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Out-of-seam dilution: Economic impacts and control strategies 161

this material results in an effective “clean coal” productivity loss, which can be quan-
tified using any production modeling tool. In this case, a simple deterministic produc-
tion model developed by the authors [10] was used. The analysis assumes that
extraction costs are the same for all sources of dilution and ignores the reality that
mining harder roof strata generally have a higher cost than mining softer coal and floor
strata due to increased bit replacement frequency and greater energy needs. In some
cases, mining floor strata may be more expensive depending on moisture and plastic-
ity characteristics. Extracted OSD incurs material handling costs as it is conveyed
from the point of extraction to the point of processing or loading for shipment to
the customer. Obviously, OSD is conveyed together with coal; however, for modeling
purposes, it is treated separately to account for higher densities characteristic of OSD,
which have a direct bearing on conveyor energy costs.

9.2.7.2 Processing costs

The processing cost impact includes several components. The purpose of the rotary
breaker at the feed end of most coal preparation plants is to remove large inorganic
material, a large percentage of which is OSD. In most instances, eliminating OSD
would eliminate the need for a rotary breaker. OSD size distributions, weight densi-
ties, and abrasive and chemical characteristics increase wear on processing equipment
and infrastructure as it flows through each processing circuit resulting in higher main-
tenance costs. OSD extracted from the floor is mostly clay, which breaks down into
ultrafine particles ultimately handled by a thickener. Eliminating OSD does not elim-
inate the need for a thickener, as was the case with the rotary breaker, but it could
allow for a smaller thickener to be used. Regardless of thickener size, OSD requires
more flocculant and flotation chemicals. In addition, clay material also affects media
viscosity in heavy-media operations, which has a negative impact on the separation
efficiency of those circuits. Cost estimates generated in a related study of these effects
[11] were utilized in this case.

9.2.7.3 Quality impact costs
Quality impacts of OSD are the result of near-gravity material in OSD being mis-
placed to the clean coal product. All coal separation devices have imperfection (I)
and probable error (E p ) ratings, a measure of the amount of misplacement they allow.
Modern heavy-media systems have lower I and E p (i.e., are more efficient) than older
water-only systems. Misplaced material results in higher ash and sulfur levels in the
final product. These translate to lower yields, reduced heating values, and higher sul-
fur emissions. For pollutants that are capped, such as sulfur, there are costs for mit-
igation or purchase of allowances. To estimate cost impacts of quality deterioration
due to increased sulfur levels in the product, an allowance price of $390 per ton of
SO 2 [12] was used. Misplaced material is typically higher in undesirable trace ele-
ments, which also has an impact on product quality. Although samples were analyzed
for several trace elements, mercury (Hg) was used as a surrogate to determine quality
impacts because it has come closest to being a regulated pollutant in the United States.