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OPAL IN INDUSTRY
& ENGINEERING |
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| Earthy
Opaline Silica |
| Common
opal is a rare material compared with the
earthy types of opaline silica formed by
biogenic activity, diagenesis or chemical
precipitation in sedimentary environments.
Elsewhere we have noted the enormous quantity
of silica which is generated in the oceans
of the world by the activity of minute organisms.
This is particularly the case with diatoms,
which also occur as large fossil deposits
in terrestrial environments.
The latter deposits occur widely, and are
given different names in different parts
of the word. In the English speaking world
they are called diatomite or diatomaceous
earth. In different countries it is known
by names such as tripoli, gaize, opoka,
infusorial earth, kieselguhr and bergmehl.
Diatomite deposits of commercial size occur
in many countries. Major producers are the
United States of America, Romania, the (former)
Soviet Union, and France. The total world
production approaches some two million tonnes
per annum. Australian production is comparatively
small, commercial deposits being mostly
in Queensland, although some production
has come from New South Wales and Victoria.
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| The properties of diatomite
and related materials which make them valuable
commercially are:
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Low
bulk density: This property
gives diatomite a low thermal conductivity,
which in turn makes excellent thermal
insulating material. |
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Chemical
stability: Being composed essentially
of silica, with some water, it is not
readily attacked by other chemicals
other than alkalis. |
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Thermal
stability: Again, being composed
essentially of silica, it is resistant
to temperatures in the region of 1000°C,
especially once fired to such temperatures.
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High
surface area: The fine and
intricate nature of the skeletal structure
of the diatom skeletons results in a
high surface area. As amorphous silica
especially has the capacity to adsorb
other materials onto its surface, diatomites
make excellent adsorbants for a variety
of materials. |
The use of diatomite for commercial purposes
appears to go back to at least Roman times.
It was used by the Greeks and Romans for
making fireproof tiles of light weight,
and, in the middle ages is said to have
been mixed with grain meal for bread making
in the belief that it had some medicinal
value. |
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Some of
the actual uses recorded are:
| 1.
As an insulating packing material
around pipes, boilers etc. The diatomite
may be pressed into bricks and other
shapes with the help of binders, and
may be fired to a temperature high enough
to render the blocks stable. |
| 2.
As an insulating refractory, the diatomite
may again be shaped with the help of
a binder, then fired to a temperature
when incipient fusion or sintering occurs.
The bricks are then of sufficient strength
to be used as a light weight insulating
structural refractory under the appropriate
conditions. It is also mixed with other
materials such as lime, magnesia, clay
etc to form refractory materials of
particular properties. |
| 3.
A major use of diatomite is as a filler.
It is added to a wide variety of materials
such as rubber, paints, paper, soap,
cleaning powders, and fabrics. |
| 4.
It is widely used as a filter medium
and clarifier for oils, beer, wine and
other liquids. Its high surface area
adsorbs unwanted contaminants, and its
physical properties are such that it
can be readily removed by settling or
filtration. |
| 5.
Its high surface area also
makes it a good adsorbant for other
purposes. For example, it has been used
to absorb nitroglycerine to make dynamite,
and as an absorbent packing around containers
of corrosive liquids. When dried and
mixed with calcium chloride, the mixture
can be used as a desiccant without formation
of liquid phases. |
| 6.
It has been used as a source of silica
in the ceramic industry because of its
relatively high reactivity. It dissolves
readily in enamels and glazes. |
| 7.
Because of the fine structure of the
diatom skeletons it breaks down easily
into very fine powders which makes it
useful as a mild abrasive and as a polishing
agent. Its inherent hardness (about
5 on Moh's scale) also adds to its suitability
for this purpose. |
| 8.
It has also been used in the chemical
industry as a suitable source of reactive
silica for the manufacture of silicates
and related compounds. |
| 9.
A less well recognised property, which
has yet to be used commercially to any
extent, is its pozzolanic activity.
When mixed with a small amount of lime
in the form of calcium hydroxide and
water, it will set like a cement. Because
of its high reactivity and low density,
if used without a filler it tends to
crack. However, there are clearly circumstances
where this property may be of considerable
use. Some data on the hydraulic activity
of tripoli and diatomite is given by
Bushinski (R1569).
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Besides diatomite
itself, there is another group of highly
siliceous sedimentary rocks which show few,
if any, remains, of siliceous organisms.
In some cases they may have been deposited
under marine environments in which the diatom
or radiolarian tests were broken up into
such fine particles that they are no longer
recognisable.
They may also be the result of the diagenesis
of deposits of biogenic silica; or the deposits
may have been the result of chemical/colloidal
deposition. These deposits frequently contain
a proportion of clay, usually kaolinite,
mixed with the fine silica, and the fine
silica itself may be opal-CT.
Such deposits are often termed opal claystones,
moler, or, in Russia, opoka footnote.
Large deposits of opal claystone occur in
south eastern United States and are mined
commercially on a large scale. Many of the
uses, such as a filler and an adsorbant,
are similar to those for diatomite; a perhaps
lesser known adsorbant use is for pet litter.
No large deposits of this type have been
exploited in Australia, although small amounts
of opal claystone have been encountered
in south western New South Wales and south
of Adelaide, South Australia. An extensive
deposit has been reported in the region
of Marree, South Australia, but is yet to
be confirmed.
Herbert has surveyed the production and
use of diatomite in New South Wales (R1636). |
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