Minerals used in Refractories and Fillers
Materials that retain their shape and chemical identity without marked expansion at high temperatures are required for lining kilns and furnaces and for many purposes in the electrical and chemical industries. These are classed as refractories and technical ceramics which require a range of specialized properties and are derived from both natural and synthetic sources. As used in the metallurgical industry, refractories should be capable of withstanding a temperature of more than 1500o C without showing any signs of fusion. They should be strong enough to bear the weight of molten metal, should not wear and tear easily, and must not react with the melt. The more important natural materials used for the manufacture of refractories are listed in the table below.
Fillers are minerals and materials used to add bulk or weight to paper, rubber, paints and other synthetic products. They are derived mainly from inert clay minerals and from barite (BaSO4). They are also used as a component of oilfield drilling muds. Barite is a common gangue mineral in hydrothermal and exhalative sulfide deposits and it occasionally forms larger concentrations.
Availability of Material in India
Fireclays are of sedimentary origin and occur as horizons below the coal seams. In India fire clays from Raniganj and Jharia coalfields are very important. The fireclays from Raniganj are excellent in quality. The super-refractories are manufactured by blending them with certain proportions of bauxite from Lohardaga. In general, the fireclays from both these coal fields occur within Barakars. Important deposits are around Kumardhubi, Mugma, Garphalbari-Dahibari region, Damagpria, Radhabhallabhpur, Pohargora Garb Dhamo, Churulia and Pathardi. The Rajhara and Daltonganj coalfields are also important suppliers of plastic fireclay. In Orissa, workable deposits of fireclay occur in Sambalpur, Cuttack, Sunde garh, Puri and Dhenkanal districts. Fireclays from Belpahar are extensively used in steel plants.
In Madhya Pradesh, highly plastic and refractory clays are reported injabalpur, Betul, Bilaspur, Drug, Hoshangabad and Satna districts. In Karnataka, good occurrences are reported in Shimoga, Bangalore and Kolar districts. Good refractory fireclays also occur in Nizamabad, Asifabad, East Godavari and Cuddapah districts (Andhra Pradesh) and North and South Arcot districts (Tamil Nadu). In Rajasthan, the fireclays are distributed in jaisalmer, Sawai Madhopur and Bikaner areas.
Quartz, Sandstone & Quartzite:
The main sources of silica are the sands and quartzites. Important occurrences in India are as follows:
Bihar: Between Rakha Mines and Kendadih (Singbhum district), Rajagoan Hills (Gaya district), Ratanpur (Monghyr district) and Bihar Shariff (Patna district).
Orissa: Khajuria-Pravasoni (Bamra district), jhargati and Garpati (Sambalpur district).
Karnataka: Dodguni area (Tumkur district).
Sands from Jabalpur (Madhya Pradesh) are also often used as refractory lining for the furnaces.
In recent years good sands have been obtained from the crushing of pure quartzose Vindhyan sandstones at several localities in Uttar Pradesh, from Gondwana (Damuda) sandstone of the Raj Mahal hills, and from Cretaceous sandstones and Archaean and other pure quartzites of some parts of Tamil Nadu and Maharashtra. For silica brick manufacture, the raw material should contain at least 96 to 98% SiO2. While magnesia and alkalies are objectionable, iron oxide in limited quantities helps in acting as a binder. Firing is done around 1,480oC in periodic kilns with 2% lime as binding material. The silica bricks have spalling with temperature fluctuations but strength is retained below the temperature of their fusion because of high thermal conductivity.
Kyanite and Sillimanite:
Kyanite and sillimanite are similar in chemical composition (Al2O3.SiO2) but differ in crystal structure. They occur in metamorphic aluminous rocks. The main occurrence of kyanite in India is in Lapsa Buru in Bihar, where it occurs with quartz along a 100 km long zone. Large deposits are also mined in Bhandara district of Maharashtra. The most important occurrences of sillimanite are in the Khasi Hills of Assam and in Meghalaya and Rewa in Madhya Pradesh.
For refractory usage, dolomite should have an equimolecular proportion of CaCO3, and MgCO3. It must be low in SiO2, Fe2O3 and Al2O3 (together less than 3%). For calcined dolomite, the specifications are very rigid and SiO2 and Al2O3 should not exceed 1% each. Calcined dolomite is distinctly preferred over the raw dolomite for maintaining the open-hearth steel furnaces. Dolomite and dolomitic limestone are used extensively for the steel furnaces owing to the low cost and easy availability in contrast to magnesite.
Workable deposits of dolomite used in refractories are reported in Orissa around Birmitrapur and Purnapani areas of Sundergarh district. These are generally high in silica and are used in furnace operations at TISCO, Rourkela and Durgapur. The dolomite occurs in a band of 300 metres width over a length of 7,000 metres and depth up to 50 metres. The estimated reserves are 252 million tonnes.
In Madhya Pradesh, the marbles of Narmadaghat (Jabalpur district) are dolomitic. Dolomite from Bilaspur and Satna districts is used in open-hearth furnaces of Rourkela and Bhilai steel plants. In Karnataka, refractory grade dolomite is worked around Sankargudda in Shimoga district and is the principal source of refractories for use in the steel plant at Bhadravati.
The total reserves of blast furnace and steel melting shop grades of dolomite in India are estimated at 720 and 428 million tonnes respectively.
The dolomite bricks are made on conversion to di-calcium or tri-calcium silicates. In the case of di-calcium silicate, the dusting encountered during cooling can be avoided by adding minor amounts of Fe2O3 within limits. The tri-calcium silicate is obtained by adding 15% serpentine to powdered dolomite and calcining the mixture at 1,600oC in a rotary kiln.
Large deposits of magnesite (MgCO3) occur in the district of Salem as veins associated with other magnesian rocks such as dolomite, serpentines, etc. The magnesite is believed to be an alteration-product of the dunites (peridotite) and other basic magnesian rocks of Salem. When freshly broken it is of a dazzling white colour and hence the magnesite-veins traversing the country have been named the Chalk hills of Salem. The magnesite of Salem is of a high degree of purity (MgO 46.4 %), is easily obtained and, when calcined at a high temperature, yields a material of great refractoriness. Other places in India also contain magnesite-veins traversing basic rocks, viz. Coorg, Coimbatore, Mysore, Almora and parts of Eastern Himalayas.
Serpentine, Olivine Rock and Chromite:
Dunites (rocks rich in olivine) occur as a product of magmatic differentiation in the form of layered masses and veins associated with ultra-basic, intrusive rocks, like chromites, peridotites, serpentines, etc. Serpentinites (rocks rich in serpentine) occur as products of weathering and alteration of olivine rich rocks. These occur in Karnataka, in several districts of Orissa (chiefly Keonjbar), and in Singhbhum. The Orissa reserves are the largest computed at over 3.5 million tons. Less important deposits have been found in parts of Tamil Nadu and in Ratnagiri in Maharashtra. Some chromite occurs in the " Chalk hills " (magnesite-veins) near Salem, but it is not worked. Large deposits of chromite occurring in dunite intrusions forming mountain-masses have been discovered in the Cretaceous volcanics of Burzil and Dras valley of Ladakh, Kashmir.
Kyanite and Sillimanite:
Kyanite and sillimanite are similar in chemical composition (Al2O3) . However, they differ in their crystal structure and
physical behaviour. They occur in metamorphic altiminous rocks. India has the richest deposits of kyanite and sillimanite in the world.
Talc and steatite occur widely in India, forming large masses in the Archaean and Dharwar rocks of the Peninsula. Workable deposits occur in Bihar, Jabalpur, Salem, Idar and Jaipur. The Rajasthan, the deposits occur as thick lenticular beds of wide extent in the schists. Some of these beds persist for miles. At most of these places steatite is quarried in considerable quantities for commercial purposes. In its geological relations, steatite is often associated with dolomite (as in Jabalpur) and other magnesian rocks, and it is probable that it is derived from these rocks by metamorphic processes resulting in the conversion of the magnesium carbonate into the hydrated silicate. In other cases it is the final product of the alteration of ultra-basic and basic eruptive rocks.
Graphite deposits in India are associated with metamorphic rocks (schists and gneisses) and limestone. Graphite graphite is being mined in Latehar and palamau districts of Jharkhand; Bargarh, Nuapada, Rayagarh and Bolangir districts in Orissa and madurai and Sivaganga districts of Tamil Nadu. Graphite mines are mostly small, except for a few underground mines in Orissa, which are faced with the problem of water seepage.
Besides the above mentioned mines, graphite also occurs in Sambalpur, Dhenkanal, Koraput, Phulbani and Kalahandi districts of Orissa;
Graphite crucibles used for steel are approximately of 50% graphite, 30% bond clay, 10% sand and 10% kaolin while those used for brass have 45% graphite, 35% bond clay, 10% grog and 10% kaolin. The graphite for crucibles should contain 80% fixed carbon. Mica, carbonates and sulphur (in the form of pyrite) are the undesirable impurities. While mica fuses causing holes in the crucibles, carbonates and pryite dissociate resulting in volume changes. Graphite is also used as a foundry facing material.
Barite is used as a pigment for mixing with white lead, as a flux in the smelting of iron and manganese, in paper-manufacture as a filler, and in pottery-glazes, etc. The whiter and better-quality barite is used in the local manufacture of paints (lithophone); the coloured variety is used in making heavy drilling mud by the oil companies.
Barite occurs in sufficient quantities at many places in India in the form of veins and as beds in shales. The chief localities for barite mining are Cuddapah, Kurnool, Khammam and Prakasum districts of Andhra Pradesh; Sirmur district Himachal Pradesh; and Udaipur district of Rajasthan. Besides these, barite also occurs in the Kurnool district of Andhra Pradesh; Alwar in Rajasthan; Salem in Tamil Nadu; and Sleemanabad (in the Jabalpur district). Barite mines in India are worked by opencast method.
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