for the Ceramic Industry
Principal mineral raw materials for the manufacture of ceramic products are the
silica in different forms and the alumino-silicates.
These minerals, in appropriate combinations, are fused at high
temperatures to obtain the required product.
While silica minerals include quartz and other forms like silica sand,
the alumino-silicates comprise of the feldspars (orthoclase, microcline and
albite) and clays. In addition,
limestone and also minor amounts of a large number of other minerals find use to
certain extent in ceramics. Feldspar and pegmatite, suitable for ceramic industry, should contain less than 9.5 per cent of Fe203; no less than 11 and 8 per cent of K2O + Na20 (in feldspar and pegmatite respecdvely); less than 1-2 per cent of CaO; and less than 10 and 30 percent of SiO2 in feldspar and pegmetite respectively.
ceramic products, the quartz should be of good quality.
Iron staining and iron oxide minerals are considered as objectionable.
The important geological sources for silica suitable for the ceramic
industry are the pegmatites (with well developed crystals of quartz), vein
quartz, sandstones of high siliceous nature, high silica sands and
India, the major producing centres of silica are in the Sankargarh, Lohargarh
and Bargarh regions (Uttar Pradesh), Bundi and Dausa in Jaipur and Adalpur in
Sawai Madhopur (Rajasthan), Shimoga district (Karnataka), Burdwan district (West
Bengal), Singbhum and Dhanbad districts (Bihar) and Guntur district (Andhra
Pradesh). Extensive deposits of
quartzite suitable for glass manufacture occur in Mayurbhunj district of Orissa.
Some of the sandstones from Himmatnagar, Padharanali and Sankhera in
Gujarat are suitable for the ceramic industry.
The common impurities are clay, slime, iron stains and iron silicate
minerals such as garnets. Feldspar
and mica are objectionable. The
raw material is ground in conventional manner and deslimed to remove the clayey
fraction. If iron is present as
surface coating, this can be removed either by scrubbing or by chemical
treatment (like heating in a dilute solution of titanous sulphate with some
hydrofluoric acid). Tabling can be
adopted for the removal of clayey minerals and the heavy minerals like ilmenite
associated impurities such as mica and iron minerals can be removed by adopting
flotation techniques. The process is carried out in acid circuits (with pH between
2 and 3). Mica is floated by using
a combination of fuel of oil and some amine acetate. Then the pulp is treated with one of the petroleum
sulphonates and iron is removed in the subsequent stage of operation.
In the final stage, feldspar is floated by using hydrofluoric acid and
are used as fluxing material in the preparation of ceramic bodies, enamels and
glazes. Commonly potash feldspars
(orthoclase and microcline) are used for this purpose and soda feldspar is
chiefly used for glazing purposes. Feldspars
for use in ceramic industry should contain a minimum of 65-72% SiO2
and not less than 4% Na2O.
is one of the dominant ingredients in acid igneous rocks, particularly of
granitic and pegmatitic nature. Pegmatites
are often zoned into distinct bands of quartz and feldspar.
In India, the pegmatites of the famous mica belts in Rajasthan, Bihar and
Andhra Pradesh are the major sources ceramic grade feldspars.
The other producing areas are in Salem and Tiruchirapalli districts of
Tamil Nadu, Burdwan and Purulia districts of West Bengal and Hassan district of
Karnataka. Indian feldspars are
exported to the U.K., France, W. Germany, Italy and Japan.
The common impurities to be eliminated are the iron-bearing minerals such as
garnet and mica. The quarried
material is broken and feldspar is removed easily by handpicking.
The picked feldspar is reduced in size by crushing in conventional
crushers and by subsequent grinding in pulverizers.
For glazing purposes and for the manufacture of ceramic ware, iron
contamination during size reduction is avoided by carrying out the grinding in
pebble mills using flint pebbles as the grinding media.
The flotation techniques, similar to silica beneficiation, are also
adopted for feldspar concentration.
have been classified on the basis of their physical properties and the
industrial usage. Of the various
clays, ball and china clay are extensively used in ceramic industry.
China clay and ball clay belong to the kaolin group.
Although the primary constituent is kaolinite in both these clays, ball
clay has greater plasticity and lesser refractoriness owing to the presence of
montmorillonite in considerable amounts.
It is normally added to china clay to achieve greater strength and the
required plasticity. These clays
are a product of weathering processes of feldspathic rocks.
During weathering, the silica and iron oxides are partially leached with
the residue (essentially of an aluminium silicate in composition) forming the
in-situ deposits. Depending upon
the efficiency of the weathering process, impurities like grit (siliceous
particles) and iron oxide minerals exist in different percentages in various
Clay: Ball Clays
with high plasticity ranges have been reported from Khajwana, Indawar, jodhpur
and Sheo areas in Rajasthan, Rampurda, Vagedia, Bagagela and Thoangadh areas in
Gujarat, Kundra in Kerala, parts of Chingleput district in Tamil Nadu and
Dwarka-Tirumala in Andhra Pradesh.
clay: Usage of china
clay for ceramic industry depends on factors such as plasticity, shrinkage
(after drying and on firing), colour on firing and refractoriness.
The grit content should normally be less than 1% and should never exceed
has extensive deposits of china clays distributed in almost all the states.
However, good deposits are in Bhagalpur, Ranchi, Singbhum and Monghyr
districts in Bihar; Mayurbhunj district in Orissa; Bankura and Birbhum districts
in West Bengal; Banda district in Uttar Pradesh; Barmer, Pali, Bikaner and Ajmer
districts in Rajasthan; Chingleput, North and South Arcot and Salem districts in
Tamil Nadu; Adilabad, Anantapur, Nellore and Guntur districts of Andhra Pradesh;
Chanda and Ratnagiri districts in Maharashtra; Shimoga and Hassan districts in
Karnataka; Sabarkantha district in Gujarat and Udhampur in Jammu and Kashmir.
Major impurities in china clay are quartz, mica, felaspar and iron oxide
minerals. Methods such as sieving,
washing, elutriation and levigation may be employed.
Normally the washing is done by 'levigation' process, which involves
passing the clay slurry through a series of troughs or channels with different
slopes. This process aids in the
settling of grit and other heavy mineral and floating of light fractions like
mica. Settling of finest quality
clay takes place in the final tank. Decolourising of the clay is also attempted for certain
clays, which are coloured due to the presence of iron and titanium oxides.
However, no elaborate beneficiation techniques are employed in India and
even the raw material that is marketed is not properly graded and specified.
This has been creating special problems to industries, particularly to
those of smaller sizes.