An Investigation of the Grindability of Alberta Subbituminous Coals

Grindability of coal has been the subject of many laboratory investigations, mainly in the United States and the United Kingdom. Several extensive studies were carried out with a view to having a method of determining the grindability of coal adopted as a standard method by A.S.T.M. Eventually, in 1951; the Hardgrove test (1) was adopted by the A.S.T.M. as the sole standard method. This was probably due, to a large extent, to its simplicity and to the rapidity with which tests could be carried out. Calcott (2) made a detailed study of the performance of a slightly modified Hardgrove machine and its applicability to U.K. coals. Fitton, Hughes and Hurley (3) investigated various factors influencing the Hardgrove test. None of these workers offered any serious criticism of the Hardgrove machine.

All of these studies dealt with high-rank coals having fairly constant ash contents, and low moisture contents varying from 0 to 5 percent. The use of low-rank coals as pulverized fuel has increased rapidly in recent years; however, these coals behave differently from the high-rank coals in a grinding mill.

Samples of Alberta subbituminous coals submitted by mine operators and by power plant engineers have, from time to time, been tested on the Hardgrove machine in laboratories outside the province. The results of these tests have shown a disturbing lack of consistency, and the question has been raised as to what the grindability index really means in grinding practice. Brown (4), using high-rank coals, tried to relate the results of the Hardgrove test to practical grinding, but had to allow for a number of difficulties. It is conceivable that the Hardgrove test, which was designed for use with high-rank coals, would need considerable modification before it could be used successfully for subbituminous coals with high moisture contents.

The present investigation of the grindability of Alberta subbituminous coals was initiated as a result of the dissatisfaction and doubt, which arose from the results of Hardgrove tests on these coals.