India has an estimated total limestone reserves of 96 billion tones (March 2005 estimates) out of which nearly 30% is either low or marginal grade which is not directly usable in cement manufacture . The quality of coal presently made available for cement manufacture i.e. of higher ash contents (>35% ash ) has further aggravated the situation , resulting in higher consumption of the scarce high grade limestone from the existing mines or increased usage of purchased high grade limestone .
At the Research & Development Division considerable studies have been carried to look into different possibilities of beneficiating low/marginal grade limestone from the limestone deposits of ACC’s different Cement Works. The beneficiation techniques include froth floatation method, bio-technological methods and dry beneficiation methods like screening, differential grinding, air classification , electro-static separation , tribo-electric separation etc .
The paper illustrates the results of some of these investigations .As most of present day cement plants are based on dry process the conventional wet process beneficiation techniques like froth floatation do not have a techno – commercial feasibility.
The paper further discusses in detail a case study related to one of the ACC works , where , in the conversion of wet process to Dry process of cement manufacture, in order to maximize the use of low grade limestone , partial use of floated limestone has been considered. This has been achieved through an innovative partial use of the floated slurry in the dry Raw Meal grinding in the Vertical Raw Mill system .
The case study discussed in the paper thus illustrates that use of low / marginal grade limestone can be maximized through selection and adoption of an appropriate limestone beneficiation technique through an innovating process adoption .