Minerals used in the Fertilizer Industry

 

During the last four decades, with the emphasis on agricultural produc­tion in India, increased attention was given to the manufacture of chemical fertilizers.  The fertilizer plants use the raw materials both from natural sources and from the chemical materials.  Gypsum, pyrite and rock phosphates form the principal mineral sources.  While the first two find extensive use in the manufacture of sulfate fertilizers, the rock phosphates are mainly utilized in the production of phosphate fertilizer.

Source of various constituents in Fertilizers

Principal nutrient

Source

Nitrogen (N)

organic fertilizers including animal manure, plant waste, seaweed, fishmeal, dried blood etc, give exchangable nitrogen

nitrogen fixing bacteria in soil or in symbiotic relationship with legumes

nitrates, non-marine evaporates of Chile

ammonia and its derivatives from petrochemical plants

Potash (K)

organic fertilisers, especially manure

bittern salts of marine evaporites

Lime (Ca)

shells and bone

carbonate rocks including limestone, tufa, calcrete

Phosphates (P)

bone meal, manure

guano (consolidated droppings of sea birds, from oceanic islands, now largely worked out)

superphosphate derived from phosphorite

basic slag byproduct of steel production

Sulfur (S)

salt domes (reduction of gypsum)

iron sulphides

 

GYPSUM DEPOSITS

Gypsum is a hydrated calcium sulfate (CaSO4.2H20 ) and its an­hydrous form (CaSO4) is known as anhydrite.  Deposits of gypsum are either of sedimentary origin (bedded type) or of marine evaporate nature.  Gypsum finds extensive use in the cement, paper, textile and paint industries.  The mineral, calcined around 200oC loses 75% of its water and the product, known by its trade name as 'plaster of paris', is widely used in building industry as a good finishing material.  It can be moulded into any shape with the addition of water and sets to a hard mass.

In India, gypsum is produced in Rajasthan.  Tamil Nadu, Uttar Pradesh, Gujarat and Maharashtra.  Of these, the first three are the important producers with Rajasthan possessing around 95% of the total Indian reserves (estimated to be around 1,000 m tonnes).  The deposits in Rajasthan are situated mainly in Bikaner, jodhpur, Nagaur and jaisalmer districts.  The Rajasthan occurrences are associated with the Vindhyan limestones.

In Tamil Nadu the deposits are situated in Tiruchirapalli, Coimbatore and Ramanathapuram districts and are associated with the shales in Cretaceous sequence.  The gypsum from these sources is chiefly utilized in the cement and pottery industry.  In Uttar Pradesh, deposits are reported in Dehradun, Garhwal and Nanital districts.  The Majhara deposit in Dehradun district and Lakshmanjhula in Garhwal district are exploited at present.

For use in fertilizer industry, gypsum with a minimum of 87% CaSO4.2H2O is preferred.  The Sindri fertilizer plant gets its main supply from the Bikaner deposits.

 

PYRITE DEPOSITS

Sulfur and pyrite (FeS2) are the principal raw materials in the manufacture of sulfuric acid which forms the back-bone of many modern industries such as fertilizers, chemicals, paints and textiles.  Pyrite is either mined as a principal mineral or is recovered from the sulfide assemblages of copper, lead-zinc, gold and other metallic ore deposits.

In India, pyrite deposits occur in Bihar, Karnataka, Rajasthan and Tamil Nadu.  However, the largest deposits are from Amjhore region in Shahabad district of Bihar where pyrite occurs as a uniform bed of about one metre thickness in the Bijaigarh black shales of Kaimur series (Vindhyan system).  The deposit (located at nine places within the Amjhore region) has a remarkable conformity with the overlying and underlying Vindhyan formations and is confined to a single stratigraphic horizon.  Two types of pyrite occurrences have been reported: the massive cryptocrystalline variety, which forms the bulk of the deposit and the fine pyrite grains, disseminated within the black shale.  About 390 million tonnes of pyrite (with 48% S) are estimated to be available within an area of 120 sq. km in Amjhore region.

In Karnataka, pyrite deposits occur around Ingaldahl in Chital­drug district.  The mineralization is localized in the ferruginous chert bands interbanded with Dharwarian greenstones in the eastern flank of an anticlinal structure.  It may be noted that copper mineralization in this region is on the western flank of this structure.  The Ingaldahl pyrite reserves are estimated to be around 2.0 million. tonnes (of 20-30% S). The deposits in Rajasthan occur at Saladipura and the inferred re­serves are of the order of85 million tonnes (with 22% S).  In Tamil Nadu, the deposits are located around Polur in the Thaniyar reserve forest.

Besides the above occurrences, pyrite is also associated with gold, copper, lead and zinc deposits as also in Tertiary coals.  In the metallic ores, after the recovery of primary minerals the tailings containing pyrite are discarded in India.  Plans are underway to recover and utilize pyrite from such tailings, Sizeable deposits of sulphur have been reported from Puga valley in Ladakh.

In India, the requirements of sulphur are of the order of 3,00,000 tonnes per year.  At present the mining activity is at Amjhore by the state-owned Pyrites, Phophates and Chemicals Ltd.  A sulphuric acid plant of 400 tonnes per day capacity has been set up at Sindri in Bihar.

 

ROCK PHOSPHATE DEPOSTS

Phosphate deposits of sedimentary origin and of economic importance are known as phosphorites or rock phosphates.  The formation of these is restricted to marine environments and is a result of deposition under specific pH conditions from the phosphorus-bearing solutions derived from the weathering of phosphate minerals like apatite of igneous source.  It is thought that redox potential does not play any role in their formation.  In all the phosphate minerals, phosphorus exists in its highest valent state,

In India, with the development of superphosphate fertilizer in­dustry, the exploration for rock phosphate deposits has been accelerated.  Deposits of economic significance are reported from Rajasthan and Uttar Pradesh.  In Rajasthan, they are located in Udaipur and jaisalmer districts.  The Udaipur deposits occur around Jhamar Kotra area, Kanpur region (Kanpur, Karbaria Ka Gurha and Maton blocks) and Dakan Kotra area.  The phosphorites are confined to limestone or cherty quartzite and are in the form of lenticular masses of variable thickness with the P2O5 content between 15 and 25%.  However, the published analyses for Maton deposits indicate P2O5 as high as 36%.  The Jaisalmer deposits are mainly located in the Fatehgarh area where the phosphorite horizon was associated with the Mesozoic sandstones.  The P2O5  content varies from 5 to 15%.  Two types of mineral associations are reported from Rajasthan deposits: pellets of collaphane and black chert in calcare­ous shaly sandstones and the banded phosphorites (of alternate collophane with quartz on calcite bands) with limestone bands.  The basic characteristic feature of these phosphorites is their association with limestones, which distinguishes the same from those of Uttar Pradesh.  Around 80 million tonnes are estimated to be available in Rajasthan.

In Uttar Pradesh, the deposits occur mainly at Mussoorie and Meldeota, although in all about 10 deposits are located.  The phos­phate-bearing zones, thickness varying from a few centimetres to about 15 metres at places, are mainly restricted to chert and carbonaceous shale formations of Lower Tals or at the contact of the underlying argillaceous limestones of Upper Krols.  Three varieties of phosphorite -- granular, pelletal and nodular -- have been reported.  The P2O5content of these deposits varies between 15 and 35%.  The deposits from Uttar Pradesh contain the collophane species of phosphates together with quartz, calcite, chert, muscovite and clay minerals.  The development of the Mussoorie deposits is beset with several difficulties such as their complex mineralogy, lack of precise estimates as regards the quantity and quality of ore available, remoteness of the deposits and the need for underground mining.

In India, the major share of the requirements for rock phosphates is met from the imports of the order of 5,75,000 tonnes per year while the production is around 6,44,000 tonnes.  A beneficiation plant is being set up at Matas near Udaipur by Hindustan Zinc Limited to upgrade the phosphorite from 25% P2O5 to >32% P2O5  with consequent reduction of silica.  It is planned to send these concentrates to the superphosphate unit at Debari smelting plant.

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