Grindability tests

1 Talc, graphite Can mark paper powdered by finger 

Each Hardgrove machine is calibrated with standard reference samples of coal of grindability indices 40, 60, 80 and 110 which are obtainable from the US Bureau of Mines in Pittsburgh or other sources. The index is determined from the amount of fines formed if a standard sample weight (50 g) is subjected to standard grinding conditions. The harder is the coal, the lower is the index number, with 100 corresponding to an original standard soft coal. 

Fso is the size at which 80% of the feed is finer (both in microns). 

The Work Index is claimed to be capable of predicting the power for a ball mill of 8 ft (2.43 m) in diameter in closed circuit wet grinding. In dry grinding applications, the value of WI has to be multiplied by an empirical constant of 1.3. The Work Index is also supposed to represent the energy in kWh required to reduce one short ton (907 kg) of mineral from a theoretical infinite size to a size distribution at which 80% is smaller than 100 microns. 

Another ball mill test worth mentioning here is the USBM (United States Bureau of Mines) Ball Mill Test57 which is in some respects similar to the Bond ball mill test. The ball mill is 8 x 8 in (203 X 203 mm — diameter x length) and fitted with three lifter bars mounted longitudinally to the inside wall equally spaced around the cylinder. The charge is 100 balls one inch (25.4 mm) in diameter each and the coal charge is 500 g. The mill is rotated at 40 revs/min and a trip counter is used to count the number of revolutions completed. 

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Manufacturers of various types of mills maintain laboratories in which grindability tests are made to determine the suitability of their machines. When grindability comparisons are made on sm equipment of the manufacturers own class, there is a basis for scale-up to commercial equipment. This is better than relying on a grindabihty index obtained in a ball mih to estimate the size and capacity of different types such as hammer or jet mihs. 

The net production in wet grinding in the Bond grindability test varies from 145 to 200 percent of that in diy grinding depending on mesh [Maxson, Cadena, and Bond, Trans. Am. Int. Min. Metall. Pet. Eng., 112, 130-145, 161 (1934)]. Ball mills have a large field of apph-cation for wet grinding in closed circuit with size classifiers, which also perform advantageously wet. 

Other Tests. The so-called tumbler tests are usually used for testing material like coke, coal, iron ore pellets or tablets. They can be divided into drum tests and ball mill type tests. The latter type is used to derive both the Hardgrove Index and the Bond s Work Index, which are often used to classify the material friability as described in Sec. 3. They are generally more suited to coarse material. The Hardgrove Grindability test requires an initial size range form 595 to 1190 microns. 

Related Calculations. Although the Bond grindability test is run dry, the work index calculated above is for wet grinding. For dry grinding, the work index must be multiplied by a factor of 4/3. 

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. 

There are many other indices and specialized grindability tests such as the Holmes grindability tests, indices for autogenous comminution or for superfine grinding but these are really beyond the scope of this guide and the reader may want to refer to a previous review of grinding equipment57. 

Sansoni, G., and Zybell, H., "Grindability Tests with Portland Cement Clinkers. Preparation and Microscopic Testing of Clinkers," Silikattechnik, Vol. 25, 1974, pp. 87-93 (in German). 

Although the resistance of coal to abrasion may have little apparent commercial significance, the abrasiveness of coal is, on the other hand, a factor of considerable importance. Thus, the wear of grinding elements due to the abrasive action of coal results in maintenance charges that constitute one of the major items in the cost of grinding coal for use as pulverized fuel. Moreover, as coals vary widely in abrasiveness, this factor must be considered when coals are selected for pulverized fuel plants. A standardized, simple laboratory method of evaluating the abrasiveness of coal would assist, like the standard grindability test now available, in the selection of coals suitable for use in pulverized form. 

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