Page 185 - Advances In Productive, Safe, and Responsible Coal Mining

164 Advances in Productive, Safe, and Responsible Coal Mining

9.3.1.2 Summary results for all samples

Table 9.2 lists average (for all sampled units) ash, sulfur, and trace element contents
for each sampled strata. These data reveal that roof rock contains significantly higher
trace element concentrations compared with either the coal seam or the floor strata.
Mercury content in the roof is almost three times that of the coal seam, while mercury
content in the floor is only slightly higher than that of the coal seam. Trace element
concentrations in the roof strata are one to two orders of magnitude higher than those
in the coal seam and are higher than those in the floor strata despite the fact that ash
content of floor strata is somewhat higher than ash content of roof strata. These results
establish that concentrations of unwanted constituents in coal, such as ash, sulfur, and
various trace elements, are significantly higher in OSD and in roof strata in particular
and that mining OSD should be minimized to the extent possible.
The “blue band” is a prominent shale parting commonly found near the bottom of
the Illinois No. 6 coal seam. Where this rock band is present, its ash, sulfur, and trace
element concentrations are significantly higher than for the coal seam. The “blue
band” is technically not OSD due to its position within the seam. Furthermore, selec-
tive elimination during mining is not practical. Nevertheless, these results suggest that
avoiding any kind of dilution, be it out-of-seam or in-seam, is beneficial from a prod-
uct quality perspective.
Results from simulated cleaning of this ROM material are presented in Table 9.3.
Although there is a high degree of variability in results, it appears that, on average,
increases in ash, sulfur, and trace element concentrations in clean coal attributable
to OSD are quite significant. Ash, sulfur, and mercury contents in clean coal are shown
to increase by 8.0%, 1.9%, and 3.8%, respectively, when OSD is present in ROM coal.
Other trace elements also increase substantially with As and Se contents increasing by
as much as 12% and 46%, respectively. These results clearly establish the hypothesis
that mining OSD has a significant and negative impact on product quality in terms of
ash content (which implies a corresponding reduction in heating value), sulfur content,

Table 9.2 Average ash, sulfur, and trace element contents in sampled
horizons of Illinois mines

Wt Ash S As Hg Pb Se
Strata (%) (%) (%) (ppm) (ppm) (ppm) (ppm)

Roof 13.53 71.55 2.77 25.77 0.29 34.97 79.59
Coal seam— 3.40 9.42 1.05 1.76 0.07 2.94 4.80
top 3in.
Coal seam— 75.50 12.94 2.00 4.77 0.11 12.06 2.93
middle bench
Blue band 4.83 53.10 4.75 10.27 0.39 128.3 5.10
Coal seam— 3.63 16.19 3.04 4.16 0.08 7.58 2.13
bottom 3in.
Floor 7.98 79.38 3.25 6.75 0.12 38.79 2.28