Chemical Tech
nology • September 2013
Containing and recovering
hazardous water
O
n 22 February 1994, 50 mm of rain fell in 30
minutes. Enough to trigger flash floods. Enough
to destabilize.
Virginia, in the Orange Free State, is a mining town
feeding the Goldfields mines in the area. In 1978 a
new tailings dam was built 320 m from the suburb of
Merriespruit. There were always problems in the early
days. The slurry had too much water making it difficult
to ensure dam stability.
In 1993, inspections noted water seepage out of the
north wall and deposition was stopped. The freeboard
– the amount of space left in the dam to accommodate
rising water – was one metre and considered within
safety limits. The wall between 4A and 4B, ponds within
the dam, broke and liquid poured across, reducing the
freeboard to only 300 mm. Despite the call for a stop-
page, excess water loaded with tailings continued to be
deposited.
Then came the rains.
Mining engineers have a colourful argot to describe
the dark, dusty, dangerous world they work in. These,
and the descriptions for tailings dam construction and
management are in the “Design and evaluation of tail-
ing dams” technical report issued by the US Environ-
mental Protection Agency.
‘Overburden’ is the rock extracted from a mine that
is unproductive. ‘Tailings’ are the waste material left
behind from productive ore. A tailings, or ‘slimes’, dam
is where tailings are contained after mining. ‘Piping’
is the subsurface erosion along a seepage pathway
in a dam embankment which results in a void space
creating additional water flow and the potential to
create a direct channel through the wall. ‘Liquefaction’
is the temporary suspension in water of fine, similarly-
sized, rock particles which behave like a liquid, passing
through narrow openings and able to flow tremendous
distances.
Modern mining techniques allow low-grade ore to be
milled to extract metals and minerals that often make
up only a few percent of the rock. What remains is a
mix of fine particles ranging from sand-like grains down
to particles only a few microns in size. Gold extraction
requires mixing this fined rock with cyanide and water
to convert it to salts and then leach it out.
After extraction, what remains – more than 90 per-
cent by volume − is the tailing; toxic, semi-liquid slime.
And it has to go somewhere.
For South Africans who live near mines, slimes
dams are a common sight as is the dust which flies off
the top of old dams, clogging the sky and dirtying the
washing. Slimes dams must be maintained long after
the commercial life of a mine has ended. It is one of
the costs that must be calculated into the operational
overhead of running a mine. Sometimes those calcula-
tions are too optimistic and then short-cuts are taken
to save money.
On 22 February 1994, one set of short-cuts was
about to have a brutal resolution.
The sudden rain squall caused the Merriespruit dam
freeboard to pass through its critical zone. The seepage
on the north wall coalesced into piping, driving water
though the wall. The sudden turbulence resulted in tail-
ing liquefaction.
The north wall failed.
Just after 7pm, a wall of slime 2,5 metres high
travelled 300 m and hit the first row of houses. Six
hundred thousand cubic metres of slime thrust its way
a further four kilometres from the dam. 17 people died.
Eighty houses were destroyed.
In the aftermath, the owner, operator, and six
employees were found guilty of negligence. It also trig-
gered slimes dam reviews and improved safety across
South Africa.
The biggest problem for tailings dams is the water.
A dam must be both sufficiently low-cost to build and
maintain, while also offering an opportunity for recovery
and containment of the water.
There are numerous ways of building and situating
such dams; from placing them in valley depressions, to
backfilling old mines, to building ring-dikes. The latter
are ‘free-standing’ in that they are built on flat ground
by Gavin Chait
Rolling thunder and the Highveld savannah are part of the joy of living in that remote, dusty
part of South Africa. Occasionally, when the rains do fall, they come all at once.
special report
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