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Out-of-seam dilution: Economic impacts and control strategies 159

(0.08m) deep channel into the cross section of the face being sampled. A brattice cur-
tain was laid on the floor to collect coal and rock material as it was chipped from the
channel. Approximately 10lb (4.5kg) of the material were collected at each sampling
location. These samples were placed in buckets and sealed with tight lids. For each
roof and floor sample and for top and bottom coal horizons, separate 2lb (0.9kg)
samples were collected, placed in airtight bags, sealed, and labeled.

9.2.2 Sample preparation

In the laboratory, samples were prepared by crushing in a jaw crusher and screening at
1mm and 100 mesh sizes to extract the 1mm 100 mesh size fraction. A Jones riffler
was used to obtain a representative ( 150g) split of this sample for conducting a
washability analysis.

9.2.3 Washability analysis
ASTM procedures [6] for conducting coal washability analyses were carefully
followed. This involved fractionation at specific gravities of 1.4, 1.5, 1.6, 1.7, 1.8,
1.9, and 2.0. Densities were prepared using typical mixtures of organic liquids such
as perchloroethylene and dibromomethane. Samples were placed in the lowest-density
liquid, and the portion that floated to the top was collected. The remaining portion was
placed in the next lowest density, and the process was repeated yielding a “float” frac-
tion for each specific gravity and a “sink” fraction for the highest specific gravity. At
each specific gravity, if a sufficient quantity of material required for further analysis
was not obtained in the float fraction, it was combined with the floated fraction from
the adjacent specific gravity, and the weighted average of densities for the combined
sample was calculated. For samples where all material sank at 2.0 specific gravity, an
additional gravity cut was made at 2.2 specific gravity to fractionate the sample. This
separated each of the 57 channel samples into as many as seven float fractions in incre-
ments of particle densities and one sink fraction with particle densities greater than 2.0
specific gravity yielding a total of approximately 450 sample fractions.

9.2.4 Advanced procedure for washability analysis
Traditionally, the average gravity of material for each fraction is calculated as the
arithmetic mean of upper and lower gravities. For example, a material that sinks at
1.4 specific gravity but floats at 1.5 specific gravity is assumed to have an average
specific gravity of 1.45. However, it is possible that all of the 1.4 1.5 density fraction
may actually be in the 1.4 1.45 density fraction with an average specific gravity
closer to 1.425. Thus, to obtain a truly accurate specific gravity requires maximizing
the number of fractions by narrowing fraction ranges to very small intervals, which
can quickly become unreasonably expensive.
To achieve optimal use of research funds, an alternate approach [7] was adopted in
Patwardhan’s study. Gravity fractions were obtained in traditional fashion and then
subjected to ash analysis. The ash content in coal is known to be a function of mineral