Measurement grindability

Grindability is a measure of the rate of grinding of material in a particular mill (discussed later). 

Chandler [Bull. Br Coal Util. Re.s. A.s.soc., 29(10), 333 (11), 371 (1965)] finds no good correlation of grindabihty measured on 11 coals with roh crushing and attrition, and so these methods should be used with caution. The Bond grindability method is described in the subsection Capacity and Power Consumption. ... 

The measured true density of clinker is typically 3150-3200 kg m the value calculated from the X-ray densities and typical proportions of the individual phases is about 3200kgm" The difference is probably due mainly to the presence of pores inaccessible to the fluids used in the experimental determination. Using mercury porosimetry. Butt et al. (B35) determined pore size distributions and discussed their relations with burning and cooling conditions and grindability. 

Some of the drum tests stress the materials much more severely. Methods 1 and 2 particularly are grindability tests producing abrasion values of up to 70%. On the other hand, methods 3 and 4 are so gentle that the results hardly differentiate between samples of different quality. Method 5 differentiates well but the relative measures of quality of materials A, B, and rock salt are switched. 

Grindability (in its many different definitions) is a composite material property, and depends on many primary material properties (e.g. particle hardness, bulk and shear moduli of elasticity) as well as its flow properties and other conditions like moisture content, humidity of the atmosphere or material composition (rank or ash content of coal, for example). It also depends on the type of mill used for its evaluation. There have been some attempts made by several authors to find correlations relating different measures of grindability the reader is referred to the literature for details of these54. 

Hardgrove grindability index Measure of the ease by which the size of a coal can be reduced. Values decrease with increasing resistance to grinding. 

The Hardgrove Grindability Index was originally developed by Babcock and Wilcox to measure the relative ease of pulverizing coal. This ASTM D409-71 method has since been employed for characterizing other particles as well. The method processes 50 g of air-dried particles screened to a size of 16 x 30 mesh (1180 X 600 pm) in a small ball-and-race mill for 60 revolutions. The amount of material (1 200) passing the 200 mesh (75 pm) screen is then measured. The result is then compared with a standard index to arrive at the HGI or the HGI is calculated from the equation... 

On account of the short residence times of the feed material in a roller mill — for example, a cycle time of about 30 seconds was measured in one such mill — these mills respond much more rapidly than tube mills to disturbing influences, e.g., variations in feed rate, grindability or moisture content of the material to be ground,... 

Grindability A term used to measure the ease of pulverizing a coal in comparison with a standard coal chosen as 100 grmdabihty. 

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