Page 290 - Advances In Productive, Safe, and Responsible Coal Mining

266 Advances in Productive, Safe, and Responsible Coal Mining

current operating costs by $0.86 (about 2.5%–3.0%) per clean ton mined over the con-
ventional practice of FCPW impoundments [10]. This analysis ignored potential pro-
ductivity increases due to increased extraction from secondary mining, which may
be significant. Regarding the latter, modeling performed in the same follow-up study
indicated that a paste backfill regimen utilizing a minimum backfill strength of
150kPa (22psi) at a minimum fill height of 50% of pillar height could significantly
increase long-term stability of coal pillars in the Illinois Basin [10].

13.2.4 Reduce, reuse, and recycle
The first priority in developing sustainable coal waste disposal practices of the future
has to be reducing the amount of waste produced. This strategy is covered in detail in
Chapter 11. Some reuse and recycling strategies were previously mentioned in this
chapter. Because of fly ash’s chemical properties, cement plants have been recycling
it for decades, using it as a substitute for Portland cement in concrete, a practice that is
coming into increasing favor as “green” concrete. Every ton of Portland cement made
the conventional way (heating limestone and clay to thousands of degrees) creates
0.8tons of CO 2 . Thus, every ton of fly ash used in concrete eliminates almost an equiv-
alent amount of the greenhouse gas. However, there are some barriers standing in the
way of that happening on a broader scale. The chief one is transportation costs. Like
aggregate, transporting fly ash over long distances destroys the economics of using it.
Cement plants need a reliable, steady supply that is local meaning that green concrete
will only be produced in close proximity to a power plant. The same can be said for
synthetic gypsum wallboard, clay bricks, and many of the other products previously
mentioned that are made from recycled fly ash.
However, fly ash has a number of other qualities that have been shown to be ideal
for high-value, specialty applications, which might change the transportation compo-
nent fly ash recycling economics. For example, hollow fly ash particles are strong
enough to be used as a lightweight additive in many metals. Researchers have devel-
oped and tested aluminum metal-matrix composites (MMCs) made with up to 50% fly
ash [25]. MMCs made with 20–30wt% fly ash were found to be as strong as metal only
but much lighter leading the US Office of Naval Research to explore whether they can
be used to make lighter armored vehicles or ships [13].
Coal ash particles also have a chemical structure that can easily be manipulated to
absorb oil. Researchers funded by the US National Science Foundation patented
“functionalized” fly ash particles that absorb oil but repel water [26]. The idea is
to place them in booms on the surface of an oil spill where they become oil-saturated,
then to recycle them back into a power plant to use the energy of the spilled oil. Testing
has shown that this process prevents leaching of toxins from the fly ash [13].

13.3 Summary

Facing the concurrent trends of increased environmental awareness among the non-
mining public and decreasing margins of profitability in the coal-mining business,
mine operators and all those who provide technical support to them must examine