1. Topic 5: Environmental and Social Concerns
From a series of 5 lectures on
Metals, minerals, mining and (some of) its problems
prepared for London Mining Network
24 April 2009
2. Outline of Topic 5:
• Focus on acid mine drainage (AMD)
Production of acid waters by oxidation of sulphide minerals
Factors influencing acid development
Impacts of AMD
Control of AMD during mining
Control and remediation of AMD after mining
• Control and remediation of uranium bearing wastes
• Mine rehabilitation case study
• Spontaneous combustion of coal
• “Sustainability” and mining – heap leaching case
3. Acid Mine Drainage:
The most serious and pervasive environmental problem related to
mine waste management is arguably acid mine drainage (AMD).
AMD is an oxidation process which takes place wherever
sulphide minerals (e.g., pyrite) are in contact with both oxygen
and water, wherever they are present on the mine.
Metallic sulphide minerals (e.g., pyrite) oxidise in the presence
of water and oxygen to:
• produce acids and
• release dissolved metals into water.
4. Acid mine drainage
+ other sulphides
Sulphuric acid (H2SO4) + Iron (Fe3+)
dissolved in water
Iron-hydroxide Fe(OH)3 precipitated
Mine dump, - water becomes more acidic
St. Kevin Gulch,
FeS2 + 15/4 O2 + 7/2 H2O Fe(OH)3 + 2 H2SO4 + energy
Pyrite Oxygen Water Iron-hydroxide Sulphuric acid heat
(solid) (dissolved) (liquid) (dissolved) (dissolved)
5. Acid mine drainage:
Sulphide oxidation is a positive feedback reaction - as the reaction
proceeds, the fluid becomes more acidic and more heat is generated,
which in turn speeds up the oxidation reaction, which produces a more
acidic fluid and more heat. The reaction will continue at an ever
increasing rate until either the sulphide or oxygen source is
6. Acid mine drainage - example
AMD seep into Slickrock Creek at
Iron Mountain Mine.
Location: Redding, Shasta County,
Photo Date: November 17, 1994
Iron Mountain Mine (IMM) has been a
source of acid mine drainage resulting
from over one hundred years of mining
activity. Though mining operations
were discontinued in 1963,
underground mine workings, waste
rock dumps, piles of mine tailings, and
an open mine pit still remain at the
NOAA Restoration Center & Damage Assessment and Restoration Program
7. Acid mine drainage - example
AMD seep into Slickrock Creek at Iron Iron Mountain Mine.
Location: Redding, Shasta County, California, USA
Photo Date: November 17, 1994
Credit: NOAA Restoration Center & Damage Assessment and Restoration Program
8. Acid mine drainage – oxidation rates of different sulphide minerals:
Different sulphide minerals are more (or less) less reactive in oxygen rich
environments (Lottermoser, 2007). Sulphides which do not contain
iron do have a significantly reduced capacity to generate
significant amounts of acid (Plumlee, 1999).
Pyrite (Iron-sulphide) FeS2 High reactivity High acidity
Marcasite (Iron-sulphide) FeS2
Pyrrhotite (Iron-sulphide) FeS
Makinawite (Iron-nickel-sulphide) (Fe, Ni)9S8
Covellite (Copper-sulphide) CuS
Millerite (Nickel-sulphide) NiS
Galena (Lead-sulphide) PbS
Cinnabar (Mercury-sulphide) HgS
Molybdenite (Molybdenum-sulphide) MoS2 Low reactivity No acidity
9. Acid mine drainage – different sulphide minerals in contact with each
other affects oxidation rate:
Pyrite in direct contact with other sulphide minerals does not oxidise as
vigorously as it does in isolation (Cruz et al., 2001), and the oxidation
of pyrite can be delayed while other sulphides are preferentially
oxidised (Kwong et al., 2003). (The industrial galvanizing of iron with zinc, to
prevent rusting of iron, takes advantage of the same electro-chemical principle)
when in contact with
High electro-conductivity less electro-conductive
Low electro-conductivity when in contact with
10. Acid mine drainage – acid buffering by non-sulphide gangue
Gangue minerals (mostly silicates and carbonates) have the capacity to
Whether the gangue minerals react with the acid depends on the pH of the
solution – different minerals react at different pH values.
Thus depending on the abundance and types of both gangue
minerals and sulphide minerals, a sulphide waste pile may, or may
not, produce acidic leachates (Lottermoser, 2007). The production of
acidic leachates is a far more common situation though.
Given all the variables (e.g., variations in the types and amounts of
sulphides and gangue minerals, oxygen and water supply, grain size
and porosity, and bacterial population) it is difficult to predict reliably the
acidity of potential drainage from waste piles.
It is more difficult to say with any certainty that the drainage will not
11. Acid mine drainage – environmental impacts:
Acid mine drainage may be released into the environment from mine sites
in two ways:
• Water drainage through sulphide-rich oxidising waste dumps, leach
heaps and tailings dams.
• Release of uncontrolled or improperly treated process-waters that into
surface drainage systems.
Erosion of waste dumps and tailings dams can cause sulphide minerals
to be transported directly into soils and streams, and therefore
dispersed away from the mine site.
Acidic, metal-bearing waters can migrate for large distances away from the
immediate mine site. The environmental impacts of such waters
(i) Surface water contamination. AMD waters have high metal and salt
concentrations that impacts on the use of waterways downstream for
fishing, irrigation, stock watering and drinking water supply.
12. Acid mine drainage – environmental impacts:
(ii) Aquatic life. The acidity of AMD can destroy the natural bicarbonate
buffer system which keeps the pH of natural waters within its normal
range. The loss of bicarbonate also affects photosynthetic aquatic
organisms that rely on bicarbonate as a non-organic source of carbon.
(iii) Heavy metals and metalloids at elevated and bioavailable
concentrations are lethal to aquatic life and are of concern to animals
and humans. Loss in biodiversity, depletion in the numbers of
sensitive species and fish kills can occur.
(iv) Groundwater contamination. AMD impacts more frequently on the
quality of subsurface waters than on surface drainage. Water seeping
from below uncapped and unlined waste repositories, or ones with
ruptured liners, form plumes of contaminated subsurface water that
allows sulphates and metals to migrate in aquifers, and subsequently
down the hydrographic gradient within the acquifer.
13. Acid mine drainage – environmental impacts:
(v) Sediment contamination. Precipitation of dissolved constituents in
AMD can cause soils, flood plain sediments and stream sediments to
become contaminated with metals, metalloids and salts.
Rum Jungle uranium
mine, Australia. Stream
channel impacted by AMD
is devoid of plant life and
encrusted with white
Figure from Lottermoser, 2007.
14. Acid mine drainage – control and remediation:
Sulphidic rock dumps are the major source of on-mine AMD
generation due to the fact that they are generally unlined, and
consisting of coarse rock material, are highly porous and permeable
to water and oxygen.
Tailings dams are associated with a somewhat lower risk of generation
and release of AMD due to the presence of liners, and the tailings
being fine-grained and less permeable. While the tailings dams are
wet (i.e., at all times during operation) the risk of AMD seepage is
higher than after decommissioning, when the tailings dams dry out,
provided surface erosion of the dams is prevented.
Sulphide oxidation of rock dumps can (potentially) be controlled during
mining, and remediated after mining, by the exclusion of one or
more of the factors that cause oxidation (water, oxygen) or
enhance oxidation (bacteria), or by the introduction of a buffering
15. Acid mine drainage – rock-dump control during mining:
Because mines operate for long period of time (decades), control
strategies that attempt to minimise AMD generation from rock dumps
during the life-of-mine should be implemented.
(i) Mixing and encapsulation. Acid generating rock Encapsulation
material can be encapsulated or mixed with benign
rock waste (e.g., oxide waste) or neutralising (e.g.,
limestone) rock waste.
(ii) Co-disposal or blending. Co-disposal refers to the
mixing of rock waste with fine grained tailings Mixing
so as to reduce the overall porosity of the dumps,
and minimise water and oxygen ingress. If an
alkaline material (e.g., lime) is added to the tailings
beforehand, the process is called blending.
Figure from Lottermoser, 2007
(iii) Bactericides may be applied to rock dumps to inhibit growth of bacteria that might
otherwise enhance the oxidation process (e.g., Kleinmann, 1999). Repeated
treatments are necessary as the chemicals are washed away by rain percolation.
The applied chemicals may cause toxicity to other organisms.
16. Acid mine drainage – rock-dump remediation after mining:
Dry covers. Capping sulphidic wastes with a thick layer of solid material,
called a “dry cover”, is the most widely used approach to countering
acid generation (by reducing water and oxygen flux into the waste
Placing a layer of neutralising materials on the surface of rock
dumps, to establish a source of alkali water percolating into the
dump, has not been successful in countering AMD generation
(Smith and Brady, 1999). Acid buffering or neutralising materials are
most effective when mixed in with the sulphidic waste.
Similar dry covers are also used to rehabilitate tailings dams and spent
17. Rock dump remediation with dry covers
Unsaturated covers are designed for semi-arid
to arid areas, to maximise rainfall run-off
and minimise water infiltration and oxygen
diffusion into the waste.
Saturated covers are designed for sites with a
wet climate. The outermost “sandy-clay”
layer remains wet permanently, providing a Fig 2.15
very good barrier against oxygen diffusion. Lottermoser 2007
Sponge covers are designed for climates with
distinctly seasonal rainfall. They aim to
capture large volumes of infiltrating rainwater
for short periods of time, and then allow the
water to drain away during dry seasons.
Problems with dry covers:
(i) The clays layers in dry covers may crack if
they dry out too rapidly.
(ii) Covers are prone to erosion on the steep
slopes of rock dumps.
Figure from Lottermoser, 2007
18. Monitoring of sulphidic rock dumps for oxidation and acid generation
Sulphidic waste rock dumps and tailings dams need monitoring during operation to
detect at the earliest time whether waste material is “turning acid”.
Rehabilitated waste repositories also need monitoring to establish the effectiveness of
the control measures used to curtail oxidation.
Pore gas sampling to using electrical
determine oxygen probes. Increasing
concentration. Decreasing Monitoring of water
concentrations indicate quality in aquifers
heat generation by
consumption of oxygen by using boreholes.
Water analysis to monitor
acid and metallic ion buildup
in drainage channels and
DUMP surface water.
19. Management of uranium-bearing wastes:
The problems associated with uranium rock and tailings wastes are identical
to those present in the case of sulphidic wastes (acid water generation
and mobilisation of metals and metalloids into the water system) with the
additional impact of mobilisation of both uranium and radium
radionuclides into waters and the release of radon gas.
Control and remediation strategies focuses primarily on the exclusion of
both oxygen and water from waste rock dumps and tailings dams (to
prevent oxidation of both sulphide and uranium bearing minerals), using
the same array of techniques discussed previously for sulphidic wastes.
Uranium tailings should be covered during operation in order to reduce
radon-222 gas emanation. A permanent water cover will reduce the
radon flux to 1% of that from dry tailings (Davy and Levins, 1984) (but
remember that a tailings dam with an overfull decant pond that encroaches on
the dam wall increases the risk of dam failure).
Other on-mine radiation hazard mitigation measures include: dust
suppression, appropriate ventilation, use of protective clothing, strict
hygiene standards, radiation dose measurements.
20. Rehabilitation case study – Sherwood Uranium Mine, Washington, USA:
The open-pit mine operated from 1976 to 1985.
Construction of the mill to process the ore was completed in 1978, and
operated until 1984. Nominal milling capacity was 2,100 tons of ore per
day, with an average design ore grade of 0.088% U3O8 (0.88 kg U3O8 per
ton of ore). Approximately 2.9 million tons of acid-leached tailings were
neutralised with lime prior to placement in a synthetically-lined tailings
impoundment. The estimated radium-226 activity in the impoundment is
470 curies (17.39 TBq).
Mill decommissioning began in 1992 and was completed in 1995.
Approximately 350,000 cubic yards of contaminated mill-site soils, building
equipment and debris were excavated from the processing-site and placed
in the tailings impoundment.
Areas disturbed were approximately 2 km2 by mining and an additional 0.8
km2 by the processing and tailings area. At end of mining, and prior to
reclamation, the pit seasonally contained surface water.
21. Sherwood Uranium Mine (1976 – 1985): mine development timeline
22. Rehabilitation case study – Sherwood Uranium Mine
Mine Closure and Reclamation Objectives:
• Maximize the potential for future retrieval of remaining ore in the deposit.
• Return the mine to a condition that will not pose a hazard to public health and safety.
• Return the site to a condition that will support wildlife habitat.
• Create a self-sustaining vegetation community.
• Enhance the visual appearance of the area.
• Use reclamation methods that are technically effective, cost efficient and employ
tested engineering practices.
Specific Closure and Reclamation Activities:
• Remove all mine-related facilities.
• Re-grade the overburden materials and mine the benches to create surfaces that
promote drainage and minimize potential for ponding of water and erosion.
• Establish stable slopes.
• Replace the topsoil and growth media and revegetate with native plant species.
• Monitor the performance during and after reclamation to ensure objectives are
23. Rehabilitation – Sherwood Uranium Mine: establishment of stable slopes
Slope modification during rehabilitation.
Shallower gradients provide greater slope
stability (less risk of collapse or slumping)
and reduce the effects of surface erosion.
Reclaimed mine-site calculated to be
erosionally stable with respect to a 100 year
storm event (2.5 inches of water over a 24
24. Rehabilitation – Sherwood Uranium Mine: post rehabilitation view
On-line documentation does
not record the nature of the
covers (if any) placed over
the rock-dumps and tailings
dams, and particularly
whether the covers are
designed to be impermeable
barriers or not.
25. South African mining-industry fatality and injury rate (all mining)
From: Mine Health and Safety Inspectorate annual report, http://www.dme.gov.za/mhs/documents.stm#3
27. Mining strategy can make a difference to rockburst activity and fatalities:
Decreasing fatality rate due to increasing use of:
• Backfill – to provide support in panels after completion of mining to reduce stress buildup
• Preconditioning – mining small portions of future mining areas in advance to allow stress
changes to occur more gradually
• Bracket and stabilising pillars
• Seismic monitoring – to provide early warning of seismically active panels
• Education – to increase consciousness of safety
Rockburst fatality rates for SA gold mines (1984 – 2002)
28. Is a zero fatality rate possible in the South Africa mining industry?
I presented the above question to two people closely involved in the South
African mining industry, one a leader in the field of mine-seismicity and
rockbursts, and the other an experienced mining engineer, having worked
on both coal and gold mines. (Their responses are included with the
course material as anonymous submissions).
The prospects for achieving zero fatalities in deep gold and platinum
mines do not look good currently, at least not until underground
operations are fully mechanised. Regardless of the mining method used
(human manpower or machine), the creation of a cavities underground
induces huge stresses we currently have no way of dissipating harmlessly
(and nature takes its course in the form of rockbursts).
However their responses (obviously expressing very personal views) make
interesting reading in illuminating a culture within mining (within their
experience) that is not conducive to the safest mining possible.
29. Is a zero fatality rate possible in the South Africa mining industry?
The submissions indicate:
(i) A culture in which (financial) reward is heavily weighted towards meeting
(ii) Senior management that may turn a blind eye to potentially risky situations, in
favour of retaining high production rates.
(iii) Lack of legal accountability of senior management in the light of decisions made
in the leadup to fatalities.
(iv) Rock mechanics and health-and-safety officers who have no power to halt
operations in dangerous situations – deferring to more senior management.
(v) Incentive (or disincentive) schemes around safety that are counter productive
– leading to situations where miners will not report injuries, or avoid going for
treatment, in order to avoid penalties.
(vi) Resistance to implement or test changes in procedures and methods that might
improve mining safety.
(vii)The better safety record of more experienced crews indicates that overall better
statistics could be obtained if all crews operated to the same standards.
30. Heap leaching and sustainability:
It has been argued that heap leaching offers a number of environmental
and social benefits. Smith (2004) claims that heap leaching meets
the seven criteria for sustainability established by the North
American MMSD (Mining, Minerals and Sustainable Development)
project (make your own judgment on MMSD and on “sustainability” of heap
(i) Engagement. Heap-leaching is the “low-technology” solution for
low-grade ores. Construction and operation technologies of heap
leaching are sufficiently “accessible” to provide local contractors
with engagement opportunities.
(ii) People. Heap-leaching is a more “hands-on” process than milling,
providing opportunity for skills transfer to local people in the areas of
pipe laying, irrigation, operation and maintenance of pumps,
surveying, earthworks, liner construction, slope and erosion control,
reclamation and revegetation, and other aspects of civil
31. Heap leaching and sustainability (continued):
(iii) Environment. Heap leach facilities have far fewer serious Acid Rock
Drainage problems than conventional mill operations because use
of the leaching approach lowers the cut-off grade and therefore
reduces the size of rock dumps, as well as reducing the overall
sulphide content in waste from copper projects. Spent heap leach
ore from gold operations is strongly alkaline and mixing waste types
can compensate for acid waste rock. Self-draining characteristics of
spent leach heaps make them more easy to reclaim than old tailings
deposits. No history of catastrophic failure of leach heaps and
dumps, in comparison to tailings dams. There is overall a reduced
reliance on conventional tailings disposal.
(iv) Economy. Allows more ore to be processed at lower cutoff grades,
allowing a longer life or larger mine operation, therefore increases
employment. Projects are less capital intensive and thus less
sensitive to commodity price fluctuations. A lower risk investment in
general. Because capital and operational costs are lower, there is
greater potential for investment into the projects by local people.
32. Heap leaching and sustainability (continued):
(v) Traditional and non-market activities. By expanding employment in
occupational areas with transferable skills, a more sustainable
workforce results. The tools and activities of heap leaching are
more directly applicable to traditional activities.
(vi) Institutional arrangements and governance. Types of problems
inherent to heap leaching projects tend to be more manageable at a
(vii) Synthesis and continuous learning. It uses technologies that are
both locally available and have more applications outside mining.
Because the projects are less capital intensive and typically subject
to expansions or revisions in the leach pad and stacking operations
annually or bi-annually, project reevaluation is a deeply engrained
part of the heap leach process, and expanding this [culture] to
include the local community should be an easy step. (!?!!)
33. Spontaneous coal combustion:
Coal seams contain large amounts of disseminated sulphide minerals.
Both coal (carbon) and sulphide minerals oxidise when exposed to oxygen,
and generate heat at the same time.
If the heat is not allowed to dissipate (for example in the interior of a coal
pile, or inside a mine), temperatures will start rising
At about 70 – 150°C, coal begins to give off small, but measurable,
quantities of gas – aerosols, hydrogen, and CO and CO2 – which are the
precursors of combustion.
As the temperature increases further – at about 315 – 370 °C – relatively
large, visible (coal) particulates are emitted.
At temperatures of about 400 – 430°C, incipient combustion, and self-
ignition and flame, will occur. (http://www.saftek.net/worksafe/bull94.txt).
34. Spontaneous coal combustion:
Sulphur and hydrogen (and methane) present in the coal contribute to the
combustion as well. Sulfur dioxide (SO2) is a major pollutant. Carbon
monoxide (CO) is poisonous and a major threat to life when burning
Once burning is underway, hydrogen will be exhausted first, followed by
sulphur, and finally coal (which although has the lowest ignition
temperature, burns the slowest).
35. Possibilities for recycling of mine waste:
Some reported secondary uses of mine wastes include (Lottermoser, 2007):
- Slag from mineral smelting is commonly used in road construction.
- Manganese tailings may be used in agro-forestry, building and
construction materials, coatings, resin cast products, glass, ceramics,
- Fertiliser for golf courses.
- Clay rich wastes can improve sandy soils or provide raw material for
- Mine water can be purified into drinking water (e.g., in arid areas)
- Mine water can be used for heating or cooling purposes.
- Mine drainage sludges can provide a resource for pigment.
- Pyritic waste rock can be a good amendment to neutralise alkali
(There is currently very limited demand for, and use of, mining waste products).