such as improved rehabilitation to
reduce the volumes and load of
impacted water and the application of
innovative treatment methods.
The role of rehabilitation
Effective rehabilitation plays an
important role in the reduction of
rainfall recharge in mines. Recharge to
rehabilitated spoils in an opencast mine
in South Africa is estimated to be
around 15 – 25% (compared to about 3%
in undisturbed conditions). Apit that is
badly rehabilitated, where all rainfall
would run-off towards the spoils may
expect a recharge of between 20% and
25%. Avery well rehabilitated pit with
free draining topography from the
spoils can expect a recharge of 10%.
Areas that have not been rehabilitated at
all, i.e. backlog areas, typically have
much higher recharge values, resulting
in increased water ingress into the pits.
This subsequently increases the risk of
decant and water quality impacts, as
well as increased pumping
requirements. This leads to increased
energy consumption and operations
costs.
Capping and vegetation cover are
important ingress control tools and are
based on the engineered use of plants
and their associated microorganisms for
hydraulic control and environmental
clean-up. Improved vegetation cover as
a result of good quality rehabilitation
will reduce groundwater recharge and
migration through spoils material as a
result of water uptake in the root zone
and subsequent evapotranspiration.
Passive treatment in the form of
evapotranspiration is considered an
effective recharge control mechanism
and has gained acceptance particularly
in the US and Europe over the last
10 – 15 yr as a cost‑effective,
non‑invasive alternative or
complementary technology for
engineering-based remediation methods
and is recommended by the
US Environmental Protection Agency as
such. The technology relies on the
abstraction of water and contaminants
as deep-rooted plants exert hydraulic
control, thereby minimising the flux of
contaminants in groundwater.
Anumber of benefits are associated
with the implementation of
phyto‑technologies including: carbon
sequestration, soil stabilisation,
improvement of functional soil
ecosystems and the possibility for the
production of high‑value wood, biofuel,
fibre, gums and dyes and essential oils.
The production of value-added
products creates opportunities for
setting up sustainable industries in
association with communities. This will
ensure that tree stands are managed
appropriately, providing assurance that
the water issue is managed effectively
into the future.
Ensuring the rehabilitation backlog is
minimised and physical rehabilitation
or vegetation cover is maximised will
reduce water ingress through mine
impacted areas. Effective and timely
rehabilitation is critical to sustainable
water management.
Water quality changes over
time
Water management post-closure is a
long-term liability that could extend into
perpetuity and is often underfunded
and generally not well understood.
Stratification may play an important
role in mine water management, as
geochemical changes result from
stratification within flooded
underground mines. Mine‑impacted
water has a higher density and will thus
form a layer beneath better quality
water. Flooding of underground
workings also restores reducing
conditions and enhances microbial
reactions, such as sulfate reduction,
particularly if methane is present,
resulting in changes in the water
chemistry. Studies have shown that
significant water quality improvement is
often evident 20 – 40 yr post-closure as a
result of natural attenuation resulting in
little or no treatment being required.
Geohydrological and geochemical
studies, which generate high confidence
level water and salt balances that inform
financial closure costing models, are
therefore imperative to ensure financial
assurance is adequate. The application
of appropriate geochemical models that
determine the evolutionary changes and
improvement of the water quality over
time will then allow the premise of ‘into
perpetuity treatment’ of impacted water
to be challenged.
Other minimisation strategies include
artificial flooding or allowing
underground workings to flood
naturally, creating oxygen reducing
conditions. Such a reductive
environment would be ideal to prevent
the formation of acid mine drainage, and
would be the ultimate goal for mine
closure.
The need for innovation
Prevention of acid mine drainage
formation at source should be the
preferable option, but this is not always
feasible and it becomes necessary to
collect, treat and discharge water, which
has significant financial implications.
There is thus a substantial need for
further investment in innovative R&D in
the management of mine-impacted
water.
Technology solutions should focus
not only on the cost-effective sustainable
treatment of mine water, but also on the
minimisation of operational water
consumption across the mining process.
The sustainability of any water
treatment process is a factor that is
becoming increasingly critical in
decision making, particularly with
regards to waste generated and energy
consumed.
There needs to be ongoing efforts to
develop processes that are energy
efficient, produce minimal waste and/or
produce byproducts that generate
revenue to offset treatment costs.
Additionally, variability and changes in
water chemistry require the
development of alternate sustainable
and cost-effective solutions.
Brine minimisation technology
Desalination is becoming increasingly
important in maintaining water quality
in production processes and in
protecting the environment. One of the
most significant issues associated with
desalination processes utilising
membrane technology is the generation
of a highly-concentrated salt stream
(brine). Brine management requires
long-term handling and storage in brine
ponds, which impacts considerably on
lifecycle costs, while remaining an
environmental liability with a
substantial footprint that is not
sustainable into the future.
82
|
World Coal
|
June 2015
