Inert Atmosphere Systems – A New Approach to controlling AMD
Discharges from Underground Mines
Nic Bourgeot1, John A. Muchan1, Andrew Sampaklis2, Nick Staheyeff2 and Jeff R. Taylor1
1Earth Systems Pty. Ltd. 2NSW Department of Industry
December 2016
Acid and Metalliferous Drainage
•
Acid and metalliferous drainage is single biggest water quality
issue facing derelict mines worldwide.
•
Key environmental and social management issue for derelict
mines.
•
AMD comprises acid and metalliferous (AMD), neutral and
metalliferous (NMD), or simply saline (sulfate-rich) water.
•
Affects mine sites across all sectors, including coal, uranium,
precious metals, base metals, iron ore, mineral sands, diamonds.
•
AMD is generated when key sulfide minerals decompose upon
exposure to air and generate acid and salinity. The acid dissolves
surrounding minerals, resulting in the release metals.
•
AMD is most commonly derived from waste rock piles, tailings
storage facilities, wallrock (pits and underground voids), heap leach
pads and slag piles.
•
For underground mines, backfilled waste rock generally represents
the largest source of AMD. Wallrock within a mine is predicted to
be a relatively small contributor to site acidity loads.
AMD Management for Underground Mines
•
There are hundreds of thousands of abandoned
underground mines globally as this was the
preferred mining method.
•
Approximately 5% of existing mines are
underground operations.
•
Few remediation techniques are available to
manage AMD discharges from derelict
underground mines.
•
Existing techniques include:
– Prevent AMD using pressure bulkheads /
hydraulic adit seals and mine flooding.
– Treat the AMD discharged in perpetuity.
– Minimise AMD using water management
approaches (diversion around workings).
Hydraulic Mine seals / plugs and mine flooding
•
Used to control mine discharge and flood the mine workings to
prevent or minimise the oxidation of the sulfidic materials within the
mine.
– Complex and expensive to engineer and install. High water
pressures common.
– Water can often migrate around the seal in fractured rock.
– Prone to catastrophic failure.
– Can be inefficient if not all sulfidic material is submerged.
Water Treatment in Perpetuity
•
Active treatment of mine discharges using alkaline reagent. In perpetuity.
•
Very high capital cost and very high ongoing reagent cost.
•
Significant quantities of treatment sludge are produced that has very little reuse
opportunity and hence requires on site disposal.
•
Socially unacceptable approach.
Inert Atmosphere Systems
•
Approach designed to exclude oxygen from inside the mine workings, while not interfering
with the mine hydrology.
•
How it works:
– Stage 1:
• Construction of mine seals (air seals only).
• Pyrite oxidation processes naturally consume the internal oxygen.
• Discharge water quality improves as AMD processes are limited by air entry into
the mine.
– Stage 2 (if required):
• Controlled injection of limited amounts of inert gas (nitrogen or carbon dioxide) to
compensate for any residual barometric pumping or residual air entry. This will
further lower internal oxygen concentration and improve mine discharge water
quality.
• Numerous technologies and methods exist for efficient inert gas generation and
injection.
Benefits of inert atmosphere approach
•
Relatively low cost – especially compared to chemical treatment in perpetuity.
•
Can be used as a closure strategy for existing underground mines as well as polluting
derelict mines.
•
Low complexity and fast to implement relative to pressure bulkheads and water treatment.
•
Passive treatment systems may be suitable if Stage 1 activities are sufficient to decrease
AMD discharges to acceptable levels.
•
If Stage 2 works are required, it can be regarded as semi-active treatment with gas rather
than commercial chemicals.
•
Can also be used in collaboration with (passive or active) water treatment to lower
ongoing costs by offsetting chemical treatment requirements.
•
Easily reversible – does not sterilise future resource or inhibit the re-opening of the mine.
•
Can simultaneously lower public safety risk by preventing access to old workings.
•
Low impact on site heritage values.
•
Likely more socially acceptable without sludge production in perpetuity.
Inert Atmosphere System at Sunny Corner Mine Site
•
Earth Systems is currently engaged by the NSW Department of Industry, Derelict Mines
Program to implement an inert atmosphere system at the Sunny Corner Site.
•
Earth Systems has developed a staged approach including the following work:
– Preliminary site investigations (completed in 2015).
– Survey of air entry points (completed in 2015).
– Design of air entry seals for each potential air-entry point (completed in 2015).
– Installation of monitoring equipment (completed in September 2016) including
internal gas (O2 and CO2), P, T, external wind speed, wind direction, rainfall, P, T,
and discharge water quality (pH, EC, ORP, TºC) and flow.
– Mine sealing works (currently in progress, to be completed in January 2017).
– 12 months of atmospheric and water quality monitoring following completion of mine
sealing.
Site background and AMD issues
•
Sunny Corner mine is an historic lead-zinc-silver-gold mine located in Central Eastern
NSW. Mining dates back to the mid-1800’s and the site has important heritage value.
•
Approximately 55 tonnes of acidity (H2SO4 equivalent) is generated from the entire site
per year. Discharge is acid and contains elevated iron, aluminium, copper, arsenic, lead,
cadmium and sulfate.
•
The majority of the AMD is generated and discharged from the underground workings via
the Level 4 Adit.
•
Water quality impacts from the site can be detected several kilometres downstream.
Site Characteristics
•
The site has five levels of underground workings, with numerous adits, shafts
stopes and open cuts.
Site Characteristics
Site Survey
Environmental Monitoring
Mine Sealing Works
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Commenced 30th November 2016 – due to finish in January 2017.
•
12 month monitoring program to follow.
Conclusions
•
Inert Atmosphere Systems offer a low complexity and cost effective alternative
approach to hydraulic seals and water treatment in perpetuity for underground
mines.
•
The technique can be applied to the closure of current or derelict underground
mines.
•
The approach could be completely passive at some sites, if air-seals alone are
sufficient to lower AMD generation to acceptable levels.
•
Mining can easily resume as the approach is reversible.
•
Simultaneously lower public safety risks with mine sealing.
•
Low impact on site heritage values.
Thank You
www.acidmetalliferousdrainage.com
www.earthsystems.com.au
www.esanalytical.com