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Out-of-seam dilution: Economic impacts and control strategies 157

efficient, OSD, having a higher specific gravity (SG) than coal, would be entirely
rejected and not influence clean coal quality; however, there is always some degree
of imperfection in even the most efficient coal preparation processes. Probable error
(E p ) is a measure of the degree of imperfection of a density-based separator based on
the amount of heavy material it misplaces to the float and the amount of light material
it misplaces to the sink. E p values for typical density-based separators range from 0.02
to 0.05 for heavy-media processes and from 0.15 to 0.25 for water-only processes.
Even though the misplaced percentages are small for heavy-media systems, large
amounts of OSD in the raw coal feed can result in significant amounts of this material
reporting to the clean coal product. The impact is particularly severe when lighter
OSD materials such as black shale (SG 1.8) are encountered. Most separators have
d 50 cut points set close to this density implying that as much as 50% of such OSD
material can report to the product. Compounding the problem, OSD material is known
to contain significantly higher amounts of pyrite, mercury, and other trace elements
[3]. Therefore, the negative impact on product quality can be very high. This aspect of
OSD’s impact is further detailed in this chapter, but additional research is still needed.
Other costs related to OSD have been highlighted by Luttrell et al. [4], who esti-
mated the cost of OSD from a blending perspective. It was argued that approximately
3tons of 35%–40% ash middlings can replace 1ton of pure rock without changing the
total mass of ash in the coal product. Thus, in coal blending operations, coal prepa-
ration middlings can be added by increasing separator cut points if OSD is reduced.
The analysis predicted that for 100tons of ROM coal, 21tons of middlings can be
substituted for a 7ton reduction in pure OSD rock. Assuming a selling price of $25
per ton, this net gain of 14tons of product per 100tons of feed results in increased
profitability of approximately $14 million per year for a 1000tons-per-hour prepara-
tion plant.

9.1.3 Importance of achieving OSD control

Increased competition due to the recent industry downturn and the resulting focus on
reducing production costs have led some mines to more closely examine the feasibility
of reducing OSD to benefit from favorable economic impacts of that strategy. At one
Illinois Basin mine, a 2.5in. (0.06m) reduction in OSD was demonstrated after mine
management, and the project team educated the face crew on the impacts of OSD [1].
Data collected during that study demonstrated that even without any focus on or incen-
tive to reduce OSD, desired levels were inadvertently achieved about 15% of the time.
This suggests that substantial reductions in OSD are possible if conscious efforts
are made.
Most of the field data presented in this chapter resulted from research conducted in
the Illinois Basin, a region that saw coal production levels decline by one-half in less
than a decade following legislation passed to curb sulfur emissions. Trying to stay
competitive, Illinois Basin coal producers focused on costly “clean coal technology”
for preparation plants and power plants instead of reducing OSD so that less
“cleaning” was necessary. Consequently, marketing decisions favored lower-cost pro-
ducers in the Powder River Basin for many years. Similar market dynamics should be