Preparation of High Ash Indian Coals for Carbonization

Abstract

The quality parameters of coke produced from a particular coal blend can be significantly influenced by the coal preparation technique adopted. In Indian coking industry, generally 8-10 types of coals are used as raw material base for carbonization and they differ widely in terms of their grindability characteristics, caking/coking properties, dulatation characteristics, petrographic analysis etc. The Indian coals contain very low percentage of vitrinite (<50%), higher content of middlings and rejects (>1.6 Sp. gr. fraction) and higher ash content (18-23%). Conventional blend crushing of these coals together for carbonization leads to concentration of inerts and middlings/rejects in the coarser size fraction (+6 and 6-3 mm) of the charge leading to heterogeneity in the properties of different size fractions. Difference thermal transformation to plastic state and shrinkage characteristics in the semi-coke state between coarser and finer fractions generates internal stresses in the coke matrix leading to poor strength characteristics.
With the use of increased percentage of low ash imported coal in the blend (15-40%), the SAIL plants are trying to contain the coal blend ash to a level of about 17%. The imported coals generally have low mineral matter content, high HGI values and increased proportion of fines (60-65% below 3 mm), These circumstances have necessitated development of new coal preparation schemes for carbonization viz. groupwise crushing, blend crushing with prescreening of fines and groupwise crushing with prescreening of fines. The paper discusses in detail the above new coal preparation techniques along with the methodology to determine the degree of homogeneity of the coal blend.