Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 687

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Runoff and/or drainage from stockpiles of ore awaiting processing;

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Runoff and/or drainage from waste rock storages including coal rejects piles;

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Seepage or overl ow from tailings storage facilities, and

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Runoff from spillages or remnants of mineral concentrates around stockpiles, bins,
conveyor transfer points, etc.

It appears that nowhere in the mining world is immune from ARD. While moisture and
temperature are essential for the reactions involved, sufi cient moisture exists virtually
everywhere and the reactions, which may be slow to initiate at low temperatures, gener-
ate their own heat so that oxidation rates can rapidly accelerate. Severe examples of ARD
have been recorded in arid and temperate Australia, in the Canadian Arctic, and in equa-
torial Indonesia.
Considerable progress has been made over the past 25 years in developing of methods
for predicting the nature, severity, and duration of ARD. Parallel developments in envi-
ronmental management include measures to prevent, isolate, and treat ARD. Despite the
considerable progress that has been made, management of ARD presents major challenges
at many mining operations throughout the world and accounts for a major part of day-
to-day environmental management and monitoring costs. It is also a major issue, some-
times the dominant issue, in mine closure. More exhaustive discussion of these topics can
be found in Environment Australia (1997), Hutchison and Ellison (eds) (1992) and MEND
(2001). The MEND Manual is a particularly comprehensive and authoritative reference,
covering all aspects of ARD including its prevention and remediation.

It appears that nowhere in the
mining world is immune from
ARD.

17.1 NATURE AND SIGNIFICANCE OF ACID ROCK
DRAINAGE

A Natural Phenomenon

ARD is, in fact, a natural process,
which takes place wherever
sulphide minerals such as pyrite
are in contact with oxygen and
moisture.

ARD is, in fact, a natural process, which takes place wherever sulphide minerals such as
pyrite are in contact with oxygen and moisture. Sulphide minerals are not only associated
with economic mineral deposits. Pyrite is widespread in nature, occurring commonly in
sediments such as organic clays, silts, and peats which deposit in reducing environments.
These sediments lithify to become shales, claystones, siltstones, and coal, all of which com-
monly contain pyrite, either disseminated or concentrated in nodules. Metamorphic rocks
such as schist and phyllite also frequently contain pyrite.
Oxidation is the predominant process involved in surface weathering which, accord-
ingly, takes place mainly in the unsaturated zone above the water table. Weathering of
bedrock usually produces a proi le with highly or completely weathered rock near the sur-
face grading to fresh or unweathered rock at depth. Rock below the water table is perma-
nently saturated and is usually unweathered except for slight weathering along fractures.
As water tables tend to l uctuate seasonally, there may be a transitional zone of incomplete
weathering between maximum and minimum water table levels. Ore bodies which cross
these boundaries may be divided into a primary sulphide zone where sulphide minerals
occur in an essentially un-oxidized condition and an oxide zone where sulphide minerals
have been largely or completely decomposed by oxidation, commonly separated by a tran-
sition zone. These features are illustrated in Figure 17.1 .
It should be noted that the existing weathering proi le may rel ect past water table loca-
tions, not necessarily those that occur under current conditions.