-97
from irradiated molybdenum.
However, p-stannic acid is the pre
ferred form for the separation of gallium-48 from germanium-68. In each
case, the required radionuclide
passes through the column, whilst
the parent isotope from which it is
being eluted is quantitatively
absorbed on to the tin oxide gel.
Ion exchange properties, including selectivity and exchange capacity, are dependent on gel preparation
conditions and can be further controlled by combining hydrous tin
oxide with oxides of other elements
such as antimony, phosphorus,
boron, silicon or molybdenum. This
versatility has led to interest in tin
oxide-based ion exchangers as
potential effluent control systems,
for removing toxic metals from
industrial effluents and other pollution sources. Recent ITRI studies in
this field have focused on the development of effluent control system
in which the active tin oxide-based
ion exchanger is dispersed on a low
cost, high surface area substrate, in
order to improve its efficiency.
The ability of tin oxide to remove
metal inns has also k e n utiIised in
the petroleum industry where akaline 51-10,colloids are used as metal
passivating agents. Tin oxide is
found to be particularly effective for
removing trace levels of vanadium
and nickel, both of which can cause
problems during crude oil cracking
prwwses.
Other UT - 3
Sigdicant quantities of anhydrous
tin oxide have found use as 'putty
powder' for polishing marble, granite, glass and plastic lenses. An interesting offshoot of this application is
the polishing of pebbles by tumbling
them with powdered SnO, to make
costume jewellery.
Electrical contact materials produced by powder metallurgical techniques find extensive application
p a r t i d u l y in the field of power en@nwrhg, Untd recently the main type of
material used was a silver - cadmium
oxide compositeF but envimmmtal
concerns have led to the development
of alternative materials based on silver
- tin oxide. Composite materials containing 5 - 20% 5110,by weight, often
with up to 5% of other metal oxides
(e.g. those of tungsten, bismuth, tantalum, molybdenum, copper, gemanium, indium or gallium), are finding
increasing usage in contact studs for
low voltage switchgear.
New areas of application for tin
oxide are being explored all the tine,
some a€ them constituting potential
new consumption of only tens of
tomes of SnO, per annum, some very
much more+But it is evident that tin
oxide, used since antiquity, is still findh g new areas of application and will
ccjninue to be an impurtant industrial
material for many years to come.
Acknowledgement
The author would like to thank Mr
Andrew Spiller (Keeling & Walker,
Stoke-on-T-nt, UK) fur supplying
much of the source material used in
the preparaticm of this article.
The Complete Range of
'TEGO'
Tin Oxides
RuhraIIee 191 / Bonsiepen 8, 0-45138Essen
TeI: +49 (0201) 8943-498 Fax: +49 (0201) 8943-503
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TIN International Vol. 72, No. 7