Acid Rock Drainage - Mining and the Environment: From Ore to Metal - page 704

engineer, hydrologist, and rehabilitation specialist. Factors considered in design of cov-
ers include:
●

Physical properties of capping materials, including particle size distribution, moisture
content, compaction, shrinkage, and permeability;

●

Surface coni guration, including angle and length of slopes, presence and slope of
berms to meet future land use requirements while being reasonably simple to construct;

●

Permeability, which should be as low as practicable, and is minimized by conditioning
and compaction;

●

Potential for cracking which depends on mineralogy, climate, depth of burial, mois-
ture content, and degree of compaction;

●

Erodibility;

●

Potential for capillary rise.

Treatment

Despite all the measures applied for storage of waste rock and tailings, there may still be
acidic streams that require treatment before discharge to the environment. During oper-
ations, these streams are commonly collected and added to process water. In the case of
most gold operations, lime is added in the process to raise the pH to 8 or more, so that this
constitutes the necessary treatment. For acidic drainage streams remaining after comple-
tion of processing, ongoing treatment may be required.
For small acidic streams, this may be as simple as the addition of lime slurry to the
stream. For larger discharges, the acidic drainage may be treated in a treatment plant,
using lime. Various other treatments exist, to remove metals as well as to neutralize the
streams. In particular there are several highly promising technologies for the extraction
of metals such as copper, zinc, and cadmium, such as Biosulphide treatment process, and
zeolite cementation (MEND 2001- Vol. 5) which in favourable circumstances may be ame-
nable to metal recovery. However, conventional neutralization using limestone (CaCO 3 )
and/or lime (Ca(OH) 2 ) remains the most widely applied and most reliable treatment tech-
nology. The lime reacts with acid to produce gypsum as follows:

Conventional neutralization
using limestone and/or lime
remains the most widely applied
and most reliable treatment
technology.

→

H 2 SO 4 Ca(OH) 2 CaSO 4 2H 2 O (17.5)

As the pH rises with addition of lime, metals will precipitate from the solution, as hydroxides.

CASE 17.4
Blending Limestone with Sulphidic Waste Rock at Grasberg, Indonesia

Limestone underlies the main waste rock dumps for the
Grasberg Copper Project in Papua Province, Indonesia. In
the initial planning, it was assumed that drainage from
the waste rock dumps would infi ltrate the numerous solu-
tion cavities in the limestone foundations and would be
neutralized at the same time. This was the case for the
fi rst few years of operations. However, ultimately, the pH

of water draining from the dumps declined, indicating that
the neutralizing capacity along the fl ow paths had been
exhausted. In the circumstances at Grasberg, encapsulation
would not be practical because of the absence of soils
suitable for construction of encapsulating layers. After a
wide range of studies into different options, it was decided
to mix crushed limestone with potentially acid-forming

wastes, particularly in near-surface zones of the dumps.
A series of laboratory column tests and fi eld trials dem-
onstrated the effectiveness of this method in achieving
in situ neutralization of acid produced by oxidation. A
limestone quarry and crushing plant were constructed
nearby, with the main objective of supplying limestone for
blending with waste rock.