Page 144 - Advanced Mine Ventilation

Generation of Respirable Coal Dust 125

The physical properties of coal that affect the production of dust, namely, resistance
2
1
to comminution, microstrength , Hardgrove grindability , etc., are in turn very much
dependent on the chemical and petrological properties of coal [4,5]. Thakur [6] found
that the following chemical and petrological properties of coal have significant influ-
ence on the physical properties of coal and consequent respirable dust production:
1. Moisture content of coal (as received).
2. Ash content (dry).
3. Volatile matter content (dry ash free [DAF]).
4. Fixed carbon content (DAF).
5. Fixed carbon/volatile matter (DAF).
6. Fusain content.
The various coal types or even coal seams can, thus, be indexed with respect to their
proneness to respirable dust production based on their proximate and petrological
analyses. Because these coal constituents are easily measurable, such indexing will
help the mining engineers in preplanning their dust control programs.
To eliminate the effect of the breaking mechanism, all the samples were subjected
to the same comminution process in Thakur’s experiment [6]. Equal masses of
uniformly sized coal particles were comminuted for a given time, and the yield of
respirable dust was measured and expressed as a percentage of the original mass
of the sample. This percentage is defined as the respirable dust index (RDI) of the coal.
Thakur [6] derived an expression for RDI as shown in Eq. (9.2).

0:714 0:191
0:95ðFCÞ ðFÞ
RDI ¼ (9.2)
0:168
ðaÞ
where FC is the fixed carbon (DAF); F is the fusain content; a is the ash content
(moisture free).
Moisture was not included in Eq. (9.2), even though it has a great impact on respi-
rable dust production, because it was highly correlated with fixed carbon. Besides, in
his experiment, the moisture contents of the samples were not truly representative of
the original moisture content of the coal. These samples were air-dried and kept in
sealed cans under vacuum to preserve its other chemical compositions. Details of
the procedure and derivation of Eq. (9.2) are as follows.

9.2 Sample Preparation and Experimental Details

Eighteen coal samples studied represented the various ranks of coal from anthracite to
lignite. Details of locations from which the samples have been drawn, type of coal, and

1
Microstrength is a measure of the resistance to the degradation of coal.
2
Hardgrove Grindability Index is a measure of relative ease of grinding coal. For details, reference can be
made to the American Society for Testing and Materials, ASTM Standard on Coal and Coke, Philadelphia,
PA, 1962, pp. 115e118.