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What is Kaolin?
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Introduction
Kaolin is a white non-swelling clay which has been formed typically by intense
weathering which has leached the source rocks into predominantly relatively pure
white kaolinite. Although kaolin is not an uncommon mineral, it is relatively
rare to find a deposit that is; (a) large enough to be commercially viable; (b)
pure enough to be economically valuable to industry; (c) relatively easily
accessible and , (d) possessing certain performance characteristics that
industry is willing to pay attractive prices for the processed material.
Kaolin geologists have to assure themselves that an ore body is sufficiently
consistent to provide prospective customers with a standard product. Only after
very extensive field testing of a prospective kaolin deposit can the confidence
levels be established to provide reasonable certainty that the new ore body will
be a commercially valuable proposition. Exploratory activity may take many years
comprising a number of drilling programmes which reveal an increasingly detailed
knowledge of the deposit. Almost invariably, a pilot plant is required to fully
characterise the products that can be made from the ore body and to define the
types of processing / refining that are required on a particular deposit. The
level of processing is also driven by the particular needs of target market
segments. The Skardon River project now has the benefit of extensive testwork
which has been conducted internally as well as by independent laboratories.
The technical
qualities of Kaolin:
Mineralogy
An Alumino
Silicate with monoclinic or triclinic crystal structure. The group includes
kaolinite, dickite, nacrite and halloysite.
Hardness : 2 -
2.5 range (Mohs scale)
Particles Size : generally below 10 microns and for coating clays generally 90%
below 2 microns.
Particle Shape : Pseudo-hexagonal crystal plates which may be stacked or
vermicular shape
Chemistry
Approximated
formula : 2H2 OAl2O32SiO2
Chemical composition : Approximately Alumina 39% Silica 46% formula water 14%
Reactivity : Inert. Good stability over pH 3-9.
The Commercial Properties of Kaolin The commercial importance of kaolin is
determined by its usefulness to the end markets of which by far the most
important, in volume terms, is the paper industry. Kaolin acts as a filler to
the paper fiber web and as a pigment used in coating the paper products. The
most important properties are mainly its optical characteristics which can be
briefly summarized as:
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Colour
The degree of whiteness is critical to the value of kaolin
pigment. Analysis is made on the brightness of the clay using the ISO (or GE)
scales. Good commercial kaolin can range from the low 80’s to, in some very
limited cases, as bright as ISO 95. Clays that exceed ISO 90 begin to compete
with titanium dixiode (the brightest of commercial mineral pigments) and can be
used in TiO2 blends in the paper and paint industries. Although TiO2 is brighter
than kaolin it is many times more expensive per tonne and is also prone to more
cyclical price variation than kaolin. As well as overall brightness the "colour"
of whiteness is very important. The kaolin industry uses the Hunter scale to
measure colour. This scale can measure all colour variation from orange hues to
green/white hues, many of which to the naked eye may simply appear to be
"white". Colour is very important to the papermaker in determining a product
blend.
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Opacity
Another equally important property of the kaolin is its
opacity (or hiding power). Most of us are all too familiar with cheap white
paint that needs 2-3 coats to hide the underlying marks on a wall whereas an
expensive paint (which probably incorporates TiO2 and /or calcined kaolin in the
mix) will do the job in one coat. Opacity is very important for the papermaker
otherwise the ink on the reverse side will show through to the obverse. This is
tolerable for cheap paper (eg school writing pads) but is unacceptable for
higher quality applications.
Other Important
Characteristics:
Viscosity describes the fluidity of the clay in a water slurry and is most
important for the papermaker. Whereas the ceramist prefers sticky clays for ease
of moulding, the papermaker demands low viscosity slurries which flow easily.
The Brookfied scale measures low shear viscosity and is important for
determining the practical levels of solids used in handling a clay slurry. A
more critical factor is high shear viscosity. The paper industry uses the
Hercules scale to accurately measure its high shear performance. Modern paper
making machines run at high speeds using high percent solids to minimise drying
time and energy costs. The clay slurry has to be capable of leaving the coating
blade as an even coat which meets quality standards with no streaking or tearing
of the paper.
The exact range
of particle size is critical in many applications. Commercial kaolins are very
fine, generally with most particles less than 2 microns in size.
It is important that the kaolin slurry does not scratch or wear the paper making
machinery. The Einlehner abrasion metre measures the abrasion level of a kaolin
sample.
Other Quality Factors Premium paper coating clays also require a number of other
quality performance characteristics including ink receptivity and hold out,
glossing attributes, etc.
Kaolin Product Types Kaolin is sold as either a hydrous kaolin or a calcined
kaolin. Hydrous clay is a natural clay that has been refined through processing
to make a range of end products. Calcined clays are thermally restructured by
means of a Kiln (calciner) usually heated at about 1050ºC. In the lake of
Skardon River, the very fine clay particle sizes require less heat to achieve
calcination. This process drives off formula water and oxides otherwise trapped
in the clay structure and enhances the whiteness, brightness and porosity of the
clay particles.
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