Review
of In-Pit Disposal Practices for the Prevention of Acid Drainage
- Case Studies
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.36.1
September 1995
EXECUTIVE
SUMMARY
Open pit mines
that have ceased production are increasingly being considered for
the permanent and environmentally acceptable disposal of mine waste
rock and tailings that are, or have the demonstrated potential to
become, sources of acidic drainage. This report addresses key aspects
that need to be taken into consideration when evaluating the in-pit
disposal of wastes and presents 12 case studies of actual and planned
in-pit disposal of mine wastes.
There are four
basic concepts for the placement of wastes in pits:
- Option 1
- Underwater disposal
- Option 2
- Elevated water tables
- Option 3
- Dry disposal
- Option 4
- Perched water tables
The four options
are described in terms of the theoretical concepts and practical
aspects; selected examples of research and field applications are
provided from the published literature.
Not all pits
are suitable for the in-pit disposal of wastes. The success of an
application would depend on many technical factors, including:
- the acid
generation potential of the wastes and pit walls;
- the geotechnical
characteristics and properties of the wastes and the pit walls;
- predicted
pore water, pit water, and groundwater quality;
- hydrogeology
of the open pit; and
- the hydrology
of the open pit.
Mine related
constraints must also be taken into consideration. These include:
limiting access to remaining mineralization below the pit; wall
stability and related safety concerns; available access; and the
proximity of underground workings to the open pit.
Consideration
must be given to both the short term and long term implications
of the in-pit disposal concept; these include ecological and human
health protection and closure planning perspectives. Each site is
unique and may have special constraints with respect to the quality
and use of surface water and groundwater, land use, and sensitive
ecological communities.
The potential
costs for future in-pit disposal of wastes should be considered
in the preliminary economic evaluations that are used to establish
final pit limits. Greater than expected waste disposal costs could
have an unfavourable impact on the future profitability of some
open pit mines.
Volume 2 of
this report presents twelve case studies: seven case studies describe
in-pit disposal programs that have been implemented; two case studies
describe proposed in-pit disposal programs; and three case studies
provide technical information from other pits which will likely
be of interest to persons evaluating in-pit disposal programs.
The following
case studies describe pits that have been used for in-pit disposal
of mine wastes:
- The
Owl Creek pit - To prevent discharge of acidic drainage
from waste rock piles, the waste rock was relocated to the pit
and flooded.
- The
Rabbit Lake pit - This pit is likely the first application
of an engineered pit disposal concept, which involved the use
of a bottom rock drain, an engineered pervious envelope, placement
of tailings as a dry filter cake, and closure with a surface lake
and a soil/sand diffusion barrier.
- The
Collins Bay B-Zone pit - Following extensive decommissioning
studies, the special waste (which contains elevated arsenic, nickel,
sulphur, or uranium content) was placed in the bottom of the pit
and the pit was flooded, creating a pit lake.
- The
Solbec pit - A crown pillar pit was filled with reactive
waste as a means of inhibiting further acidic drainage.
- The
Udden pit - The pit was backfilled with reactive waste
rock and allowed to flood.
- The
Stratabound CNE pit - The in-pit disposal of reactive
waste rock was planned at the design stage, and the waste rock
has been relocated to the pit and clay capped.
- The
Cluff Lake "D" Zone pit: The pit flooded after
the end of mining operations, and an extensive monitoring program
has been carried out to evaluate the physical and chemical changes
occurring in the pit lake water column.
In-pit disposal
programs have been proposed at the following case study sites:
- The
Island Copper pit - As part of planned closure, it is
proposed that the pit be flooded to create a meromictic lake.
- The
Deilmann pit - Insufficient capacity exists within an
existing surface tailings management facility to accommodate future
tailings production therefore tailings disposal options are being
reviewed; one option involves the conversion of the Deilmann pit
to a full side drain and under-drain tailings management facility.
Additional
case studies which may provide useful information to persons evaluating
in-pit disposal programs are:
- Robinson
Mining District - Extensive numerical modelling was carried
out to evaluate the hydrogeochemistry and to support the prediction
of environmental impacts from several pit lakes.
- The
Gunnar pit - This pit lake is interesting because of unique
and well-documented limnological characteristics.
- The
Berkeley pit - This mine site is part of a U.S. Superfund
site and is a well-known example of a serious acidic drainage
problem in a flooding open pit
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