2. Acid mine drainage occurrence
Acid Mine Drainage results from the oxidation of
sulfide minerals inherent in some ore bodies and the
surrounding rocks.
Iron sulfide minerals, especially pyrite (FeS2),
chalcopyrite (FeS.CuS) and also pyrrhotine(FeS)
contribute the most to formation of Acid Mine
Drainage.
Oxygen (from air or dissolved oxygen) and water (as
vapor or liquid) which contact the sulfide minerals
directly cause chemical oxidation reactions which
result in the production of sulfuric acid.
3. Acid mine drainage occurrence
Overall chemical reactions associated with pyrites
can be described as follows;
- Pyrite is initially oxidized by atmospheric oxygen
producing sulfuric acid and ferrous iron (Fe2+)
according to the following reaction:
- FeS2 + 7/2 O2 + H2O > Fe2+ + 2SO4
2- + 2H+…….. (1)
Conversion of ferrous iron to ferric iron.
- Fe2+ + 1/4 O2 + H+ > Fe3+ + ½ H2O .........(2)
4. Acid mine drainage occurrence
Ferrous iron may be further oxidized by oxygen releasing
more acid into the environment and precipitating ferric
hydroxide.
- Fe2+ + 1/4 O2 + 5/2 H2O > Fe (OH) 3 + 2H+........(3)
As acid production increases and the pH drops (to less
than 4), oxidation of pyrite by ferric iron(Fe3+) becomes the
main mechanism for acid production
- FeS2 + 14Fe3+ + 8H2O > 15Fe2+ + 2SO4
2-+ 16H+……. (4)
This reaction is catalyzed by the presence of Thiobacillus
ferrooxidans which accelerates the oxidation of ferrous
iron into ferric iron (reaction 2) by a factor of 106:1.
5. Acid mine drainage occurrence
The sulfuric acid produced in these reactions
increases the solubility of other sulfide minerals in
the solid surfaces. Ferric iron in acidic solution can
oxidize metal sulfides per the following reaction:
MS + 2Fe3+ > M2+ + S + 2Fe2+ …………………………..(5)
Where MS = metal sulfide (galena, sphalerite, etc.)
6. Acid mine drainage occurrence
Metals commonly solubilized from sulfides in AMD
include aluminum, copper, lead, manganese, nickel,
and zinc. Metals in the form of carbonates, oxides, and
silicates may also be mobilized, often aided by
biological catalysts.
AMD may also leach uranium, thorium, and radium
from mine wastes and tailings associated with uranium
mining operations
7. AMD effects on environment
AMD harms the environment by lowering soil and
water pH hence increase heavy metals drainage from
rocks and soil colloid materials into water bodies and
the environment in general
At low pH levels heavy metals release from soil colloids
increase
Accumulation of heavy metals in soil and water leads
to increase bio-concentration and bio-accumulation in
plants, fish, livestock and humans through the process
of eating and being eaten.
8. AMD Prevention and Mitigation
Minimizing oxygen supply because of diffusion or
advection
Minimizing water infiltration and leaching (water
acts as both a reactant and a transport mechanism)
Minimizing, removing, or isolating sulphide
minerals
Controlling pore water solution pH
Maximizing availability of acid neutralizing
minerals and pore water alkalinity
Controlling bacteria and biogeochemical processes
9. AMD Control Strategies
Containment and isolation
- Soil covers- by imported materials e.g. clay, soil
- low sulphide waste-rock if compactable
- Geo-textile fabrics
- Water cover- creation of a permanent lake or swamp
- use of an existing lake
- flooding of underground tunnels
- submarine disposal
-Blending- mixing of acid and non acid forming waste rock
Treatment- Using AMD remediation technologies
10. AMD Control Strategies
Soil Covers
Soil covers generally involve the use of granular earthen
materials placed over mine wastes. The objectives of a
soil cover varies from site to site, but generally include
(i) dust and erosion control; (ii) chemical stabilization
of acid-generating mine waste (through control of
oxygen or water ingress); (iii) contaminant release
control (through improved quality of runoff water and
control of infiltration); and/or (iv) provision of a
growth medium for establishment of sustainable
vegetation
11. AMD Control Strategies
Key factors to consider in the design of a soil cover
include
The climate regime at the site
The reactivity and texture of the mine waste material
The geotechnical, hydrologic, and durability
properties of economically available cover materials
The hydrogeologic setting of the waste storage facility
Long-term erosion, weathering, and evolution of the
cover system
13. AMD Control Strategies
Limitations of soil covers
Soil covers do not stop infiltration and may not stop acid
drainage.
Permeability of water infiltration barriers may increase
with time when subjected to climate and vegetation.
Oxygen barrier covers are especially vulnerable to
relatively small imperfections in the cover – such as
differential settlement, holes caused by animal burrows,
desiccation cracking etc.
Soil covers may be prone to erosion and long-term
maintenance requirements
Soil covers may be vulnerable to vegetation, animal, and
human activity including vehicle traffic
15. AMD Remediation Technologies
Active remediation technologies
One of the most common ways used is chemical
precipitation
These systems usually include:
1. equipment for introducing the neutralizing agent used
to treat the Acid Mine Drainage
2. means for mixing the two streams
3. procedures for ensuring iron oxidation
4. settling ponds for removing iron, manganese, and
other co-precipitates
16. AMD Remediation Technologies
Passive remediation technologies
Require little to no maintenance
Typically in the form of a wetland
Cuts down on the price immensely
However must remove the buildup of metal
accumulations
Very common method used because of its ability
to cut down on price and the utilization of nature
and not chemicals to remediate the water
17. Abiotic remediation
Active technologies
The most widespread method used to mitigate acidic
effluents is an active treatment process involving
addition of a chemical-neutralizing agent.
Addition of an alkaline material to AMD will raise its
pH, accelerate the rate of chemical oxidation of ferrous
iron (for which active aeration, or addition of a chemical
oxidizing agent such as hydrogen peroxide, is also
necessary), and cause many of the metals present in
solution to precipitate as hydroxides and carbonates
18. Abiotic remediation
Active technologies cont…
Various neutralizing reagents have been used,
including lime (calcium oxide), slaked lime,
calcium carbonate, sodium carbonate, sodium
hydroxide, and magnesium oxide and hydroxide.
Neutralizing agents vary in cost and effectiveness;
for example, sodium hydroxide is some 1.5 times as
effective but is about nine times the cost of lime.
19. Abiotic remediation
Passive technologies- Anoxic limestone drains
An alternative approach for addition of alkalinity to
AMD is the use of anoxic limestone drains (ALD).
The objective of the systems is to add alkali to AMD
while maintaining the iron in its reduced form to avoid
the oxidation of ferrous iron and precipitation of ferric
hydroxide on the limestone), which otherwise severely
reduces the effectiveness of the neutralizing agent.
In an ALD, mine water is constrained to flow through a
bed of limestone gravel held within a drain that is
impervious to both air and water (generally constructed
of a plastic bottom liner and a clay cover).
22. Abiotic remediation
Open Limestone Channels
Open limestone channels are long channels or ditches
lined with limestone to increase the alkalinity of the
water.
The acid water flows down the limestone bed and
AMD is treated bylimestone dissolution.
The construction of an OLC is determined by the flow
rate, channel slope, and influent acidity concentration
and this information will dictate the weight of
limestone, the cross-sectional area and the length of
the channel, and the in-channel retention
23. Abiotic remediation
Open limestone channel cont…
One of the primary design factors is to have a steep
enough slope and large enough limestone particle size to
prevent iron and aluminum hydroxides from plugging
up limestone pores.
The ideal limestone size is 15-30cm in diameter.
Another important factor when designing an OLC is the
residence time. The quantity of time that the limestone
is in contact with the AMD water is central and is a
function of influent acidity and flow. Residence time of
the AMD water with limestone is calculated as;
R.T (hours) = Length (ft). * Width (ft.) * Depth (ft)* Void Ratio (%) / Flow (cfs) *
3600
25. Biological remediation
Passive biological systems- Constructed wetlands
Constructed wetlands utilize soil- and water-borne
microbes associated with wetland plants to remove
dissolved metals from mine drainage.
Aerobic wetlands are generally constructed to treat mine
waters that are net alkaline.
This is because the main remediative reaction that occurs
within them is the oxidation of ferrous iron and subsequent
hydrolysis of the ferric iron produced, which is a net acid
generating reaction (Eqs. (1) and (2)).
- 4Fe2+ + O2 + 4H+> 4Fe3+ + 2H2O........(1)
- 4Fe3++ 12H2O> 4Fe (OH) 3 + 12H+ ....(2)
26. Biological remediation
Constructed wetland cont…
If there is insufficient alkalinity in the mine water to
prevent a significant fall in pH as a result of these
reactions, this may be amended by the incorporation
of, for example, an anoxic limestone drain.
In order to maintain oxidizing conditions, aerobic
wetlands are relatively shallow systems that operate by
surface flow.
An aerobic wetland comprises channels or basins with
an impermeable bottom (to limit seepage loss), a
medium to sustain vegetation, and a shallow water
depth (10-50cm) in order to allow water to contact the
27. Biological remediation
Constructed wetland cont…
Anaerobic wetlands are designed to treat net acidic
and metal-laden waters.
They can be used only if a large enough area of land is
available.
Anaerobic wetlands usually consist of a limestone layer
overlain by an organic compost layer, over which the
AMD flows.
Water quality in wetlands is improved by filtration of
suspended and colloidal materials, and by adsorption
of metals to the soil substrate or other organic-based
substrates.
29. Biological remediation
Constructed wetland cont…
Constructed wetlands are efficient when the
effluent is low in its acidity loading, but often
show reduced efficiency and even fail under high
acid loading. For this reason, alkalinity additions
to wetland influent may precede constructed
wetland.
30. Biological remediation
Sulfate Reducing Bioreactors
Sulfate reduction has been shown to effectively treat
AMD/ARD containing dissolved heavy metals, including
aluminum, in a variety of situations. The chemical
reactions are facilitated by the bacteria desulfovibrio in
sulfate-reducing bioreactors
The sulfate-reducing bacterial reactions involve the
generation of:
- Sulfide ion (S-2), which combines with dissolved metals
to precipitate sulfides
- Bicarbonate (HCO3), which has been shown to raise the
pH of the effluent
