Most of the grinding of coal for combustion burners is performed in roller mills, but air — swept dry-grinding ball mills operating in closed circuit with separators are also used. Coal is fed to the ball mill with heated air. The air is drawn through the mill by a fan
located in the classifying section. The heat resulting from dry grinding and from the heated air is sufficient to drive off any moisture in the coal. Coal drying prevents coating in the mill, the classifier, and the air ducts. The oversize from the classifier is returned to the ball mill, and the fines are fed direct to the burners in the combustion unit.
The grindability of coal can be obtained from Bond (1952) or Hardgrove (1938) grindability tests. The Hardgrove test involves grinding 50 g of a prepared sample in a miniature pulverizer for a specified time and determining the amount of material that is retained on a 75-p. m sieve in the product. It is a widely used test for coal, but the Bond test is preferred for most other minerals.