Limestone & Cement

Limestone is a sedimentary rock composed mainly of calcium carbonate (CaCO3) in the form of the mineral calcite. The two most important constituents are calcite and dolomite. Limestone often contains magnesium carbonate, either as dolomite Ca, Mg(CO3) 2 or magnesite (MgCO3) mixed with calcite. Such rocks are termed as 'dolomitic' or 'magnesian' limestone. The limestone which is used by industries in bulk quantity is a bedded type sedimentary limestone.

LIMESTONE RESOURCES

1. RESOURCES The total reserves/resources of limestone of all categories and grades as per NMI data based on UNFC system as on 1.4.2015 have been estimated at 203,224 million tonnes, of which 16,336 million tonnes (8%) are placed under reserves category and 1,86,889 million tonnes (92%) are under remaining resources category. Karnataka is the leading state having 27% of the total resources followed by Andhra Pradesh and Rajasthan (12% each), Gujarat (10%), Meghalaya (9%), Telangana (8%), Chhattisgarh and Madhya Pradesh (5% each) and remaining 12% by other states. Grade wise, cement grade (Portland) has leading share of about 70% followed by Unclassified grades (12%) and BF grade (7%). Remaining (11%) are others.
2. About 97% of the total production of limestone during 2015-16 was of cement grade, 2% of iron & steel grade and the rest 1% consisted of chemical grade. Rajasthan was the leading producing state accounting for (22%) of the total production of limestone, followed by Madhya Pradesh (12%), Andhra Pradesh (11%), Karnataka & Chhattisgarh (9% each), Gujarat & Telangana (8% each), Tamil Nadu (7%), Himachal Pradesh & Maharashtra (4% each). 

MINING & MARKETING

1. In India, limestone mines are worked by opencast method. Captive mines are mechanized and supply feed to cement and iron & steel units. Some mines have well laid road-cum-rail routes. The large mines are developed by forming benches in overburden and limestone bed. The face length, width and height of the benches correspond to the mining machinery deployed and production schedule. Heavy earth-moving machinery like 3.3 to 4 cu m capacity hydraulic excavators in combination with 10-35 tonnes dumpers are normally used. Other mines are mainly worked by semi-mechanized and manual opencast mining methods.
2. The threshold value of limestone has been revised by IBM, through a Notification in 2009, as follows: (i) For limestone deposits in Chhattisgarh, Gujarat, Himachal Pradesh, Madhya Pradesh, Maharashtra, Rajasthan, Uttarakhand & Uttar Pradesh - CaO - 34% (min), MgO - 4% (max). (ii) For limestone deposits of Andhra Pradesh, Jharkhand, Karnataka, Kerala, Odisha & Tamil Nadu - CaO - 35% (min), MgO - 4% (max), SiO2 - 18% (max) & Alkalies - 0.5% (max).

The principal use of limestone is in the Cement Industry. Other important uses are as raw material for the manufacture of quicklime (Calcium Oxide), slaked lime (Calcium hydroxide) and mortar.

SPECIFICATIONS OF LIMESTONE

FOR CEMENT INDUSTRY

1. Cement is a binder, a substance used in construction that sets, hardens and adheres to other materials. Cement used in construction are usually inorganic, often lime or calcium silicate based. Magnesia, sulphur and phosphorus are regarded as deleterious elements. Limestone should have less than 3% magnesium oxide (MgO), maximum tolerance being 5 percent. The presence of P as P2O5 more than 1% slows down considerably the setting time of Portland Cement. Indian cement manufacturers prescribed that the limestone should have CaO 42% (min), Al2O3 1 to 2%, Fe2O3 1 to 2%, SiO2 12 to 16% and MgO 4% (max).
2. The broad chemical specifications of cement grade limestone (r.o.m.) for cement manufacture suggested by the National Council for Cement and Building Materials, New Delhi, are in the table above.
3. Portland cement: Portland cement is by far the most common type of cement in general use around the world. This cement is made by heating limestone (calcium carbonate) with other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which then chemically combines with the other materials that have been included in the mix to form calcium silicates and other cementitious compounds. The resulting hard substance, called 'clinker', is then ground with a small amount of gypsum into a powder to make 'ordinary Portland cement', the most commonly used type of cement (often referred to as OPC). Portland cement is a basic ingredient of concrete, mortar and most non-specialty grout. The most common use for Portland cement is in the production of concrete. Concrete is a composite material consisting of aggregate(gravel and sand), cement, and water. As a construction material, concrete can be cast in almost any shape desired, and once hardened, can become a structural (load bearing) element. Portland cement may be grey or white.

The Clinker invariably composed of Tricalcium Silicate, Dicalcium Silicate, Tricalcium Aluminate & Tetra calcium Alumino-ferrite.

FOR IRON & STEEL INDUSTRY

1. In Iron & Steel Industry, limestone is used both in blast furnace (BF) and steel melting shop (SMS) as a flux after calcining. It is also added as flux in self-fluxing iron ore sinters. It has two basic functions in steel making, first to lower the temperature of melting and second, to form calcium silicate which comes out as a slag, as it combines with silica in iron ore. For use in the blast furnace, the calcium carbonate (CaCO3) content in limestone should not be usually less than 90 percent. The combined SiO2 and Al2O3 should not exceed 6% though up to 11.5% is allowed; MgO should be within 4% and sulphur and phosphorus as low as possible. In Steel Melting Shop (SMS), insoluble in limestone should not exceed more than 4 percent.
2. Good fluxing limestone should naturally be low in acid constituents like silica, alumina, sulphur and phosphorus. Limestone should be dense, massive, preferably fine-grained, compact and non-fritting on burning.

FOR CHEMICAL INDUSTRY

The calcium carbide manufacturers generally prefer lime containing 95% CaO (min) with limitations of not more than 3% SiO2, not more than 0.95% phosphorus and other impurities not exceeding more than 2%. For the manufacture of bleaching powder, lime containing 95% and above CaO is required. Total Fe2O3 +Al2O3 +MnO2 should be less than 2%; MgO should be below 2% and SiO2 less than 1.5%. Bleaching powder is prepared by absorption of chlorine by dry hydrated lime. The hydrated lime should not contain more than 2% excess water. Iron and manganese oxides lead to unsuitability of the product and iron oxides tend to discolor the bleached material. Magnesia renders the bleaching powder hygroscopic. Silica and clay impede solution and settling of bleaching powder.

FOR INDUSTRY & CONSUMPTION

India was the second largest cement producing country in the world after China. There were 210 large cement plants having an installed capacity of 410 million tonnes in 2015-16 in addition to more than 350 mini cement plants having estimated capacity of around 11.10 million tonnes per annum. The total installed capacity of cement in 2015-16 was thus about 421.10 million tpy against 356 million tpy in the preceding year. Besides, there are three white cement plants having a total 990,000 tpy capacity. The total production of cement reached 283.45 million tonnes in 2015-16.

Cement was the major consuming industry accounting for 94% consumption, followed by iron & steel (4%) and chemical (2%). The remaining consumption was reported by aluminum, alloy steel, sugar, paper, fertilizer, glass, metallurgy, foundry, etc.

The Cement Industry in India is among the core Industries that is vital for economic growth and development. India the second largest producer of cement after China in the world. Cement is the basic building material and is used extensively in urban housing, industrial sector and infrastructure development. It has become synonymous with construction activity and the per capita consumption of cement is accepted as an important indicator of the country's economic growth.

READY MIX CONCRETE (RMC)

Ready-mix Concrete (RMC) is a relatively nascent market in India accounting for only about 0.5% of the demand. RMC is ready-to-use concrete blend of cement, sand & aggregate and water mixed in convenient proportion. It was first launched in Mumbai a few years ago and is gaining ground in other metros in India. RMC is a corollary to bulk handling and transportation of cement. It has several advantages. It is produced under controlled conditions and hence has consistency in quality and it can be directly powered in the required form which would not only save time but also would improve the quality of construction.

OTHER DYNAMICS OF CEMENT MANUFACTURING

Operating Cost

The cement capacity in the country is mostly concentrated near the main raw material source, i.e., limestone. Other important raw material is coal (0.25 tonnes required per tonne of cement). Many cement plants are situated near the coal belts in eastern Madhya Pradesh, primarily due to two reasons, namely, (i) less freight cost incurred to transport coal, and (ii) inability of domestic coal producers to fulfil supply-requirements of cement plants due to fall in production and prioritized supply to power plants. However, limestone reserves have been the primary consideration in choosing the location of plants. Presence of clusters of capacity and high transportation cost make the cement market regional in nature with the producers supplying cement to areas around the location of the plant. Power is a major parameter that influences the operating cost. Grid power purchased from SEBs is costlier than captive power from coal-based plants by more than 25-30 per cent. Where conditions are favourable, setting up captive wind power farms has become a realistic option for cement plants.

Coal Distribution

Coal being a low value, bulk product with regional concentration of deposits entails freight costs that constitute a substantial part in the production cost of cement. Though, rail is the predominant form of transport, road transport is commonly used by plants located close to pitheads. The Government in its notification to the Cement Industry has permitted cement plants to operate their own captive coal mines. Many cement plants have expressed interest in taking up coal blocks on lease and operating the mines for coal. As proposed by the Government, cement is one of the core sectors for which captive mining blocks would be allocated. As per Cement Manufacture Association's A.R 2015-16, inadequate availability of coal to cement industry is a major constrain. The supply of linked coal during 2002-03 was about 69% of total consumption, this has come down to about 23% during the year 2015-16, mainly due to the diversion of coal to the power sector.

Power Availability

The Industry's average energy consumption is estimated to be about 725 kcal/kg clinker thermal energy and 80 kWh/t cement electrical energy. The best thermal and electrical energy consumption presently achieved by the Indian Cement Industry is about 667 kcal/kg clinker and 67 kWh/t cement which are comparable to the best reported figures of 660 kcal/kg clinker and 65 kWh/t cement in a developed country like Japan. Since the controls were lifted, aggregate power requirements have grown rapidly with rising cement capacity without commensurate growth in power generating capacity in the country. To offset the power crisis situation, many cement plants have set-up installations for captive power generation. Further, as part of reform process in Coal Sector, the Government has also permitted 100% FDI in captive coal blocks in Cement Sector along with Power and Steel Sectors to facilitate and augment power availability.

Disclaimer: The views expressed are personal and after interactions with Industry Experts

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