Download Feldspar

MME391 Ceramic
Materials
Asst. Prof. Dr. Ayşe KALEMTAŞ
Bursa Technical University, Metallurgical and Materials Engineering Department
Office Hours: Tuesday, 10:00-11:00
[email protected], [email protected]
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
Clay Products – Main Components
Clay
Feldspar
Silica
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
Clay
When mixed with water
the crystals can easily
slide over each other
(like a pack of cards),
and this phenomenon
gives rise to the
plasticity of clays.
Provides plasticity,
when mixed with
water Hardens upon
drying and firing
(without losing the
shape)
Adding water to clay
-- allows material to
shear easily along weak
van der Waals bonds
-- enables extrusion
-- enables slip
casting
Clay products – Main Components
Silica
Silica, SiO2, is mixed
with clay to reduce
shrinkage of the ware
while it is being fired,
and thus prevent
cracking, and to
increase the rigidity of
the ware so that it
will not collapse at
the high temperatures
required for firing.
Silica is useful for this
purpose becasue it is
hard, chemically
stable, has a high
melting point and can
readily be obtained in
a pure state in the
form of quartz.
Feldspar
Feldspars are used as a
flux in the firing of
ceramic ware. When a
body is fired, the
feldspar melts at a
lower temperature
than clay or silica, due
to the presence of Na+,
K+ or Ca2+ ions, and
forms a molten glass
which causes solid
particles of clay to
cling together: when
the glass solidifies it
gives strength and
hardness to the body.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
Fluxes are raw materials with high amount
of alkaline oxides, mainly K2O and Na2O,
whereby, in reaction with silica and alumina,
promote
liquid
phase
formation
that
facilitates the densification.
The liquid phase surrounds the solid particles
and
by
surface
tension
enables
rearrangement of particles and decreasing
the porosity.
Feldspars are used as a flux in the firing of
ceramic ware. When a body is fired, the
feldspar melts at a lower temperature than
clay or silica, due to the presence of Na+ , K+
or Ca2+ ions, and forms a molten glass
which causes solid particles of clay to cling
together: when the glass solidifies it gives
strength and hardness to the body. The
molten glass also reacts with the silica and
clay particles.
Feldspar
Feldspars are used as a
flux in the firing of
ceramic ware. When a
body is fired, the
feldspar melts at a
lower temperature
than clay or silica, due
to the presence of Na+,
K+ or Ca2+ ions, and
forms a molten glass
which causes solid
particles of clay to
cling together: when
the glass solidifies it
gives strength and
hardness to the body.
Asst. Prof. Dr. Ayşe KALEMTAŞ
ORES AND MINERALS
Earth’s Crust
Earth’s crust refers to the outher siliceous shell of
the earth which is about 16 km thick.
Metallic ores and mineral products are produced
from this crust. Therefore, the original source of
all metals is the earth’s crust.
Ores are naturally occuring deposits in the
earth’s crust. The ores are mined and treated
by various
• unit operations (mechanical processes)
• unit processes (chemical metallurgical
processes)
to extract metals and to convert them into
the metallic form.
Mine : It is a large area having
an abundant quantity of
mineral deposits that can be
easily and economically
extracted.
The mineral content of the ore must be in sufficient
concentration to make its extraction commercially
viable.
Asst. Prof. Dr. Ayşe KALEMTAŞ
ORES AND MINERALS
Minerals are the constituents of rocks, which make up the
entire inorganic, solid portion of the earth. Rocks are usually
not composed of a single mineral but, rather, an aggregate of
two or more minerals. Broadly speaking, geologists divide
rocks into three types:
 igneous,
 metamorphic, and
 sedimentary.
Rocks:
• They are a combination of minerals along with impurities.
A rock can contain either a single mineral or a number of
minerals.
• Although over 2000 minerals have been discovered, only
a few are abundantly found in most of the rocks.
Asst. Prof. Dr. Ayşe KALEMTAŞ
ORES AND MINERALS
Igneous rocks form when magma cools and solidifies. Magma is a complex
molten material that originates deep within the earth.
The word igneous comes from the Latin word ignis, which means “fire”;
igneous rocks then are “formed from fire.”
Magma is rich in the elements Si, O, Al, Na, K, Ca, Fe, and Mg.
Feldspars are alumino-silicate minerals found in nearly all igneous rocks.
Metamorphic rocks have undergone structural and/or chemical transitions
(metamorphism or metamorphosis) from their original form as a result of high
temperatures and pressures deep beneath the earth’s surface. These transitions
occur in the solid state without melting and result in the formation of new
minerals, such as kyanite, staurolite, sillimanite, andalusite, and some garnets.
Other minerals, such as some of the igneous minerals, may be present in
metamorphic rocks but are not necessarily the result of metamorphism. The
word “metamorphic” has a Greek origin coming from meta, meaning
“change,” and morpe, meaning “shape”.
Asst. Prof. Dr. Ayşe KALEMTAŞ
ORES AND MINERALS
Sedimentary rocks are formed when small particles or precipitated
crystals become cemented together. Sedimentary rocks are classified as
either clastic or chemical.
Clastic sedimentary rocks form when rock particles produced by
mechanical and chemical weathering are transported by water, ice, and
wind to new locations where they become cemented together.
Chemical sedimentary rocks form when highly soluble ions, such as, Na+,
K+, Ca2+, Mg2+, Cl-, F-, (SO4)2-, (CO3)2-, (PO4)3- from existing rocks are
dissolved in water and subsequently precipitate, forming layers in
oceans and lakes, where they become cemented together.
The composition of sedimentary rocks depends on:
▪ Composition of original source rocks
▪ Chemical and mechanical resistance of each mineral
component
▪ Distance traveled
Asst. Prof. Dr. Ayşe KALEMTAŞ
ORES AND MINERALS
Resistant minerals such as quartz are common constituents of
sedimentary rocks, and some more rare minerals (e.g., garnet, rutile,
zircon) have similar properties.
cooling and
solidification of
magma
Rocks that have
undergone
structural
and/or chemical
transitions
Sedimentary
Rocks formed by
Metamorphic
Igneous
Feldspar is less resistant but is so common that it is a major
constituent of many sedimentary rocks. Precipitated minerals include
the carbonates (e.g., calcite, dolomite), the sulfates (e.g., gypsum,
anhydrite), chlorides, and chalcedonic silica (e.g., chert, flint).
Rocks formed
when smaller
particles become
cemented
Asst. Prof. Dr. Ayşe KALEMTAŞ
Average Analysis of the Earth’s Crust
It is clear that :
 Oxygen
 Silicon
 Aluminum
 Iron
are the 4 most
abundant elements
Most common
elements in
the crust
Since engineering metals Al, Fe,
Mg and Ti are far more
abundant
than
the
other
metals, there is never likely to
be a shortage of those metals
due to exhaustion of the ore
deposits.
The commonly known metals
like Au, Pb, Sn are in so small
quantities in the earth’s crust
that their commercial recovery
is impossible.
Asst. Prof. Dr. Ayşe KALEMTAŞ
A group of aluminosilicates, tetrahedra form
three-dimensional
frameworks with Ca, Na
and K as the balancing
cations.
Feldspars
Feldspars
Feldspars
The very abundant
feldspar are subdivided in
K-Na bearing alkali
feldspars and the Ca-Na
solid-solution series called
the plagioclase feldspars.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Feldspar Group
light silicates (K-Na-Ca, Al)
K-feldspar
Most common mineral group
Orthoclase
Plagioclase
2-directions
of cleavage
(at 90 degrees)
Ca/Na-feldspar
Asst. Prof. Dr. Ayşe KALEMTAŞ
hard
K-feldspar Most
Has
multiply
colors
Feldspars
Is very
Feldspars
Feldspars
Feldspars
common
mineral
group
orthoclase KAlSi3O8 ...
... weathers to kaolinite:
Al2Si2O5(OH)4
... which can
weather further
to bauxite:
AlO(OH)
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
of ceramic ware.
Feldspar produces a
glassy phase during
firing increasing the
strength and
translucency of the
Feldspars are
alumino-silicate
minerals found in
nearly all igneous
rocks.
Feldspars
a flux in the sintering
Feldspars
Feldspars
Feldspars are used as
The plagioclase
feldspars are solid
solutions of albite
and anorthite.
Na+ Si4+ is replaced
by Ca2+ Al3+
body.
are solid solutions
of albite and
orthoclase.
Na is replaced by K
There is virtually no
solid solution
between anorhite
and orthoclase.
Feldspars
The alkali feldspars
Feldspars
Feldspars
The glass industry uses
most of the feldspar
produced. Feldspar is a
source of Al2O3, which
improves the
mechanical properties
of glass such as its
scratch resistance and
its ability to withstand
thermal shock.
Asst. Prof. Dr. Ayşe KALEMTAŞ
minerals are likely
to enter the
composition of
silicate ceramic
pastes. They are:
Orthoclase, a
mineral rich in
potassium with the
composition
K2O.Al2O3.6SiO2
Feldspars
Four feldspathic
Feldspars
Feldspars
Feldspars
Albite, a mineral
rich in sodium
with the
composition
Na2O.Al2O3.6SiO2
rich in calcium with
the composition
CaO.Al2O3.2SiO2
Petalite, a mineral
rich in lithium with
the composition
Li2O.Al2O3.8SiO2
Feldspars
Anorthite, a mineral
Feldspars
Feldspars
Feldspars constitute
an abundant mineral
group and make up
an estimated 60% of
the earth’s crust. They
are present in many
sedimentary deposits
and are found in
almost all igneous and
metamorphic rocks.
Asst. Prof. Dr. Ayşe KALEMTAŞ
occur in nature as
pure minerals.
Albite and anorthite
form a complete
solid solution series.
Feldspars
Feldspars rarely
Feldspars
Feldspars
Feldspars
Even though
orthoclase and albite
form only limited
solid solutions,
deposits of
orthoclase always
contain some albite.
syenite is a mixture
of orthoclase, albite
and nepheline with
minor impurities.
typically ground to
a relatively coarse
powder, on the
order of 70 to 100
m, for use in
Feldspars
The rock nepheline
Feldspars
Feldspars
These materials are
The Republic of
Korea is the largest
producer of feldspar
in the world.
ceramic (or glass)
production.
Asst. Prof. Dr. Ayşe KALEMTAŞ
albite, which form
eutectics with
silica, respectively,
at 990 and
Anorthite is rather
regarded as a
substitute to chalk.
1050°C, are
Natural feldspars used for the
3.5% of albite mass, whereas
anorthite and a small quantity
of orthoclase, between 0.5 and
3.2%, are often present in the
available sodium feldspars.
Feldspars
Feldspars
can contain between 2.5 and
especially owing to
its negative
expansion
coefficient, is
Potassic feldspar is particularly
preparation of ceramics are
commercial potassium products
The use of petalite,
marginal.
widely used as flux.
mineral mixtures. Thus, the
Feldspars
Orthoclase and
Feldspars
Feldspars
Feldspars
appreciated by ceramists because its
reaction with silica leads to the
formation of a liquid whose
relatively high viscosity decreases
slightly when the temperature
increases. This behavior is
considered as a guarantee against
the excessive deformation of the
pieces during the heat treatment.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars


The tendency to form a glass is strongly correlated to the viscosity of a melt.
In general, molten feldspars are rather viscous which is ascribed to the existence of
polymerized silicon-aluminum-oxygen tetrahedra in the liquid.
Despite lower melting points, the alkali feldspars produce much more viscous liquids
than anorthite.

In the case of albite this is interpreted as evidence for a higher degree of
polymerization in the melt.



In the case of orthoclase it is due to the formation of leucite, KAlSi2O6, crystals.
In all cases glasses are produced under the cooling rates normally
ceramic processing.
encountered in
Albite melts at the lower temperature than orthoclase, but the addition of anorthite
increases the melting temperature of soda feldspar while decreasing that of the potash
feldspar (down to a minimum at about 22% anorthite). Similarly a 50%: 50% mixture
of albite and orthoclase melts at a lower temperature than either end member.

Often mixtures of fluxes are employed in order to take advantage of
melting.

eutectic
Lithium
bearing minerals are often very effective fluxes when used in conjunction with
feldspar since such combinations form deep eutectics.
Asst. Prof. Dr. Ayşe KALEMTAŞ
temperatures or in
the presence of a
large quantity of
primarily the result
of the use of raw
materials free from
iron and titanium or
flux are generally
Feldspars
the least porous.
containing only
as well as the green strength, but often
leads to discoloration as a result of
contamination by iron-bearing
opposite end of the
spectrum where aesthetics
take priority. China clays
are used in these
formulations, because they
are nearly phase pure and
bearing solid solutions (as
transition metals.
clay generally improves the plasticity
Fine china represents the
do not occur as iron
small contents of
Increasing the relative amount of ball
Feldspars
treated at higher
Whiteness is
Feldspars
The materials
Feldspars
Feldspars
Feldspars
do the smectites and
illites).
The extent of glass formation affects
properties such as dimensional stability
and degree of densification. In systems
which require high dimensional
stability such as structural clay
products (i.e., large ceramic pipes and
accessory minerals. Therefore
tiles) the extent of glass formation is
applications where green strength is at
kept to a minimum. In contrast,
a premium, and color is of less
dental porcelains must fuse at low
importance, employ larger amounts of
temperatures to be compatible with
ball clay.
metal substructures and therefore may
contain in excess of 80% feldspars.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
The greatest consumption of
ceramics industries because
of their chemical
composition, particularly for
their alkali content. The
consumption of feldspars
within these two industries
Feldspar is also used in
fillers and abrasive
industries but not as
much as in the ceramic
and glass industries.
Feldspars
used in the glass and
Feldspars
Feldspars
Feldspars are commercially
feldspar raw materials is in
ceramic products. Ceramic
products comprise sanitary
ware, ceramic tiles,
dinnerware, ceramic glazes,
and electrical porcelain. In
ceramic industries feldspar
is a basic component in the
is estimated at 85–90 % of
raw material batch for
the market.
ceramic bodies.
The typical feldspar content
wt.% in white ware, chemical
porcelain and hotel china,
25–35 wt.% in sanitary
ware and 30–50 wt.% in
electrical porcelain. Feldspar
is also used in ceramic glazing
in amounts ranging from
30–50 wt.%; by providing
alumina and alkali it also acts
as a flux in fine ceramics.
In pottery, a high amount of
In addition, feldspar also has
various significant usages such
as for ceramic shields. The
NASA space programme has
used a ceramic thermal shield
to protect spacecraft during
the re-entry to the
atmosphere.
Feldspars
10–55 wt.%, from 15–30
Feldspars
Feldspars
in floor and wall tiles is from
potash feldspar increases
their strength. Potash
feldspar is also involved in
the ceramic materials which
increase the efficiency of gas
turbines. A particular
application that consumes a
significant amount of feldspar
is porcelain industry, which
uses 60–80 %.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Beneficiation
Beneficiation is the process that most minerals need to go through before
they can be used in producing ceramics.
Physical beneficiation includes crushing and grinding of coarse rocks. The
particle size of the raw material may affect subsequent steps in the
production process. An example that we use is producing alumina from
bauxite, a process that involves a chemical reaction.
Chemical beneficiation includes processes of separating the desired mineral
from unwanted waste products—for example, by dissolution in a suitable
solvent followed by filtration. The Bayer process for producing alumina is
also a good example of chemical beneficiation. Bauxite contains many
impurities. The purity of the raw materials is reflected in the composition of
the final product. For many ceramics, careful control over purity is
required. For these applications, the raw materials are synthesized.
Furthermore, several important ceramics do not occur naturally in mineral
form and must be fabricated chemically. Synthesis of ceramic powders can
have advantages not only in purity but also in allowing the generation of
fine particle sized powders having a well-defined morphology.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
• Structural clay products : bricks, sewer pipe, roofing tile
• Triaxial bodies: Whiteware, porcelain, chinaware, sanitary ware.
Asst. Prof. Dr. Ayşe KALEMTAŞ
Traditional Ceramics
Triaxial whiteware chemical composition
Asst. Prof. Dr. Ayşe KALEMTAŞ
Feldspars
most abundant
forming minerals in
the nature. They
are components of
many rock types,
including:
pegmatite, granite,
syenite (especially
nepheline syenite),
feldspathic sand
and sandstone
(arkose).
Feldspars
Feldspars
group of rock-
For example, in the
value of feldspars rests
production of ceramic
on their chemical
wares of high
composition, especially
whiteness (porcelain),
on the content of
the content of Fe2O3
alkalis, i.e. K2O (min.
10%) – preferred in
the production of
whiteware, or Na2O
(min. 7%) – in glass
and ceramic tile
Feldspars
Feldspars are the
The major commercial
and TiO2 cannot
exceed 0.15% and
0.05% respectively,
while in the case of
high quality glass –
max. 0.08% Fe2O3 is
manufacture. In the
acceptable. The term
majority of
‘feldspar raw material’
applications the lowest
or ‘feldspar’ refers to
possible content of
both K and Na
colouring oxides, in
feldspar-rich grades as
particular of Fe2O3
well as to feldspar-
and TiO2, is required.
quartz ones.
Asst. Prof. Dr. Ayşe KALEMTAŞ
QUESTIONS
Asst. Prof. Dr. Ayşe KALEMTAŞ