AMD in South Africa

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Oupa Nhambe
Student No: 587899
CHMT 4013: Wastewater Engineering
Assignment No: 1
Submission date: 30 March 2015

Table of Contents

The causes and consequences of Acid Mine Drainage in the Witwatersrand
basins and the consequences for South Africa ................................................................1
1. Introduction .....................................................................................................................1
2. Literature survey............................................................................................................2
2.1.

The Witwatersrand basins and gold mining in South Africa .......................2

2.2.

The occurrence and chemistry of AMD ..............................................................3

3. Critique of the issues .....................................................................................................4
3.1.

Environmental issues ............................................................................................4

3.2.

Economic issues .......................................................................................................4

3.3.

Social issues .............................................................................................................5

3.4.

Legal issues ..............................................................................................................5

4. Discussion.........................................................................................................................5
5. Conclusions ......................................................................................................................6
6. References ........................................................................................................................7

The causes and consequences of Acid Mine Drainage in the
Witwatersrand basins and the consequences for South Africa
1. Introduction
Pieter Jacob Marais was the first man to discover alluvial gold in the Transvaal
Republic now known as the republic of South Africa. This great feat was
achieved in 1853 in the Crocodile and Jukskei rivers. After this discovery, the
first commercial gold mine on the Witwatersrand was registered in 1886
September (Durand, 2012). Mining operations led to the development of the
conurbation which is now Johannesburg. A mercury amalgam was initially used
for gold extraction, but with time, this method became uneconomical. Another
method was developed; the new method used cyanide to extract gold from the
Witwatersrand ores. Large mine dumps (sand dumps) were used to deposit
tailings from the mercury extraction process and tailings from the cyanide were
dumped at disposal sites known as slimes dumps (Naicker et al., 2003; Tutu et
al., 2008).
Secondary mining activities in the Witwatersrand basins left an unpleasant
legacy of well recorded form of water pollution, which is called acid mine
drainage (AMD) (Johnson & Hallberg, 2005; Kalin et al., 2006). CSIR defines
AMD as “highly acidic water, usually containing high concentrations of metals,
sulphides, and salts as a consequence of mining activity” (CSRI, 2009). The issue
of AMD has environmental, social and economic implications for South Africa.
This essay will attempt to present in further details the causes and consequences
of AMD in the Witwatersrand basins and the consequences for South Africa.

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2. Literature survey
2.1.

The Witwatersrand basins and gold mining in South Africa

The Witwatersrand basins are located in Gauteng province. There is the west
rand basin (Krugersdorp area), the central basin (Rooderport to Boksburg area)
and the east rand basin (Springs, Brakpan and Nigel area). Gold mining
commenced in the Witwatersrand basin in September 1886; with gold extraction
achieved by using a mercury amalgam. The tailings produced from this process
were deposited dumps, commonly referred to as sand dumps. As gold mining
went deeper, the industry was faced with the challenge of un-oxidised pyritic
ores. This rendered the processes inefficient and uneconomical. The mercury
amalgam extraction process was substituted by cyanide extraction process. This
occurred in 1915 (Tutu et al., 2008).
For the newly developed cyanide extraction process, the gold ore had to be
transported to the surface, and there it is crushed. Using the cyanide the gold is
then extracted. After gold recovery, the solid waste material produced is dumped
on waste pile referred to as slimes or tailings dumps. The gold containing ore or
conglomerate contains about three per cent (3%) of pyrite, otherwise known as
“fool’s gold”. This compound ends up on the waste heaps dumped after gold has
been recovered. During rainy weather, the pyrite is oxidised to form sulphuric
acid which filters through the porous layer of the surface. During this slow
percolation movement, the acid dissolves some of the heavy metals, including
uranium. This acidic solution finds its way to the ground water stream as a
pollution source (McCarthy, 2011).
The problem of acid forming pyrite also occurs underground. Some gold
containing ores are left un-mined to assist miners or because their economic
value is too low for productive extraction. In the presence of oxygen, this pyrite is
oxides to form the acid.

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Some of the polluted waters are washed to steams draining the tailings. Such
streams are characterised by high pH and high sulphate and heavy metals
concentrations (Rosner & Schalkwyk, 2000).

2.2.

The occurrence and chemistry of AMD

Acid mine drainage (AMD) is produced when sulphide containing material is
exposed to water and oxygen. The formation of AMD occurs naturally, but
mining activities can encourage AMD formation through exposure of the
sulphides. There are bacteria that occur naturally, and can speed up the process
of AMD generation by assisting in the breakdown of sulphide minerals. Low pH
and elevated heavy metals concentrations and a variety of toxic elements are
observed for sites affected by AMD (Akcil & Koldas, 2006).
The occurrence of AMD is demonstrated by Akcil and Koldas (2006), showing the
oxidation of pyrite (FeS2). Firstly, the sulphide compound is oxidised to form
dissolved iron, sulphate and hydrogen:
FeS2 + 7/2O2 +H2O → Fe2+ + 2SO2-4 + 2H+

(1)

The products formed increases the concentration of dissolved solids, and the
acidity of the medium is decreased. If enough oxygen is present, the ferrous iron
formed will further oxidise to ferric iron as follows:
Fe2+ + 1/4O2 + H+ → Fe3+ + 1/2H2O

(2)

At lower pH ferric iron will form a precipitate, leaving the solution with very
little ferric iron, and the pH is further lowered:
Fe3+ + 3H2O → Fe(OH)3s + 3H+

(3)

The little ferric iron left is used to oxidise more of the pyrite as follows:
FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO2-4 +16H+

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(4)

These reactions show how the highly acidic solution is formed. The overall
reaction scheme is as follows:
4 FeS2 +15O2 + 14 H2O → 4 Fe(OH)3s + 82SO2-4 + 16H+

(5)

3. Critique of the issues
Most of the mines on the Witwatersrand basin shut operations a while ago.
During operation, the continuous seepage of water into the groundwater was
pumped out to avoid flooding. When operations were halted, pumping of the
water was stopped. So the void created by the mines started filling with water.
This is mostly rain water that seeps through the surface towards the
groundwater table. The central basin is flooding at 60ML per day, with no ways
of disturbing the acidic waters before coming into contact with groundwater and
start decanting in Boksburg. In August 2002, the Krugersdorp-Randfontein area
began to decant. The amount of decanting volume is at the moment between 1836ML in a day (McCarthy, 2011; Durand, 2012).

3.1.

Environmental issues

The problem of AMD leads to the destruction of aquatic life in the rivers, lakes,
ponds etc., if the mine effluent is discharged. The streams which receive the
discharged effluent see an increase in salinity, turbidity, toxicity and
concentration of radioactive and heavy metals increases. The pH is lowered.
Some toxins accumulate in organisms in the food chain. This accumulation can
have a deadly effect on higher trophic levels (Durand, 2012; Luis et al., 2008).

3.2.

Economic issues

The reduction of the environmental effects from AMD is very enormous. The
South African government is faced with the biggest environmental liability.
Treatment of acid mine drainage is very costly, and with increasing effluent
discharge from decanting old mines, costs are expected to increase (Oelofse,
2012). According to Johnson and Hallberg (2005), the proposed treatment
processes are very expensive for the South African government.

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3.3.

Social issues

The ground water from the karstic aquifer is the sole source of water for a couple
of towns, some rural settlements and farms situated on the karst system. If it
was not for the pollution from acid mine drainage, this aquifer would also supply
water to the people and industries of the Witwatersrand (Durand, 2012). Other
social impacts includes exposure to radioactive elements such as uranium, these
elements are mutagenic and also carcinogenic.

3.4.

Legal issues

According to Oelofse (2010), the issue of acid containing water from mine tailings
is the most costly socio-economic and environmental problem in South Africa.
The government has been trying to liaise with current mine owners for the
rehabilitation of the places affected by mining operations, but mines are refusing
to take responsibility, claiming that they had inherited the AMD issue when
they took over mining operations.

4. Discussion
The issue of AMD is costly environmental problem facing the South African
government, since most the mines were abandoned years back, and are
ownerless. AMD is characterised by elevated concentrations of heavy metals and
lower pH values, and can severely pollute ground and surface water sources.
It is therefore imperative to address this issue before an ecological disaster
occurs. There are a couple of treatment options proposed for the treatment of
acidified waters from mine dumps. Johnson and Hallberg (2005) proposed active
and passive treatment methods for acid mine drainage. For the formation of
AMD water and oxygen is required, so preventing either of these can minimise
the production of AMD. This can be achieved by flooding and sealing mines left
with no owners. Dissolved oxygen available in the water can be consumed
biochemically by microbes.

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Active treatment technologies are systems that are engineered water treatment
schemes. Chemical alterations are used for treating water to achieve certain
specified standards. Addition of alkaline materials to AMD raises the pH and
increases the rate of chemical reaction of ferrous iron, and the metals present in
solution will precipitate out as carbonates and hydroxides. Iron-rich sludge is
produced as a result. Active remediation processes have the potential of treating
AMD but they are costly. In passive technology an alkaline substance is added to
AMD and the iron is kept in its reduced form, this prevents armouring from
occurring.
Carbon partial pressure is increased, and the rate of reaction increases as a
result. The downside with passive technologies, they are suited for all AMD
waters.
AMD can also be remediated using biological strategies. Bioremediation uses
microorganisms to increase alkalinity and immobilise metals, and thus reversing
the reaction from the formation of AMD. Microbial activities that give rise to
overall net alkalinity are at most reductive processes. These processes include
de-nitrification, methanogenesis, sulphate reduction, and magnesium and iron
reduction. Material for these processes is scarce; their net contribution to AMD
treatment is of minor importance (Johnson & Hallberg, 2005).

5. Conclusions
In South Africa there are three hot spots concerning the issue of AMD. These are
the west rand basin, central basin and the eastern basin. It can be concluded
that water must be removed from the three priority areas and treated. This will
help keep the water levels low. Also ways to reduce or stop the ingress of water
into the mine voids must be implemented. By so doing the amount of water that
needs to be pumped and treated will be minimised. These will only cater for
short term solutions while permanent solutions are still being studied.

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6. References
Akcil, A., & Koldas, S. (2006). Acid Mine Drainage (AMD): causes, treatment and
case studies. Journal of Cleaner Production, 14(12), 1139-1145.
CSIR (2009, August). Briefing note. Acid mine drainage in South Africa. Dr. Pat
Manders. Director, Natural resources and the Environment
Durand JF (2012). The impact of gold mining in the Witwatersrand on the rivers
and karst system of Gauteng and Northwest Province, South Africa, Journal of
African Earth Science 68: 24-43.
Johnson, D. B., & Hallberg, K. B. (2005). Acid mine drainage remediation
options: a review. Science of the total environment, 338(1), 3-14.
Kalin M., Fyson A., & Wheeler WN. (2006). The chemistry of conventional and
alternative treatment systems for the neutralization of acid mine drainage.
Science of the total environment, 366: 398-408
Luis AT., Teixeira P., Almeida SFP., Ector L., Matos JX., & Ferreirra da Silva
EA (2008). Impact of Acid Mine Drainage (AMD) on Water Quality, Stream
Sediments and Periphytic Diatom Communities in the Surrounding streams of
aljustrel mining area, water air soil pollut, 1-21.
McCarthy, T. S. (2011). The impact of acid mine drainage in South Africa. South
African Journal of Science, 107(5-6), 01-07.
Naicker, K., Cukrowska, E., & McCarthy, T. S. (2003). Acid mine drainage
arising from gold mining activity in Johannesburg, South Africa and environs.
Environmental Pollution, 122(1), 29-40.

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Oelofse S (2010). The pollution reality of gold mining waste on the
Witwatersrand, Waste Water management, ReSource, 51-55.
Rosner T., & van Schalkwyk A. (2000). The environmental impact of gold mine
tailing footprints in the Johannesburg region, South Africa. Bull Eng Geol Env,
59: 137-148

Tutu, H., McCarthy, T. S., & Cukrowska, E. (2008). The chemical characteristics
of acid mine drainage with particular reference to sources, distribution and
remediation: The Witwatersrand Basin, South Africa as a case study. Applied
Geochemistry, 23(12), 3666-3684.

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