With limited life left for the use of limestone as a major raw material for cement, every day there is a new challenge to use the material. Mainly the quality of limestone is a challenge.
A majority of the limestone deposits available for cement Industry in India are marginal grade, many of the deposits that have come up or are likely to come up for auction are having high silica and low to sub grade CaO content and many of the deposits are high in MgO content as well. This type of deposit cannot be utilized for cement manufacturing process without blending with high grade limestone. About 97 per cent of Limestone is used in Cement Industry, 3 per cent in other industries like Iron and Steel and Chemical. Hence major consumer for limestone is cement industry.
Sedimentary limestone is normally associated with soft deleterious components like clay and shale. Owing to difference in hardness these softer components become powdery on blasting and are separated through beneficiation process like screening and washing. Metamorphic limestone on the other hand is normally associated with hard igneous intrusive like pegmatite, granite, amphibolite etc. and at times with dolomites/ dolomitic limestone which all are equally hard as metamorphosed limestone. On blasting, all these components along with limestone are obtained in almost the same fraction size and are therefore difficult to segregate in the course of beneficiation in mechanised mining.Marginal Grade limestone
Many of the deposits that have come up or are likely to come up for auction are marginal grade having high silica and low to sub grade CaO content found mainly in Rajasthan, Chhattisgarh, Gujarat and Maharashtra from cement manufacturing perspective which alone will not be able to sustain the process requirement for manufacture of clinker.
The deposits that have come up for auction were high in MgO content as well. These too will not be able to sustain the clinker manufacturing process owing to more than desirable limits of MgO in clinker manufacturing (Rajasthan). This will definitely require blending with high grade limestone whose availability is very limited. Deposits with inherent high silica cannot be utilized for cement manufacturing process without blending with high grade limestone.
NCCBM norms for prospecting limestone suggest limiting value of CaO to be minimum 40 per cent. The scope of beneficiation or addition of small quantity of sweetening material may make otherwise sub-grade limestone suitable for cement manufacture. The broad chemical specification is given below:
The scope of beneficiation or addition of small quantity of sweetening material may make otherwise sub-grade limestone suitable for cement manufacture.
Role of fuel in the cut off limit of limestone: Fuel being used in the cement industry is petcoke, imported coal and Indian coal. Thus, industry uses Indian coal along with imported coal/ petcoke so that average run of mine is maintained at 43-45 per cent CaO.
Role of additives in the cut off values of RoM limestone: Besides Limestone, cement raw mix requires 8 to 10 per cent additive material like laterite, bauxite, red ochre, iron ore etc.
Day-by-day, quality of additives are deteriorating i.e. increase in silica. The quality also deteriorates as we go down the depth. The top 4 to 5 m is high grade with an average CaO of 46 per cent. Remaining limestone is varying from 40 to 42 per cent CaO. Remaining marl and low grade limestone is having CaO 31 to 40 per cent.– VIKAS DAMLE