Limestone scenario in India
The Indian cement industry occupies a pride of place in the cement universe. The annual cement manufacturing capacity is now close to 500 million tonne per annum (MTPA) and thus has the distinction of being the second biggest producer in the world. Broadly speaking, cement is termed as a binder, a substance used for construction that sets, hardens and adheres to other materials. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Cement is the most widely used material in existence and is only behind water as the planet's most-consumed resource.
Any cement raw material investigator needs to keep a few important points in mind starting from the fact that this industry is very capital intensive, large gestations time, varied raw materials in large quantities are handled in the plant involving different processes such as mining, crushing, grinding, pyro-processing and finished grinding, to name a few. Perhaps, because of these reasons, there is always a need to build inventory at every stage of processing to ensure uninterrupted operations. Cement manufacturing process could itself be divided into clinkering and grinding processes, where clinker becomes an intermediate product involving calcination of limestone.
Limestone is used in clinkering process by finely grinding it into powder along with admixture of either/or clay, sand, bauxite, iron, etc. is calcined (heated up) at a very high temperature approximately 140 degree Celsius resulting into limestone (CaCO3) dissociated into CaO and CO2. Lime (CaO) then chemically reacts with available silica (SiO2), alumina (Al2O3) and iron (Fe2O3) to form silicates, aluminates and ferrites of lime.
From the point of economy of size, currently, clinkering units of 6,000 tonnes per day (TPD) to 10,000 (TPD) capacity are preferred that on an annualised basis works out to nearly 2 to 3.5 (MTPA) of clinker entailing huge capital cost in the range of Rs 1,500 to Rs 3,500 crore. Such large upfront capital investment calls for assured life of more than 30 years of plant for any investor to get economically viable returns. When such large quantity of limestone is processed day in and day out, the tolerance/fluctuations in terms of quality and quantity needs to be minimised for a very smooth and therefore economical operation of the clinkering unit.
As a rule of thumb, in order to produce a tonne of clinker about 1.6 tonnes of limestone is processed, i.e., around 3.5 to 5.5 MTPA of limestone is required and assuming a minimum plant life of 30 years each modern clinkering unit would need an assured limestone deposit of more than 100 to 150 MT. Such large quantity of limestone handling on an annual basis necessitates that the cement plants (at least the clinkering unit) should preferably be located in proximity to the cement grade limestone deposit.
This is perhaps the main reason as to why in India cement plants occur in clusters. A cursory glance of map of India with cement plant locations would show clusters like, for example: Kadapah in Andhra Pradesh/Telangana; Ariyalur in Tamil Nadu; Wadi and Gulbarga in Karnataka; Damoh, Katni, Satna and Rewa in Madhya Pradesh; Raipur and Bilaspur in Chattisgarh; Bhavnagar, Amreli and Junagadh in Gujarat, so on a so forth.
Further compared to other parts of world, the Indian limestone deposits are considered to be of inferior grade meaning marginal or sub-marginal in quality and therefore need additional processing step/s to enrich lime content alternately it may call for adding high grade limestone to enhance overall lime content before it is ground and fired in the kiln. The problem of low grade in limestone gets further compounded by the fact that the Indian coal, which is used as a source of heat, is also poor grade meaning low in heat value and high in ash.
Limestone classification generally adopted in the Indian cement industry is two folds: one based on its origin such as igneous (Carbonetite), metamorphic (Marble) and sedimentary and other being based on its geological age. Like Achaean (>2,500 million years), i.e., those limestone deposits that were formed much before any life form existed, Proterozoic (between 2,500 and 600 MY) Palaeozoic (600 to 250 MY), Mesozoic (250 to 65 MY) and Cenozoic (65 to Present).
In India, occurrences of igneous type limestone deposits are very rare and none of the cement plant is exclusively relying on this type of limestone.
The metamorphic limestone deposits, generally in India, are of Archaean Age and are located in Coimbatore, Salem and Madurai regions of Tamil Nadu, Dharwar region of Karnataka and Aravalli region of Rajasthan. Such occurrences exhibit complex type of limestone deposits meaning non-uniformity in terms of shape and size, intricately mixed with waste material and inconsistency in quality. Because of these complexities, such deposits are relatively lower grade and thus invariably calls for additional processing ' be it selective mining, screening either before or after crushing, magnetic separation, adding higher/better quality stone that is generally termed as "Sweetener" to in some rare cases going all the way to froth floatation.
From the above, it may be obvious that such deposits are not preferred because of high mining/manufacturing cost, but still we do find some cement plants in India derive their limestone deposit because of its proximity to the consumption centres. So from this description it is evident that the selection of plant site is not solely dependent on limestone deposit but it is a complex interplay of many other criteria chief among them being "total logistical cost" of input materials as well as finished product.
One would not be wrong in making a statement that the backbone of the Indian cement industry is Proterozoic limestone deposits starting from dense cluster around Cuddapah and Kornool region, Bhima basin in Gulbarga, Katni, Satna and Damoh regions of Vindhya Range, etc. are considered to be the ideal source of limestone. They are sedimentary type; therefore predictable in shape and size, fine grained therefore easy to burn/react in chemical process, large to very large size that could support modern mega cement plants and are near or about uniform cement grade limestone.
These features make such limestone deposits as mainstay of the Indian cement industry. The paleozoic age sedimentary rock formations in India are known for extensive coal deposits in Bihar, Bengal, Chhattisgarh, Odisha and Maharashtra; unfortunately however barring a few small limestone deposits no major limestone deposits are found in Indian Peninsular region. Though economically insignificant, a few limestone deposits are reported in the Himalayan region.
Similar is the situation in Mesozoic Period except sedimentary limestone deposits occur in Tiruchirappalli, Tamil Nadu. The limestone deposits are though patchy but its aerial extent is large and hence many large cement plants are situated in Tiruchirappalli, Perambular and Cuddalore districts of Southeastern Tamil Nadu. These limestone deposits are banded and inter-bedded with marl and other clayey formations. Limestone deposits of this region exhibits better quality in terms of CaO content however at times it gets contaminated with marl and thus brings down the overall quality of limestone.
In addition of above limestone deposits, extensive limestone deposits in Himachal Pradesh and Uttarakhand are said to be of Mesozoic age indicating that their age is doubtful. As mentioned above, these limestone deposits are large, mostly occur in hilly areas, sedimentary type. From the quality perspective, they are invariably high in CaO content at times, however some minor oxides are at elevated levels. These limestone deposits support almost all major cement plants in Himachal Pradesh and a plant in Uttarakhand (CCI Poanta Sahib).
One of the youngest formations belonging to Cenozoic Period, in India is found along the periphery of continental margins. Extensive limestone occurrences in Kachchh district of Gujarat is one such example. This deposit currently supports many recently constructed large cement plants. The limestone deposits of this region is considered to be high grade as far as CaO content is concerned however, minor oxides especially SO3, Na2O and K2O are high and therefore in some cases alkali and/or sulphur bypass arrangement is required.
Another extensive occurrence of limestone of this period is in Northeastern State of Meghalaya and in that also the large limestone deposits in Jaintia district are notable. Such large deposits have tremendous potential of supporting large cement plant and in fact a few are already in operations. These limestone deposits are high to very high in CaO content with a little less problem of minor oxides as compared to Kachchh District.
Finally, the limestone deposits that occur along and almost parallel to the Saurashtra coast and support many cement plants belongs to almost recent age, i.e., these deposits were formed at about two million years back. These limestone deposits are very soft that could be cut even with an axe and because of this reason many houses in Saurashtra region cut and dress into brick like blocks and used as a primary building material. Like above, quality of limestone is very good and easy to mine. However being close to sea coast, certain restrictions are imposed due to environmental considerations.
Summarising above discussion, it could be said that in-spite of many challenges, raw material situation is reasonably good to support not only current cement capacity in India, but also are confident that future expansions are well supported till the middle of this century.
ABOUT THE AUTHOR:
MG Damle did his post graduation from the Department of Geology, University of Rajasthan and joined ACC as a Junior Geologist. After nearly four decades of active service with the company, he retired as Director ' Raw Materials and Mine Planning. He was associated with many exploration campaigns ' for ACC and other external clients. He has led to establishing many modern cement plants in India. In addition, he was also associated with iron ore, chromite, talc, and other mineral explorations in India and abroad.