1
Geological background and site
For over
135 years, the family-run company Strobel Quarzsand GmbH has been extracting
kaolinitic silica sands and processing them to high-quality raw materials for
the glass and construction chemical industries. The silica sand plant is
located north of Amberg in the Upper Palatinate, Bavaria. Geologically, the
deposit lies in the outcrop of what is known as the Dogger Sandstone, which
can be attributed to the Brown Jurassic, especially the Dogger beta. The
formation of the silica sands goes back to delta-like coastal deposits around
170 mill. years ago. Starting from crystalline source areas from the area of
the Bohemian Massif, the fine-grained silica sands were deposited together with
kaolinitic fines in a thickness of just under 100 m. The deposit reserves total
around 150 mill. tonnes. On account of the generally low iron content, whitish
grey sediment colours dominate. On a minor scale, there are yellowish as well
as pinkish red to intensive red colours. During washing of the silica sands,
the fines < 40 µm are deslimed and dewatered on modern membrane filter
presses to a residual moisture content of around 20 mass %. Here considerable
quantities of kaolin filter cakes are produced as a recycled product with
long-term assured supply. The filter cake has a plastic consistency without any
problematic coarse components, impurities or organic substances.
2 Mineralogical
composition
The filter
cake has a well-graded particle size distribution with an average fines content
of d < 2 µm = 39 mass %. The content d > 63 µm averages 20 mass %. In
respect of the mineral phases contained, the filter cake features a considerable
phyllosilicate content with ∑ TM = 50 mass %, well-ordered kaolinite
significantly dominating compared to illite/mica. Innercrystalline swelling
clay minerals are not detected. With regard to the series of crystalline
weathering residue, quartz is the only phase. Iron is present as an accessory
phase in the form of haematite. In respect of the element distribution, silica
dominates clearly in front of alumina and iron oxide. The filter cake has an
exceptionally low content of alkaline and alkaline earth flux. Potassium
dominates with 0.38
mass % in
front of magnesium with 0.1 mass % and calcium with 0.07 mass %. Sodium is
below the detection limit of 0.03 mass %. The content of organically bonded
carbon is also very low and amounts to TOC = 0.09 mass %.
3 Ceramic
characteristics
The filter
cake is characterized by a soft to stiff plastic consistency and can be used in
conventional brickwork preparation plants. It exhibits slightly plastic
properties and can be dried without any problem. In the firing process, the
filter cake first demonstrates an exceptionally low release of OH groups from
adsorption and interlayer water. Up to a range of around 400° C, the material
is therefore very firing-insensitive. Typical for the content of kaolinite are
the clear endogenous peak of the water of crystallization at 533° C as well as
the exothermal reformation reaction at 990° C. Already at around 700° C, the
maximum weight loss of 5.7 mass % is reached, all degassing having been
completed. On account of the low content of flux, the filter cake has an
extremely wide sintering interval and only starts sintering at a late stage.
This is reflected in a very high refractoriness and correspondingly low linear
firing shrinkage, which only starts at around 1100° C with 0.2%. The water
absorption of the fired body varies depending on the peak temperature between
17.4 mass % at 900° C and 15.4 mass % at 1150° C. On account of the low content
of haematite, the fired colours range from pink to cream, the fired colour becoming
lighter with increasing peak temperature. Owing to the very low thermal
conductivity of the body of 0.24 W/mK, its main use is in the manufacturing of
high thermal insulation backing bricks. Practical experience in the roofing
tile sector has been available continuously since 2003. Use in the production
of facing bricks and clinkers is also easily possible. It can be used as a
slightly plastic additive with a batch content of 10 to 30% to increase the
content of the medium grain and improve refractoriness as well as to lower the
thermal conductivity of the body in backing brick bodies. It is also suitable
for lightening the colour of red-firing bodies.
4 Volume
availability and final remarks
The filter
cake is produced throughout the whole year without any winter break in two
heated press facilities. The available annual volume currently amounts to over
80 000 t. The specified data are only guide values and are subject to natural
fluctuations. This information is not binding. Supplementary details are
available for download here .
Source
Ziegelindustrie International
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