HARDGROVE GRINDABILITY INDEX ESTIMATION APPARATUS

The grindability of coal is a measure of the ease with which it can be ground fine enough for use as a pulverized fuel and as such it reflects some of the physical properties of coal like hardness, strength, tenacity and fracture Of the two methods developed for determining grindability of coals, that developed by R. N. Hardgrove is better known and simpler. The Hardgrove Grindability test serves as a means for estimating how various coals behave in the commercial pulverizers. This method is based upon the relative ease of grinding in a batch machine where the grinding is stopped before appreciable crushing from finely crushed material occurs. The relation between grindability and capacity of any pulverizer is a straight line if plotted on a graph, provided fines are removed from the pulverizing zone before the crushing effect becomes dominant.

A general relationship exists between 9findability of coal and its rank as shown by the degree of metamorphism in the natural series from brown coals and lignites to anthracite. Coals that are easiest to grind (having highest grindability index) are those of about 14-30 per cent volatile matter content on the dry mineral-matter free basis Coals of either lower or higher volatile content are more difficult to grind.

It consists a stationary grinding bowl of iron, with a horizontal track in which run 8 steel balls each of 25 mm. in dia. The balls are driven by an upper grinding ring  which is rotated at19-21 revolution per min. The upper grinding ring is connected to a spindle which is driven by an electric . motor through reduction gears. Weights are added to the driving spindle so that the total vertical force of the balls due to weights, shaft, top-ring and gear is equal to 29 kg ± 0225 kg. The machine is fitted with an auto­matic counter for stoppirlg the machine after 60 ± 0,25 revolutions,