Mining Methods Vary Widely - Mining and the Environment: From Ore to Metal - page 232

Further details of the design and operation of heap leaching systems are provided in
Hutchison and Ellison (1992), from which much of this section has been summarized.
Three approaches to leach pad design have been utilized, namely: reusable pads, expand-
ing pads and valley leach dumps. These are depicted schematically in Figures 5.5 and 5.6 .
The features and requirements of each system are compared in Table 5.2 . The reusable pad
system requires that the leached and rinsed ore be relocated to a separate spent ore dump.
Environmentally, the most critical design considerations for heap leach systems relate
to the liners, which provide containment for leach pads, solution ponds and any drainage
channels used for process solutions. Common liner types include single liners composed
of high density polyethylene (HDPE) or polyvinyl chloride (PVC), compacted clay liners,
and composite liners involving compacted clay and geomembranes (Hutchison and Ellison
1992). Very low permeabilities may be achieved using geomembranes; however, the use of
suitable clay soils, appropriately conditioned and compacted avoids the risks of rupture,
puncture or tearing that apply to geomembranes. Selection of liner design depends on
many factors including leach system, local environmental conditions, local availability of
suitable soils, and availability of skilled geomembrane installation personnel.

The most critical design
considerations for heap leach
systems relate to the liners,
which provide containment for
leach pads, solution ponds and
any drainage channels used for
process solutions.

Impacts
Construction and operation of heap leaching structures involve most of the same short-
term and long-term impacts as other mining facilities, including loss of vegetation and
faunal habitat, modii cation of landforms, soil proi le changes, land use changes, and mod-
ii cations to surface and subsurface drainage. These impacts can be largely reversed by
rehabilitation during and following operations.
Heap leaching does involve additional risks of serious environmental damage includ-
ing: (1) leakage of pregnant solution through the pad liner, or through solution pond or
drain liners, causing contamination of underlying groundwater; and (2) discharge from or
overtopping of solution ponds due to excess water, pump failure or damage to structures,
causing contamination of downstream surface water.
These risks are minimized by: careful calculation of water balances under all conceiv-
able operating scenarios; sizing of solution ponds to accommodate all predicted inl ows
plus an appropriate freeboard to provide additional security; selection of appropriate liner
systems; close attention to construction control (particularly compaction of embankments
and clay liners, and welding of geomembranes); and installation of appropriate instrumen-
tation to monitor key parameters and provide early warning and/or automatic shutdown
in the case of malfunction ( Case 5.6 ).
Heap leaching operations are less common in areas of high rainfall because of the dif-
i culty of storing and treating the large volumes of water entering the system from pre-
cipitation. However, successful heap leaching operations have been conducted in both
Indonesia and the Philippines.
Arguably heap leaching also offers a number of environmental and social benei ts.
Smith (2004) uses The Seven Questions to Sustainability: How to Access the Contribution of
Mining and Minerals Activities ( Table 5.3 ) as framework for evaluating mineral develop-
ment involving heap leaching in terms of the goal of sustainable development and pro-
vides the following seven answers (for full text see original publication).
1.

Heap leaching operations are
less common in areas of high
rainfall because of the diffi culty
of storing and treating the large
volumes of water entering the
system from precipitation.

Engagement - Since the construction technologies are more accessible to local contrac-
tors, there are more opportunities for partnering between the owner and contractors,
and less reliance on imported technologies and equipment. Thus, a heap leach operation
is more accessible to the local community, improving opportunities for engagement.

2. People - Heap leaching is an inherently more 'hands-on' process then milling. It is the 'low

technology' solution for low grade ores, and as such requires more people doing things