Manufacturing Cordierite Mullite and Mullite Filters from Sanitary Ware Kiln Wastes for Use in Metal Casting Industry

Full text

Journal of Materials Science and Engineering B 8 (5-6) (2018) 129-135
doi: 10.17265/2161-6221/2018.5-6.005
Manufacturing Cordierite Mullite and Mullite Filters from
Sanitary Ware Kiln Wastes for Use in Metal Casting
Industry
Eray Çaşın1, İskender Işık2 and Mustafa Yaman1
1. Ece Banyo Gereçleri San. ve Tic. A.Ş., Çorum 19100, Turkey
2. Metallurgy and Material Science Engineering, Dumlupınar University, Kütahya 43100, Turkey
Abstract: The metallic melts shaped with casting method include impurities within the product. The ceramic foam filters are used for
removing these impurities. Proverbially, the mullite based filters is suitable for high temperature applications as its technical features
such as low coefficient of thermal expansion, chemical and mechanical stability at higher temperature and thermal shock resistance.
Due to these properties, mullite is the potential matrix material for relatively higher temperature applications. In this study, the
wasted sanitary ware kiln materials are utilized to investigate as the source of material for mullite and cordierite mullite ceramic filter.
In this context, Ece Banyo Sanitary Ware Plant’s kiln roller materials and kiln plates are used (both of which are cordierite-mullite
and alumina mullite) in the manufacturing ceramic foam filter. This work is a preliminary study about how to utilize wasted kiln
materials in order to manufacture value-added materials. Physical, chemical, mineralogical, thermo gravimetric, micro-structure and
sintering behavior tests are made over these prescription samples. Also these tests have been made for nowadays marketing
filters-mullite and cordierite mullite-for benchmarking. On the basis of the test results, the best suitable prescription specimen is
being selected and shaped with foaming process. Furthermore, the shaped filters are being tested in casting process. It is concluded
that a mullite-cordierite based foam filter production which can be used in the metal casting industry is proven to be realized by
recycling mullite and cordierite-mullite based ceramic materials that are used in the ceramic sanitary ware industry.
Key words: Sanitary ware, cordierite, mullite, waste recovery.
1. Introduction
Mullite, which is rarely natural phase, is the most
common solid phase of alumina and silicate. It is
preferred in high temperature applications because of
its low coefficient of thermal expansion, high
chemical stability and mechanical toughness. It is the
most important technologic material which is the
mid-phase of Al2O3-SiO2 system. In general, it can be
produced as transparent, translucent and opaque.
The mullite—3Al2O3·2SiO2—is the silicoaluminous
refractory material for excellence. Its high melting
point (≈1,800 °C), good thermal expansion coefficient
(≈3-5 × 10-6 °C), and excellent characteristics to
Corresponding author: Eray Çaşın, Ph.D., ceramic
engineer research fields: ceramic materials, raw materials,
mineralogy, sintering reactions in ceramic materials, refractory
materials, ceramic processes, re-using.
support moderate thermal shocks make it irreplaceable
in most of silica-alumina refractory formulations [1].
The different crystal phase transformation shows up
in ceramic suspension depending upon thermal
efficient and mineral concentration. Kaolinites
transform into meta-kaolinite at 500-600 °C. Plasticity
is changed due to endothermic reaction at this stage.
The composed meta-kaolinite transforms to mullite
and cristobalite with strongly exothermic reaction at
830 °C. The terminal breakdown pressure of the
anhydrous phase is form to enstatite, sillimanite and
quartz at 830 °C [2].
Cordierite and mullite materials have high
sensitivity, elevated thermal and chemical resistance
and stability, very low coefficient of thermal
expansion with features to be used as high temperature
application materials. Also they can be used as
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Manufacturing Cordierite Mullite and Mullite Filters from Sanitary
Ware Kiln Wastes for Use in Metal Casting Industry
130
variable temperature application.
The mullite crystal system is orthorhombic and
needle like crystal shaped structure in (001) plane. In
Fig. 1, the SEM image of mullite crystals originates,
indicating the secondary mullite crystal formations in
vitreous phase.
Mullite is used with high temperature applications
due to its low thermal coefficient of expansion and
high thermal shock resistance. Also mullite has high
mechanic resistance and chemical stability at high
temperature [3].
Cordierite is the important phase for
MgO-Al2O3-SiO2 phase system. It has low coefficient
of thermal expansion, perfectly thermal shock
resistance, high chemical strength, high refractoriness
and high mechanical strength. The sol gel and glass
crystallization methods are used in cordierite synthesis.
The most commonly used methods are solid phase
reaction synthesis and crystallization of glass dusts.
High purity cordierite is synthesized with chemical
methods like sol-gel.
Metal cast filters are sold on the market in two
ways. One of these is used as a honey pellet and is
mainly used to provide melt flow into the melt in the
mold. The melt-flowed molten metal is poured into
the mold without causing any layer or air defect, and
the surface quality of the product is better than that of
products that are not used in the filter-free molds.
Another type of filter is foam filtration. The main
task of foam filters is to remove the inclusions in the
molten metal rather than making the flow into a
laminate. In this regard, the melt ensures that the slag
and non-metallic materials in the metal are retained in
the metallic material without mixing and that a high
quality product is obtained.
Ceramic foam filter is a product with low bulk
density, high porosity and three-dimensional web like
structure. Ceramic foam filter has excellent properties
such as high temperature resistance, strong chemical
corrosion resistance, and large surface area as a result
of high porosity; it is widely used in molten metal
filtration to remove undesirable nonmetallic inclusions
in the melt.
Metal casting process begins by creating a mold,
which is the reverse shape of the part. The mold is
made from a refractory material such as silica sand.
Silica sand is usually used in mold production. The
metal casting mold has a feeder which is used for
molten metal to enter into mold. The feeder includes
filtration part which is needed for ceramic filter (Fig. 2).
Molten metal presents many opportunities for
contamination, often with particles as small as a few
microns. This contamination may be slag, dross, or
pieces of refractory from the melting crucible. Metal
that contains impurities will reduce strength, and this
usually requires a heavier section thickness to
compensate for the lower strength. Impurities also
present serious stress points if they are located on the
surface of the castings that are subjected to
mechanical forces.
Fig. 1 SEM image of secondary needle like mullite phase originated in vitreous phase.

Manufacturing Cordierite Mullite and Mullite Filters from Sanitary
Ware Kiln Wastes for Use in Metal Casting Industry
131
Fig. 2 The view of metal casting mold.
Fig. 3 The view of metal casting process and inclusions over ceramic filter [4].
Filters will collect dross particles and inclusions
that are larger than the filter hole or pore size on their
upstream face. These particles are unable to pass
through the casting cavity due to their physical size
[4].
Metal casting foam filters are used for filtration in
metal casting process. This foam filters provide
smooth surface due to laminar flow of metal melt.
Also filters filtrate the inclusions in metal melt.
Metal casting filters are shaped as honeycomb,
foam and cellular. Ceramic foam filter is the most
common and efficient used in metal casting industries.
Foam filters gained advantages due to its 3D network
shape as high porosity clinker collection, flat surface
area, low flow resistance and good filtration. Ceramic
foam filters create a simplified feeder to melted metal.
Also it regulates the metal product’s metallurgic
structure and perfect surface quality after engraving
process [3].
Ceramic foam filter’s chief production method is
polymer foam shaping method. In this technique, the
polymer foam dips into ceramic suspension and makes
a burning process before sintering. The polymer
foam’s surface quality is important for continuous and
smooth coating. The adhesive materials are used for
ceramic suspension stick over the foam material’s
surface (Fig. 3).
The ability of a ceramic filter to remove inclusions
is obviously an important parameter. Ceramic filters
are generally more efficient at removing micro
inclusions than traditional methods such as extensive
running systems and whirl gates (Table 1) [3].
The ceramic foam filters requirements are as follow:
 Excellent stiffness and toughness,
 Filtrate the ashes and clinkers in melted metal,
 Optimum mesh size for permeability,
 Reduce molten metal turbulence and improve
molten metal distribution and flow.

Manufacturing Cordierite Mullite and Mullite Filters from Sanitary
Ware Kiln Wastes for Use in Metal Casting Industry
132
Table 1 Advantages and disadvantages of ceramic foam filters [4].
Advantages Disadvantages
High porosity percentage Expensive
The filtration of inclusion increase because of its network shaped Contamination in melted metal because of its thin network shape
and sharp edges
Chemical filtration
Screening efficiency is high
Fig. 4 Roller kiln’s cordierite roller materials [5].
2. Experimental Procedure
In this study, the waste sanitary ware kiln materials
are used for mullite and cordierite-mullite foam filter.
Within this scope, Ece Banyo Gereçleri San. ve Tic.
A.Ş.’s which is the 3rd biggest sanitary ware
manufacturer in Turkey, cordierite and mullite wasted
kiln materials are utilized. The roller bars and kiln
plates are checked routine and these materials are
replaced with new ones. This process takes nearly 3
months for all the materials to be replaced. Also the
cordierite-mullite plates used in roller kilns replaced
nearly 1 year. These materials are grained and used in
foam filter manufacturing (Fig. 4).
Ece Banyo has 3 roller sanitary ware kilns and one
tunnel kiln in its production area. The kiln materials
used in this kilns are replaced periodically. The
materials are changed due to abrasion and cracking.
First, these materials are grounded into ball mill and
dried in dryers. The cordierite-mullite dusts are used
in ceramic foam filter suspension. Ten different filter
prescriptions are being prepared and rheological,
physical, mineralogical, chemical, thermal and
micro-structure analyses are done.
The market filter has been supplied by the People’s
Republic of China lately. Although these filters are
disposable and partly portable, they are costly for
users due to freight costs and shipping costs. Iron
casting companies using these filters are limiting their
use of filters due to high freight rates.
First of all, the literature search has been done. In
this period, the technical specialties and scientific
articles and studies are searched. Also the market
cordierite and mullite based filter’s chemical,
mineralogical, micro-structures and sintering
behaviors are/will be examined. The metal casting
companies are/will be investigated in Çorum province.
The right prescription is chosen against market
filter’s characterization results. The chosen
prescription’s rheological and physical specifications
are done based on ceramic casting suspension input
control test method. These methods are as follow:
 Determination of density test;
 Determination of viscosity test;
 Determination of dry and fired shrinkage;
 Water absorption;
 Loss of ignition;
 Deformation test;
In the second period of this project, the prescription

Manufacturing Cordierite Mullite and Mullite Filters from Sanitary
Ware Kiln Wastes for Use in Metal Casting Industry
133
studies are made for filter manufacturing. The raw
materials used in prescription are determined.
The filter’s prescriptions are sintered at different
temperatures in experimental kiln in Ece Banyo.
Sintered materials will determine the mineralogical
changes of foam filtrate at different temperature
ranges. Also the sintering behavior tests are made with
optical dilatometer.
These tests are performed in Ece Banyo R&D
Center’s Ceramic Materials Development Laboratory.
Chemical, thermal and mineralogical tests are/will be
made in Ceramic Research Center in Anadolu
University. Sintering behavior and thermal expansion
tests are/will be made in Ece Banyo.
The characteristics of the mullit-based ceramic
foam filter to be produced are/will be the same as the
mullit-based ceramic filters on the market. The aim of
the project is to produce a value-added product to
reduce the input costs.
About 21 tons of cordierite mullite bars and 8 tons
of mullite kiln plates cast away to wastage per year.
The cost percentages of these materials are calculated
as 150 k€ which is relatively high in budget.
The prescriptions are/will be made based on
cordierite based kiln plates and alumina based kiln
rollers. Also a plastic clay and kaolinite based kaolin
are/will be used to bond the refractory based materials
in suspension. The kaolinite based raw materials have
high Al2O3. Also plastic clays have some impurities
which are Fe2O3 and TiO2. In literature, the cordierite
mullite based filters have nearly 6% of MgO and
40-45% Al2O3 and SiO2. Clays and kaolin based raw
materials are suppressed by the cordierite formation.
The calcined magnesia is/will be used to generate
cordierite inside the glassy phase. Other MgO based
materials are/will be used such as dolomite or talc.
The properties of the various raw materials used in
the prescriptions will be examined with the
cordierite-based refractory materials used as the
recycling material. The sintering attempts to be made
in this context will be between 1,230 and 1,290 °C,
which is the formation temperature range of the
secondary mullites.
Ceramic sanitary ware materials have a sintering
temperature of approximately 1,200-1,230 °C. Tunnel
kilns are usually used in bigger production sites. The
burner array in the tunnel kilns is placed in the lower
parts of the kiln cars and in the upper parts so as the
fire will not come into the products. In this type of
kiln design, the temperatures in the lower parts of the
kiln cars are 1,250-1,270 °C. Cordierite-based foam
filters to be produced within the scope of the study are
sintered in the ceramic sanitary ware production
furnace according to the sintering temperature
resulting from the experimental works and the desired
structure. Thus, no energy costs will be incurred for
the construction of the filter.
Mullite is a Al2O3-SiO2 based material which is
generated at high temperature with these oxides. The
secondary mullite phases begin to form within the
vitreous phase in a hexagonal loop at 1,230 °C. The
secondary mullite phases are mainly due to meta
kaolin transformation. Secondary mullite phases are
intimately related to the presence of the glassy phase.
The secondary mullite crystal’s height growths in
glassy phases are depended on grain size distribution
of mullite particles.
The middle part of a ceramic filter is depended
pressure imposed by the molten metal flow. Filter
edges are embedded in the mold and are not in contact
with the molten metal. The density of the middle part
of the filter is different from the edges and is
commonly referred to as the filter bulk density [6].
The bulk density of ceramic foam filter will be
obtained due to sample edges.
In third part of the project, the produced foam
filter’s characterization specialties will be compared
with market filters. The experiments will be carried
out in a factory that performs metal casting by taking
samples from the produced ceramic foam filters. The
parameters that will be observed in these process are:
 Filtration productivity of ceramic foam filter,

Manufacturing Cordierite Mullite and Mullite Filters from Sanitary
Ware Kiln Wastes for Use in Metal Casting Industry
134
Fig. 5 XRD patterns of the prescription.
 Deformation in thermal heaviness,
 Thermal strength of ceramic filter,
 Maximum temperature strain,
 Blockage of ceramic filters pores with melted
metal,
 Thickness of impurities over foam filter.
The shaping and modeling process of ceramic foam
filter are difficult and important. Because of filtration
efficiency, filtrating part’s particle size and blockage
of filter’s pores are depended to the particle size and
modeling of the filter. The ceramic foam filters mesh
size will determined during the project due to casting
tests.
Cordierite, mullite and corundum phases were
encountered in the market filter where characterization
analyzes were made (Fig. 5). According to chemical
analyzes, the MgO ratio is sufficient with respect to
cordierite formation. Based on these results, recipe
studies will be carried out and a metal casting foam
filter will be made which will be the same feature. The
thermal expansion coefficient values, resistance
temperatures and pore structure of the prepared filter
will be examined and the metal casting industry will
be tested.
According to the characterization analysis of the
first recipe studies, it was observed that cordierite
formation was supported due to the increase of MgO
ratio in the prescriptions enriched with calcined
magnesite. Especially when the results of the chemical
analysis and the results of the market filter analysis
are compared, it is seen that the percentage of the
MgO ratios is similar.
In prescriptions using dolomite (CaO-MgO-SiO2),
shrinkage values and glow losses were found to
increase due to the presence of CaO in raw material
and recipe.
3. Conclusion
After this work, a mullite-cordierite based foam
filter production which can be used in the metal
casting industry will be proven to be realized by
recycling mullite and cordierite-mullite based ceramic
materials that are used in the ceramic sanitary ware
industry. In this respect, it will be ensured that the
industrial wastes taken into the environment will be
evaluated to protect the environment and become a
highly usable product with added value. The input
costs (excluding labor and grinding costs) of a foam
filter were calculated to be 0.1 €. Mullite-cordierite
filter is imported between 0.5 € which is more
economical.
Cordierite mullite based filters are used in grey iron,
ductile iron, aluminum alloy and non-ferrous alloys
casting. The expected maximum work temperature is
about 1,500 °C. This gives the high temperature
resistance and high thermal shock resistance.
The right prescription was selected due to
prescription tests. Foam materials were supplied from
the market. Based on the FFC slip rheology used in
laboratory-based rheology analysis operations, the
FFC slip will form the foam material and will have the
ability to obtain a better thickness and homogeneous

Manufacturing Cordierite Mullite and Mullite Filters from Sanitary
Ware Kiln Wastes for Use in Metal Casting Industry
135
shape in different rheological properties. After the
determined prescription is prepared in laboratory and
done in a large scale, the rheological values
determined in the rheology studies will be applied and
shaped on the foam material.
Foam filters will be sintered at the sintering
temperatures determined in the sanitary ware sintering
furnaces of Ece Banyo. Sintered products will be
tested in metal casting companies and their usability
in the sector will be determined.
“Development of alumina-mullite and mullite filters
for metal casting industry from fired ceramic sanitary
ware wastes” project was done in 2015 at Ece Banyo
R&D Center supported by TUBİTAK. This project
was planned to be finalized in 2018. All experiments
were finished. The casting tests indicate that
honeycomb filter which is produced with sanitary
ware wastes has perfect filtration productivity. The
casting temperature was at 1,386 °C which was not
deformation after casting.
Ece Banyo R&D Center started its activities in
2015 with the approval of Ministry of Science,
Industry and Technology. The activities are
continuing with 20 researchers, 12 technicians and 2
support staff. Since the establishment of Ece Banyo
R&D Center which has knowledge and experience on
ceramic materials, high technology ceramics,
innovation of ceramic materials, product design,
design and development of ceramic machines, have
been started in-house projects and ministry supported
projects. The projects outcomes are started to use or
marketing from these projects.
Acknowledgment
This project is a project made within the scope of
TÜBİTAK 1501. Dumlupınar University Ceramic
Engineering Department is also a work done in PhD
program. The cordierite mullite based filter produced
by recycling technique, the prescriptions and recycling
process will be protected by Ece Banyo Gereçleri A. Ş.
Recycling materials, recycling techniques, recipe
operations and shaping procedures will be carried out
within the premises of Ece Banyo Gereçleri A. Ş. and
Dumlupınar University and the work done is
confidential. Any articles to be taken from this article
should be taken with permission. All legal, intellectual
property rights are on Ece Banyo Gereçeleri San. ve
Tic. A. Ş. All rights are reserved.
This study has been submitted and supported by
TUBİTAK TEYDEB 1501.
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