Silica And Allied Minerals
quartz crystals or high-class silica sand are fused for the
manufacture of ophthalmic glasses and lenses.
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Silicon in the amorphous form was prepared by Berzelius in 1823AD.
Silicon in the crystalline form was prepared by H St Cdeville in
The Present Scenario
Quartzite, sandstone, quartz and other siliceous rocks like mica
schists are used in the manufacture of silica bricks. Quartz and
quartzite are used in making ferro-silicon for the manufacture of
is the most abundant mineral found in the crust of the earth. It forms
an important constituent of practically all rock-forming minerals. It
is found in a variety of forms, as quartz crystals, massive forming
hills, quartz sand (silica sand), sandstone, quartzite, tripoli,
diatomite, flint, opal, chalcedonic forms like agate, onyx etc., and
in with numerous other forms depending upon colour such as purple
quartz (amethyst), smoky quartz, yellow quartz or false topaz
(citrine), rose quartz and milky quartz. Only pure quartz crystal or
rock crystal, untwinned, clear, free from any inclusion, has an
It expands (mechanically) under the influence of electric current and
conversely pressure induces a measurable electric current. This
property is known as piezoelectricity. The current thus developed is
called piezoelectric current.
This property resulting from the asymmetry of its atomic groups makes
quartz an effective transducer for coverting electrical energy into
mechanical energy and vice-versa. This property in quartz crystals was
discovered in 1880-82 by Pierre and Jacques Curie and remained a
laboratory curiosity till in 1921 when W.G. Cady, a physicist,
discovered that quartz plates could be used to control the frequency
of wireless transmission circuits.
This discovery marked the dawn of quartz crystal application in
modern communication equipments. A very thin plate of quartz is so cut
that the frequency of the oscillating circuit corresponds with the
quartz plate and when such plate is inserted in a radio receiving set
or radio transmitter it prevents frequencies from wandering and
deviation and greatly reduces interference.
Quartz plate is used in controlling frequencies in air and water
media as well. It is largely used in radio circuit, radar, ultrasonic
and in multiple telephone lines. Quartz plates keep the bradcast on
the right beam.
Quartz cystals cut into prisms, wedges and lenses are used for
microscopes and other optical instruments. Quartz wedge is the
commonest accessory which students use in the petrological microscope.
A number of other crystals giving piezoelectricity are known but none
compares with quartz. Chemically prepared Rochelle salt and Barium
titanate have been found good substitutes for piwzoelectric quartz.
However, the crystal - quartz because of its chemical and physical
stability and high elasticity has remained indispensable so far. The
consumption of quartz plate pieces has tremendously increased with the
increase in the manufacture of modern receiving sets.
Occurrences and Consumptions
alone uses a few million pieces every year for use in
receiving sets for combat forces.
Clear quartz crystals or high-class silica sand are fused for the
manufacture of ophthalmic glasses and lenses. Most of the world supply
of flawless, untwinned radio-grade quartz comes from Brazil
It is found occurring in thousands of square km. in the States of
, and Bahia
Quartz deposits are found in the form of veins, pipes, pockets, stock
works and vugs. All crystals are not of radio grade. Clear crystals
Only clear pyramidical crystals are picked up for tests, Grey or
milky quartz being unsuitable. Small production of piezoelectric
quartz crystals is also reported from Mozambique
The commonest use of quartz and glass-sand, also referred to as
silica-sand, is in the manufacture of glass. Great advancement has
been made in the manufacture of translucent, transparent, coloured and
clear glass in sheets or in glassware.
The size of the sand grains is important in glass industry. It should
be between 40 to 80 mesh (BSS). It should be of high purity containing
a minimum of 98% SiO2
. In the manufacture of colourless
glass the iron content (Fe2
) should not
exceed 0.04%. for optical purposes, the presence of iron oxide (Fe2
more than 0.008% is not tolerated.
Iron and chromium are both objectionable impurities in glass-sand.
The minutest presence of these impurities gives colour effect in the
glass melt. Glass is manufactured by melting a mixture in suitable
proportion of felspar, dolomite, limestone and soda ash together with
glass-sand at 1400º - 1500ºC in the furnace when clear
molten glass is formed.
The average composition of the mixture by parts is: soda ash 1;
silica sand 2½; limestone / dolomite ¼; borax 1/320;
saltpetre 1/160; felspar 1/80 and selenium 1/36000. Selenium is added
to neutralise the colour effect imparted by the presence of iron
Twelve grams of selenium are sufficient to neutralise the colour
effect of one tonne glass containing 0.15% Fe2
Borax is used to manufacture borosilicate glass of low alkali content
including laboratory wares and optical glass. Boron confers a low
co-efficient of expansion, increases resistance to mechanical and
thermal shocks and gives a bright and pleasing appearance to glass.
Glass-sand free from organic and clayey impurities is used in the
manufacture of sand-paper, abrasive cloth etc. generally sands crushed
from sandstone and quartzite are used. River-borne sands are
unsuitable as they do not possess the angular faces.
Quartzite, sandstone, quartz and other siliceous rocks lime mica
schists are used in the manufacture of silica bricks. Quartzite
contains mainly silica and has high refractoriness. The purity of raw
material i.e., the high silica content is essential with least
. Phase diagram study has sown
that even about 5% Al2
present brings down
the refractoriness from 1728ºC to 1545ºC when eutectic is
formed. The presence of 0.01% Al2
refractoriness of silica by 4.8ºF and 0.4% of Al2
by 140ºF. Silica-rock of metamorphic origin is better than that
of igneous origin because silica grains cemented with cristobalite and
tridymite are stable phases of silica. Silica bricks are used in the
steel industry. The importance of silica bricks is fast losing ground
because of a gradual change in the practice of manufacturing steel by
basic hearth furnace. L.D. process also eliminates much use of silica
bricks as instead dolimite bricks are used. At present the estimated
rate of consumption of silica bricks in the domestic steel plants in
open hearth process using silica roof is 8 kg. per ingot tonne of
steel as against 2.8 kg. consumed in the advanced countries.
Quartz and quartzite are used in making ferro-silicon for the
manufacture of silicon steel. The specification of quartz and
quartzite provided by the industry manufacturing ferro-silicon is as
Rounded pebbles of chalcedony are used in ball mills for finer
crushing of minerals like felspar, calcite, and barytes. Chalcedony
being the cryptocrystalline variety of silica is very hard and has
good abrasive effect on the material to be pulverized. Some of the
ball mills use steel shots for this purpose. Chalcedony pebbles are
used to advantage where product absolutely free from iron
contamination is required.
The agate pieces after cutting to requisite sizes and shapes are
utilized in the manufacture of fulcra of scientific balances and
making edges, planes and bearing of precision instruments. Cambay and
Varanasi are important agate cutting centres. Big pebbles are used for
making mortar and pestle.
Glass-sand in large quantities is used as moulding sand in the
foundry industry, as body constituent in the ceramic industry in the
preparation of glazes and sometimes added to the raw material for
cement manufacture to balance SiO2
percentages. Silica flour made by
grinding silica sand is used in paints. Paint manufacturers generally
prefer diatomite powder which has a wide covering powder.
Silica is a specialist semiconductor distribution business offering
an extensive technology and product portfolio from a highly focused
linecard. Serving most Western and Eastern European countries, Silica
markets through pan-European agreements from the leading suppliers of
Silica offers one of the most comprehensive product portfolios
available from any distributor. With local teams of application
engineers and technology specialists, it is dedicated to supporting
its OEM customers technically and to providing the design-in expertise
needed for customers to compete successfully.
As a division of Avnet Electronics Marketing (EM) in Europe, Silica
is supported by Avnet Logistics for warehousing, programming and other
value added services, including laser marking, tape & reel and
inspection, fast turn-round prototyping, dry packaging and baking. The
Avnet Logistics Programming Service Centre, one of the largest of its
kind in Europe, has a capacity of more than 30 million units a year.
A full suite of value added supply chain management services, such as
buffer stock management, replenishment services (Kanban), EDI and
supply chain consulting, is available to Silica's European customers
in order to reduce inventory, to lower costs and reduce time to market
Silica is a division of Phoenix-based Avnet Inc. (NYSE:AVT), a
Fortune 500 company with annual sales exceeding $ 10,2 billion (fiscal
year 2004). The world's largest distributors of semiconductors,
interconnect, passive and electromechanical components from the
leading manufacturers, Avnet EM markets, inventories and adds value to
these products and provides world-class supply-chain management
services. Avnet EM serves customers in 68 countries.
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